Application Note 66

AN66-1

Linear Technology Magazine Circuit Collection, Volume IIPower Products

Richard Markell, Editor

August 1996

INTRODUCTION

Application Note 66 is a compendium of power circuitsfrom the first five years of Linear Technology. The objectiveis to collect the useful circuits from the magazine intoseveral applications notes (another, AN67, will collectsignal processing circuits into one Application Note) sothat valuable gems will not be lost. This Application Notecontains circuits that can power most any system you canimagine, from desktop computer systems to micropowersystems for portable and handheld equipment. Also

, LTC and LT are registered trademarks of Linear Technology Corporation.

included here are circuits that provide 300W or more ofpower factor corrected DC from a universal input. Batterychargers are included, some that charge several batterytypes, some that are optimized to charge a single type.MOSFET drivers, high side switches and H-bridge drivercircuits are also included, as is an article on simple thermalanalysis. With these introductory remarks, Ill stand asideand let the authors describe their circuits.

ARTICLE INDEXREGULATORSSWITCHING (BUCK)High Power (>4A)

Big Power for Big Processors: The LTC1430 Synchronous Regulator ............................................................. 4Applications for the LTC1266 Switching Regulator ............................................................................................ 5A High Efficiency 5V to 3.3V/5A Converter ......................................................................................................... 7High Current, Synchronous Step-Down Switching Regulator ............................................................................ 8

Medium Power (1A to 4A)1MHz Step-Down Converter Ends 455kHz IF Woes ......................................................................................... 10High Output Voltage Buck Regulator ................................................................................................................ 11The LTC1267 Dual Switching Regulator Controller Operates from High Input Voltages................................... 12High Efficiency 5V to 3.3V/1.25A Converter in 0.6 Square Inches .................................................................... 13LT1074/LT1076 Adjustable 0V to 5V Power Supply ....................................................................................... 14Triple Output 3.3V, 5V and 12V High Efficiency Notebook Power Supply ........................................................ 15The New SO-8 LTC1147 Switching Regulator Controller Offers High Efficiency in a Small Footprint ............... 17The LT1432: 5V Regulator Achieves 90% Efficiency ........................................................................................ 20

Low Power (

Application Note 66

AN66-2

REGULATORSSWITCHING (BUCK/BOOST)5V Converter Uses Off-the-Shelf Surface Mount Coil ..................................................................................... 27Switching Regulator Provides Constant 5V Output from 3.5V to 40V Input Without a Transformer ................ 28Switching Regulator Provides 15V Output from an 8V to 40V Input Without a Transformer ......................... 29

REGULATORSSWITCHING (INVERTING)High Efficiency 12V to 12V Converter ............................................................................................................ 32Regulated Charge Pump Power Supply ............................................................................................................ 34Applications for the LTC1265 High Efficiency Monolithic Buck Converter ........................................................ 22LTC1174: A High Efficiency Buck Converter ..................................................................................................... 35

REGULATORSSWITCHING (FLYBACK)Applications for the LT1372 500kHz Switching Regulator ............................................................................... 25

REGULATORSSWITCHING (POWER FACTOR CORRECTED)The New LT1508/LT1509 Combines Power Factor Correction and a PWM in a Single Package ...................... 37

REGULATORSSWITCHING (DISCUSSION)Adding Features to the Boost Topology............................................................................................................ 39Sensing Negative Outputs ................................................................................................................................ 40

REGULATORSSWITCHING (MICROPOWER)3-Cell to 3.3V Buck/Boost Converter ................................................................................................................ 41LT1111 Isolated 5V Switching Power Supply ................................................................................................... 41Low Noise Portable Communications DC/DC Converter ................................................................................... 43Applications for the LT1302 Micropower DC/DC Converter ............................................................................. 44Clock-Synchronized Switching Regulator Has Coherent Noise ........................................................................ 49Battery-Powered Circuits Using the LT1300 and LT1301 ................................................................................. 51LTC1174: A High Efficiency Buck Converter ..................................................................................................... 35Battery-Powered Circuits Using the LT1304 Micropower DC/DC Converter with Low-Battery Detector ........... 54Automatic Load Sensing Saves Power in High Voltage Converter .................................................................... 57

REGULATORSSWITCHING (MICROPOWER)Backlight

High Efficiency EL Driver Circuit ....................................................................................................................... 58A Low Power, Low Voltage CCFL Power Supply .............................................................................................. 60All Surface Mount EL Panel Driver Operates from 1.8V to 8V Input ................................................................. 61A Dual Output LCD Bias Voltage Generator ...................................................................................................... 62LCD Bias Supply............................................................................................................................................... 63

REGULATORSSWITCHING (MICROPOWER)Switched Capacitor

Regulated Charge Pump Power Supply ............................................................................................................ 34REGULATORSSWITCHING (MICROPOWER)VPP Generator

LTC1262 Generates 12V for Programming Flash Memories Without Inductors ............................................... 64Flash Memory VPP Generator Shuts Down with 0V Output ............................................................................. 64

Application Note 66

AN66-3

REGULATORSLINEARLow Noise Wireless Communications Power Supply ....................................................................................... 65An LT1123 Ultralow Dropout 5V Regulator ...................................................................................................... 66

REGULATORSLINEARMicroprocessor Power

LT1580 Low Dropout Regulator Uses New Approach to Achieve High Performance ....................................... 67LT1585: New Linear Regulator Solves Load Transients ................................................................................... 68

BATTERY CHARGERSCharging NiMH/NiCd or Li-Ion with the LT1510 ............................................................................................... 70Lithium-Ion Battery Charger ............................................................................................................................. 71Simple Battery Charger Runs at 1MHz ............................................................................................................. 73A Perfectly Temperature Compensated Battery Charger ................................................................................... 74A Simple 300mA NiCd Battery Charger ............................................................................................................ 75High Efficiency (>90%) NiCd Battery Charger Circuit Programmable for 1.3A Fast Chargeor 100mA Trickle Charge.................................................................................................................................. 76

POWER MANAGEMENTLT1366 Rail-to-Rail Amplifier Controls Topside Current .................................................................................. 78An Isolated High Side Driver ............................................................................................................................ 79LTC1163: 2-Cell Power Management ............................................................................................................... 80LTC1157 Switch for 3.3V PC Card Power ........................................................................................................ 81The LTC1157 Dual 3.3V Micropower MOSFET Driver ...................................................................................... 82The LTC1155 Does Laptop Computer Power Bus Switching, SCSI Termination Power or5V/3A Extremely Low Dropout Regulator ......................................................................................................... 82A Circuit That Smoothly Switches Between 3.3V and 5V.................................................................................. 84A Fully Isolated Quad 4A High Side Switch ...................................................................................................... 85The LTC1153 Electronic Circuit Breaker ........................................................................................................... 86LTC1477: 0.07 Protected High Side Switch Eliminates Hot Swap Glitching ............................................... 87

MISCELLANEOUSProtected Bias for GaAs Power Amplifiers ....................................................................................................... 88LT1158 H-Bridge Uses Ground Referenced Current Sensing for System Protection........................................ 89LT1158 Allows Easy 10A Locked Antiphase Motor Control .............................................................................. 91All Surface Mount Programmable 0V, 3.3V, 5V and 12V VPP Generator for PCMCIA ...................................... 92A Tachless Motor Speed Controller .................................................................................................................. 93LT1161...And Back and Stop and Forward and RestAll with No Worries at All ............................................ 95Simple Thermal AnalysisA Real Cool Subject for LTC Regulators ............................................................... 98

ALPHABETIC INDEXBy Major Categories ....................................................................................................................................... 101

Application Note 66

AN66-4

similar class processor and the input is taken from thesystem 5V 5% supply. The LTC1430 provides the pre-cisely regulated output voltage required by the processorwithout the need for an external precision reference ortrimming. Figure 1 shows a typical application with a3.30V 1% output voltage and a 12A output current limit.The power MOSFETs are sized so as not to require a heatsink under ambient temperature conditions up to 50C.Typical efficiency is above 91% from 1A to 10A outputcurrent and peaks at 95% at 5A (Figure 2).

Figure 1. Typical 5V to 3.3V, 10A LTC1430 Application

RegulatorsSwitching (Buck)High Power (>4A)

BIG POWER FOR BIG PROCESSORS:THE LTC1430 SYNCHRONOUS REGULATORby Dave Dwelley

The LTC1430 is a new switching regulator controllerdesigned to be configured as a synchronous buck con-verter with a minimum of external components. It runs ata fixed switching frequency (nominally 200kHz) and pro-vides all timing and control functions, adjustable currentlimit and soft start, and level shifted output drivers de-signed to drive an all N-channel synchronous buck con-verter architecture. The switch driver outputs are capableof driving multiple paralleled power MOSFETs withsubmicrosecond slew rates, providing high efficiency atvery high current levels while eliminating the need for aheat sink in most designs. The LTC1430 is usable inconverter designs providing from a few amps to over 50Aof output current, allowing it to supply 3.3V power to themost current-hungry arrays of microprocessors.

A Typical 5V to 3.3V Application

The typical application for the LTC1430 is a 5V to 3.xVconverter on a PC motherboard. The output is used topower a Pentium processor, Pentium Pro processor or

Pentium is a registered trademark of Intel Corporation.

Figure 2. Efficiency Plot for Figure 1s Circuit. Note ThatEfficiency Peaks at a Respectable 95%

+

+ +

AN66 F01

VOUT 3.3V

C30.1F

M2MTD20N03HL

CIN220F10V 4

L12.5H/15A

COUT330F6.3V 6

M1BMTD20N03HL

M1AMTD20N03HL

PVCC1IMAX

FREQ

SGND

SHDN

COMP NC

SS

PVCC2SVCC

PGND

PGNDSGND

PGND AND SGNDCONNECTED AT A SINGLE POINT

L1 =CIN =

COUT =

6 TURNS #16 WIRE ON MICROMETALS T50-52B CORE4 EACH AVX TPSE 227M010R01006 EACH AVX TPSE 337M006R0100

*TRIM TO OPTIMIZE TRANSIENT REPONSE

SGND

R116k

RC*33k

C10.1F

CC*3300pF

100pF*CSS0.01F

C210F

R2100

D11N4148

G1

IFB

G2

VTRIM

+SENSESHUTDOWN

NC

R31k

VIN4.5V TO 5.5V

LTC1430

SENSE

LOAD CURRENT (A)

40

70

100

90

80

50

60EFF

ICIE

NCY

(%)

10

AN66 F02

0.1 1

VCC = 5VTA = 25CVOUT = 3.3V

Application Note 66

AN66-5

20mV/DIV

5A/DIV

Figure 3. Transient Response: 0A to 5A Load StepImposed on Figure 1s Output

largest value, lowest ESR capacitors that will fit thedesign budget and space requirements. Several smallercapacitors wired in parallel can help reduce total outputcapacitor ESR to acceptable levels. Input bypass capaci-tor ESR is also important to keep input supply variationsto a minimum with 10AP-P square wave current pulsesflowing into M1. AVX TPS series surface mount tantalumcapacitors and Sanyo OS-CON organic electrolytic ca-pacitors are recommended for both input and outputbypass duty. Low cost computer grade aluminumelectrolytics typically have much higher series resistanceand will significantly degrade performance. Dont counton that parallel 0.1F ceramic cap to lower the ESR of acheap electrolytic cap to acceptable levels.

The 12A current limit is set by the 16k resistor R1 fromPVCC to IMAX and the 0.035 ON resistance of theMTD20N03HL MOSFETs (M1A, M1B).

The 0.1F capacitor in parallel with R1 improves powersupply rejection at IMAX, providing consistent current limitperformance when voltage spikes are present at PVCC.Soft start time is set by CSS; the 0.01F value shown reactswith an internal 10A pull-up to provide a 3ms start-uptime. The 2.5H, 15A inductor is sized to allow the peakcurrent to rise to the full current limit value withoutsaturating. This allows the circuit to withstand extendedoutput short circuits without saturating the inductor core.The inductor value is chosen as a compromise betweenpeak ripple current and output current slew rate, whichaffects large-signal transient response. If the output loadis expected to generate large output current transients (aslarge microprocessors tend to do), the inductor value willneed to be quite low, in the 1H to 10H range.Loop compensation is critical for obtaining optimumtransient response with a voltage feedback system likethe LTC1430; the compensation components shownhere give good response when used with the outputcapacitor values and brands shown (Figure 3). The ESRof the output capacitor has a significant effect on thetransient response of the system. For best results use the

APPLICATIONS FORTHE LTC1266 SWITCHING REGULATORby Greg Dittmer

Figures 4, 5 and 6 show the three basic circuit configura-tions for the LTC1266. The all N-channel circuit shown inFigure 4 is a 3.3V/5A surface mount converter with theinternal MOSFET drivers powered from a separate supply,PWR VIN. The VGS(ON) of the Si9410 N-channel MOSFETsis 4.5V; thus the minimum allowable voltage for PWR VINis VIN(MAX) + 4.5V. At the other end, PWR VIN should bekept under the maximum safe level of 18V, limiting VIN to18V 4.5V = 13.5V. The current sense resistor value ischosen to set the maximum current to 5A according to theformula IOUT = 100mV/RSENSE. With VIN = 5V, the 5Hinductor and 130pF timing capacitor provide an operatingfrequency of 175kHz and a ripple current of 1.25A.

Figure 5 shows an LTC1266 in the charge pump configu-ration designed to provide a 3.3V/10A output from a singlesupply. The Si4410s are new logic level, surface mount,N-channel MOSFETs from Siliconix that provide a mere0.02 of on-resistance at VGS = 4.5V and thus provide a10A solution with minimal components. The efficiencyplot shows that the converter is still close to 90% efficientat 10A. Because the charge pump configuration is used,the maximum allowable VIN is 18V/2 = 9V. Due to the highAC currents in this circuit we recommend low ESROS-CON or AVX input/output capacitors to maintain effi-ciency and stability.

Figure 6 shows the conventional P-channel topside switchcircuit configuration for implementing a 3.3V/3A regula-tor. The P-channel configuration allows the widest pos-sible supply range of the three basic circuit configurations,

AN66 F03

Application Note 66

AN66-6

3.5V to 18V, and provides extremely low dropout, exceed-ing that of most linear regulators. The low dropout resultsfrom the LTC1266s ability to achieve a 100% duty cyclewhen in P-channel mode. In N-channel mode the dutycycle is limited to less than 100% to ensure proper start-up and thus the dropout voltage for the all N-channelconverters is slightly higher.

The three application circuits demonstrate the fixed 3.3Vversion of the LTC1266. The LTC1266 is also available infixed 5V and adjustable versions. All three versions areavailable in 16-pin SO packages.

+

+

AN66 F04a

LTC1266-3.3

D1MBRS140T3

CIN100F20VOSCON 2

VIN3.5V TO 14V

VOUT3.3V/5A

PINV

PWR VINPWR VIN

(SEE TEXT)

TDRIVE

BINHBINH

VIN

CT

ITHCC3300pF

0.1F

CT130pF

RC470

RSENSE0.02

L5H

1000pF

COUT330F10V 2

SENSE

3

2

1

4

5

6

7

8

14

15

16

13

12

11

10

9

LBOUT

PGND

BDRIVE

Si9410DY

Si9410DY

LBIN

SGND

SHDN SHDN

NC

SENSE +

Figure 4a. All N-Channel 3.3V/5A Regulator with Drivers Poweredfrom Seperate Power VIN (PWR VIN) Supply

LOAD CURRENT (A)

90

85

80

95

100

EFFI

CIEN

CY (%

)

10

AN66 F05b

0.01 0.1 1

VIN = 5V

Figure 5b. Efficiency for Figure 5as Circuit

+

+

AN66 F05a

LTC1266-3.3

D1MBRS340T3

CIN100F10VOS-CON 3

VIN4V TO 9V

VOUT3.3V10A

PINV

PWR VIN

TDRIVE

BINHBINH

VIN

CT

ITHCC3300pF

0.1F

CT220pF

RC470

RSENSE0.01

L5H

1000pF

COUT330F10V 3 SENSE

3

2

1

MBR0530T1

4

5

6

7

8

14

15

16

13

12

11

10

9

LBOUT

PGND

BDRIVE

Si4410DY

Si4410DY

LBIN

SGND

SHDN SHDN

NC

SENSE +

Figure 5a. All N-Channel Single Supply 5V to 3.3V/10A Regulator

Figure 4b. Efficiency for Figure 4as Circuit

LOAD CURRENT (A)

90

85

80

95

100

EFFI

CIEN

CY (%

)

5

AN66 F04b

0.01 0.1 1

VIN = 5V

Application Note 66

AN66-7

+

+

AN66 F06a

LTC1266-3.3

D1MBRS140T3

CIN100F25V

VIN3.5V TO 18V

VOUT3.3V3A

PINV

PWR VIN

TDRIVE

BINHBINH

VIN

CT

ITHCC3300pF

0.1F

CT220pF

RC1k

RSENSE0.033

L10H

1000pF

COUT220F10V 2 SENSE

3

2

1

4

5

6

7

8

14

15

16

13

12

11

10

9

LBOUT

PGND

BDRIVE

Si9430DY

Si9410DY

LBIN

SGND

SHDN SHDN

NC

SENSE +

Figure 6a. Low Dropout 3.3V/3A Complementary MOSFET Regulator

LOAD CURRENT (A)

90

85

80

95

100

EFFI

CIEN

CY (%

)

3

AN66 F06b

0.01 0.1 1

VIN = 5V

Figure 6b. Efficiency for Figure 6as Circuit

A HIGH EFFICIENCY 5V TO 3.3V/5A CONVERTERby Randy G. Flatness

The next generation of notebook and desktop computersis incorporating more 3.3V ICs alongside 5V devices. Asthe number of devices increases, the current require-ments also increase. Typically, a high current 5V supply isalready available. Thus, the problem is reduced to deriving3.3V from 5V efficiently in a small amount of board space.

High efficiency is mandatory in these applications, sinceconverting 5V to 3.3V at 5A using a linear regulator wouldrequire dissipating over 8W. This wastes power and boardspace for heat sinking.

The LTC1148 synchronous switching regulator controlleraccomplishes the 5V to 3.3V conversion with high effi-ciencies over a wide load current range. The circuit shownin Figure 7 provides 3.3V at efficiencies greater than 90%

Figure 7. LTC1148-3.3 High Efficiency 5V to 3.3V/5A Step-Down Converter

AN66 F07

TANTALUMSANYO (OS-CON) 20SA100M ESR = 0.037 IRMS = 2.25AAVX (TA) TPSE227K01R0080 ESR = 0.080 IRMS = 1.285ASILICONIX PMOS BVDSS = 20V DCRON = 0.100 Qg = 50nC SILICONIX NMOS BVDSS = 30V DCRON = 0.050 Qg = 30nCMOTOROLA SCHOTTKY VBR = 30VKRL NP-2A-C1-0R020J Pd = 3WKOOL M CORE, 16 GAUGECOILTRONICS (408)241-7876KRL BANTRY (603) 668-3210SILICONIX (800) 554-5565KOOL M IS A REGISTERED TRADEMARK OF MAGNETICS, INC.

C1 = C3 = C6 =

Q1, Q2 = Q3 = D1 = R2 = L1 =

SHDN

ITH

CT

LTC1148-3.3

VIN

SENSE +

SENSE

+

C70.01F

+

C20.1F

R1470

C43300pF

C5680pFNPO

10

6

4

1

8

7

14

Q2Si9430DY

Q3Si9410 D1

MBRS140T3

C3100F20V 2

L1

27H

R20.02

VOUT3.3V5A

C6220F10V 2

3

11

12

PDRIVE

NDRIVESGND PGND

+

Q1Si9430DY

C11F

VIN5V

0V = >2V =

NORMALSHUTDOWN

Application Note 66

AN66-8

Figure 8. Efficiency for 5V to 3.3V Synchronous Switcher

AN66 F08

OUTPUT CURRENT (mA)1

70

EFFI

CIEN

CY (%

)100

10 10000100

90

1000

80

maximize the operating efficiency at low output currents,Burst ModeTM operation is used to reduce switching losses.Synchronous switching, combined with Burst Mode op-eration, yields very efficient energy conversion over a widerange of load currents.

The top P-channel MOSFETs in Figure 7 will be on 2/3 ofthe time with an input of 5V. Hence, these devices shouldbe carefully examined to obtain the best performance. TwoMOSFETs are needed to handle the peak currents safelyand enhance high current efficiency. The LTC1148 candrive both MOSFETs adequately without a problem. Asingle N-channel MOSFET is used as the bottom synchro-nous switch, which shunts the Schottky diode. Finally,adaptive anti-shoot-though circuitry automatically pre-vents cross conduction between the complementaryMOSFETs which can kill efficiency.

The circuit in Figure 7 has a no-load current of only 160A.In shutdown mode, with Pin 10 held high (above 2V), thequiescent current decreases to less than 20A with allMOSFETs held off DC. Although the circuit in Figure 7 isspecified at a 5V input voltage, the circuit will function from4V to 15V without requiring any component substitutions.Burst Mode is a trademark of Linear Technology Corporation.

from 5mA to 5A (over three decades of load current). Theefficiency of the circuit in Figure 7 is plotted in Figure 8.

At an output current of 5A the efficiency is 90%; thismeans only 1.8W are lost. This lost power is distributedamong RSENSE, L1 and the power MOSFETs; thus heatsinking is not required.

The LTC1148 series of controllers use constant off-timecurrent mode architecture to provide clean start-up, accu-rate current limit and excellent line and load regulation. To

HIGH CURRENT, SYNCHRONOUSSTEP-DOWN SWITCHING REGULATORby Brian Huffman

The LTC1149 is a half-bridge driver designed for syn-chronous buck regulator applications. Normally a P- andN-channel output stage is employed, but the P-channeldevice ON resistance becomes a limiting factor at outputcurrents above 2A. N-channel MOSFETs are better suitedfor use in high current applications, since they have asubstantially lower ON resistance than comparably pricedP-channels. The circuit shown in Figure 9 adapts theLTC1149 to drive a half-bridge consisting of twoN-channel MOSFETs, providing efficiency in excess of90% at an output current of 5A.

The circuits operation is as follows: the LTC1149 providesa P-drive output (Pin 4) that swings between ground and10V, turning Q3 on and off. While Q3 is on, the N-channelMOSFET (Q4) is off because its gate is pulled low by Q3through D2. During this interval, the Ngate output (Pin 13)turns the synchronous switch (Q5) on creating a lowresistance path for the inductor current.

Q4 turns on when its gate is driven above the input voltage.This is accomplished by bootstrapping capacitor C2 offthe drain of Q4. The LTC1149 VCC output (Pin 3) suppliesa regulated 10V output that is used to charge C2 throughD1 while Q4 is off. With Q4 off, C2 charges to 5V during thefirst cycle in Burst Mode operation and to 10V thereafter.

Application Note 66

AN66-9

(TA) LOW ESRNICHICON (AL) UPL1J102MRH, ESR = 0.027, IRMS = 2.370A SANYO (OS-CON) 10SA220M, ESR = 0.035, IRMS = 2.360A PNP, BVCEO = 30VNPN, BVCEO = 40VSILICONIX NMOS, BVDSS = 60V, RDSON = 5

C3CIN

COUTQ1Q2Q3

VIN12V TO 36V

VINPGATE

SENSE

NGATE

P-DRIVE

SENSE+

ITH

CT

SHDN2

SHDN1

VCC

VCC

CAP

C33.3F

+

SGND PGND RGND

LTC1149-5

3

5

16

10

15

7

6

11

1

4

9

8

13

2

0V = NORMAL>2V = SHUTDOWN

R11kC4

3300pFX7R

CT820pF

NPO 12 14

C40.001F

Q3VN2222LL

D21N4148

R5100

R6100

R3470

D11N4148

C10.1F

+

Q12N3906

R210k

Q22N2222

R4220

C20.1F

D3MBR160

Q4MTP30N06EL

L150H

RSENSE0.02

COUT220F10V 2

+5V5A

CIN1000F63V

Q5IRFZ34

NMOS, BVDSS = 60V, RDSON = 0.05SILICON, VBR = 75VMOTOROLA SCHOTTKY, VBR = 60VKRL NP-2A-C1-0R020J, PD = 3WCOILTRONICS CTX50-5-52, DCR = 0.21, IRON POWDER COREALL OTHER CAPACITORS ARE CERAMIC

Q4, Q5D1, D2

D3RSENSE =

L1 =

+

AN66 F09

Figure 9. LTC1149-5 (12V-36V to 5V/5A) Using N-Channel MOSFETs

When Q3 turns off, the N-channel MOSFET is turned on bythe SCR-connected NPN/PNP network (Q1 and Q2). Re-sistor R2 supplies Q2 with enough base drive to trigger theSCR. Q2 then forces Q1 to turn on, supplying more basedrive to Q2. This regenerative process continues until bothtransistors are fully saturated. During this period, thesource of Q4 is pulled to the input voltage. While Q4 is on,its gate source voltage is approximately 10V, fully enhanc-ing the N-channel MOSFET.

Efficiency performance for this circuit is quite impressive.Figure 10 shows that for a 12V input the efficiency neverdrops below 90% over the 0.6A to 5A range. At higherinput voltages efficiency is reduced due to transitionlosses in the power MOSFETs. For low output currentsefficiency rolls off because of quiescent current losses.

OUTPUT CURRENT (A)0.1

50

EFFI

CIEN

CY (%

)

80

100

1 5

AN66 F10

60

70

90

36V

24V

12V

Figure 10. LTC1149-5 (12V-36V to 5V/5A) High Current Buck

Application Note 66

AN66-10

RegulatorsSwitching (Buck)Medium Power (1A to 4A)

1MHz STEP-DOWN CONVERTERENDS 455kHz IF WOESby Mitchell Lee

There can be no doubt that switching power supplies andradio IFs dont mix. One-chip converters typically operatein the range of 20kHz to 100kHz, placing troublesomeharmonics right in the middle of the 455kHz band. Thiscontributes to adverse effects such as desensing andoutright blocking of the intended signals. A new class ofswitching converter makes it possible to mix high effi-ciency power supply techniques and 455kHz radio IFswithout fear of interference.

The circuit shown in Figure 11 uses an LT1377 boostconverter operating at 1MHz to implement a high effi-

ciency buck topology switching regulator. The switch isinternally grounded, calling for the floating supply ar-rangement shown (D1 and C1). The circuit converts inputsof 8V through 30V to a 5V/1A output.

The chips internal oscillator operates at 1MHz for loadcurrents of greater than 50mA with a guaranteed toleranceof 12% over temperature. Even wideband 455kHz IFs areunaffected, as the converters operating frequency is wellover one octave distant.

Figure 12 shows the efficiency of Figure 11s circuit. Youcan expect 80% to 90% efficiency over an 8V to 16V inputrange with loads of 200mA or more. This makes the circuitsuitable for 12V battery inputs (thats how Im using it), butno special considerations are necessary with adapterinputs of up to 30V.

AN66 F11

LT1377

CTX20-2P*

2k

1.24k

5V1A

*CTX20-2P, COILTRONICS 20H**OS-CON, SANYO VIDEO COMPONENTS

47nF

MBRS130

150F6.3VOSCON**

100nF

D11N5818

4.7nF

C12.2F

SHDN

VSW

NFBNC

8V TO 30VINPUT

SG

3.57k 101N41484

32

6 1 7

5 8

PGVC

V+

PFB

+

+

+100F

Figure 11. Schematic Diagram: 1MHz LT1377-Based Boost Converter

IOUT (mA)

50

60

70

80

90

100EF

FICI

ENCY

(%)

1000

AN66 F12

2000 400 800600

VIN = 8V

VO = 5V

VIN = 12V

VIN = 16V

Figure 12. Efficiency Graph of theCircuit Shown in Figure 3

Application Note 66

AN66-11

HIGH OUTPUT VOLTAGE BUCK REGULATORby Dimitry Goder

High efficiency step-down conversion is easy to imple-ment using the LTC1149 as a buck switching regulatorcontroller. The LTC1149 features constant off-time, cur-rent mode architecture and fully synchronous rectifica-tion. Current mode operation was selected for itswell-known advantages of clean start-up, accurate currentlimit and excellent transient response.

Inductor current sensing is usually implemented by plac-ing a resistor in series with the coil, but the common modevoltage at the LTC1149s Sense pins is limited to 13V. If ahigher output voltage is required, the current sense resis-tor can be placed in the circuits ground return to avoid

common mode problems. The circuit in Figure 13 can beused in applications that do not lend themselves to thisapproach.

Figure 13 shows a special level shifting circuit (Q1 and U2)added to a typical LTC1149 application. The LT1211, ahigh speed, precision amplifier, forces the voltage acrossR5 to equal the voltage across current sense resistor R8.Q1s drain current flows to the source, creating a voltageacross R6 proportional to the inductor current, which isnow referenced to ground. This voltage can be directlyapplied to the current sense inputs of U1, the LTC1149.C12 and C4 are added to improve high frequency noiseimmunity. Maximum input voltage is now limited by theLT1211; it can be increased if a Zener diode is placed inparallel with C12.

Figure 13. High Output Voltage Buck Regulator Schematic Using LTC1149

+

+

+

AN66 F13

VIN26V TO 35V

C13

C1

16

15

14

13

12

11

10

9

1

2

3

4

5

6

7

8

P-GATE Q2RFD15P05

Q3RFD14N05

Q1VN2222LL

D1MBRS140

D31N4148

L1150H

VIN

VCC

P-DRIVE

VCC

CT

ITH

SENSE

CAP

SHDN

RGND

N-GATE

U1LTC1149

U2ALT1211

PGND

SGND

VFB

SENSE+

C80.047F

C71F

C5220pF

R4510

R1312k1%

R61001%

R51001%

R9100

24V2A

R12220k1%

R10100

R80.05

R9100

1

8 3

2

4C63300pF C11100pF

C21000pF

C100.1F

C120.1F

C90.068F

Application Note 66

AN66-12

THE LTC1267 DUAL SWITCHING REGULATORCONTROLLER OPERATES FROMHIGH INPUT VOLTAGESby Randy G. Flatness

Fixed Output 3.3V and 5V Converter

A fixed LTC1267 application circuit creating 3.3V/2A and5V/2A is shown in Figure 15. The operating efficiencyshown in Figure 14 exceeds 90% for both the 3.3V and 5Vsections. The 3.3V section of the circuit in Figure 15comprises the main switch Q1, synchronous switch Q2,inductor L1 and current shunt RSENSE3.

The 5V section is similar and comprises Q3, Q4, L2 andRSENSE5. Each current sense resistor (RSENSE) monitorsthe inductor current and is used to set the output currentaccording to the formula IOUT = 100mV/RSENSE. Advan-tages of current control include excellent line and loadtransient rejection, inherent short-circuit protection andcontrolled start-up currents. Peak inductor currents for L1and L2 are limited to 150mV/RSENSE or 3.0A. The EXT VCCpin is connected to the 5V output increasing efficiency athigh input voltages. The maximum input voltage is limitedby the MOSFETs and should not exceed 28V.

Adjustable Output 3.6V and 5V Converter

The adjustable output LTC1267-ADJ shown in Figure 16 isconfigured as a 3.6V/2.5A and 5V/2A converter. The resis-tor divider composed of R1 and R2 sets the output voltageaccording to the formula VOUT = 1.25V (1 + R2/R1). Theinput voltage range for this application is 5.5V to 28V.

OUTPUT CURRENT

60

70

80

90

100

EFFI

CIEN

CY (%

)

1A 2A

AN66 F14

0.001 0.01 0.1

LTC1267VIN = 12V

5V SECTION

LTC1267VIN = 12V3.3V SECTION

Figure 14. LTC1267 Efficiency vs Output Currentof Figure 15 Circuit

+

+

+

+++

1000pF 1000pF

1N41481N4148

PDRIVE3

SENSE+3

SENSE3

SGND3 CT3 ITH3 ITH5 CT5 SGND5NGATE5

SENSE5

SHDN3

SENSE+5

PGATE5

SHDN5

NGATE3

PGATE3

PDRIVE5

VCC3 EXT VCCVINVCCCAP3 CAP5MASTERSHDN

VCC5

PGND5PGND3

LTC1267

CT5270pF

7 11 9 10 15 16 20 22RC51k

CC33300pF

CC53300pF

CT3 270pF

RC31k

0.15F

0.1F

1 238 27 26 28 21

25

24

17

18

19

23

4

5

14

13

12

6

VOUT5 5V2A

COUT5220F

10V 2

RSENSE50.05

Q3P-CH

Si9435DYL2

33H

D2MBRS140T3

Q4N-CH

Si9410DY

0.1F

3.3FCIN5100F50V33F

Q1P-CHSi9435DY

Q2N-CHSi9410DY

0V = RUN>2V = SHUTDOWN

0V = RUN>2V = SHUTDOWN

D1MBRS140T3

COUT3220F10V 2

L120H

RSENSE30.05

VOUT33.3V

2A

5.5V < VIN < 28V

CIN3100F50V

0.15F

AN66 F15

R SENSE,:KRL SL-C1-1/2-R050J L1:COILTRONICS CTX20-4 L2:COILTRONICS CTX33-4

KRL (603) 668-3210COILTRONICS (407) 241-7876

Figure 15. LTC1267 Dual Output 3.3V and 5V High Efficiency Regulator

Application Note 66

AN66-13

+

+

+

+++

1000pF 1000pF

1N41481N4148

PDRIVE1

SENSE+1

SENSE1

SGND1 CT1 ITH1 ITH2 CT2 SGND2PGND2

SENSE2

SHDN1

SENSE+2

PGATE2

NGATE2

NGATE1

PGATE1

PDRIVE2

VCC1 EXT VCCVINVCCCAP1 CAP2MASTERSHDN

VCC2

VFB2VFB1

LTC1267-ADJ

CT2270pF

14 10 8 9 15 16 20 19RC11k

CC13300pF

CC23300pF

CT1 270pF

RC11k

0.15F

0.1F

1 237 27 26 28

100pF

21

25

24

17

18

23

22

4

5

13

12

11

6

VOUT2 5V2A

COUT2220F

10V 2

RSENSE20.05

P-CHSi9435DY

L233H

D2MBRS140T3N-CH

Si9410DY

0.1F

3.3FCIN2100F50V33F

P-CHSi9435DY

N-CHSi9410DY

0V = RUN>2V = SHUTDOWN

D1MBRS140T3

COUT1220F10V 2

L120H

RSENSE10.04

R2100k1%

R2 150k1%

R149.9k1%

R152.3k1%

VOUT13.6V2.5A

5.5V < VIN < 28V

CIN1100F50V

0.15F

100pF

AN66 F16

R SENSE1,: KRL SL-C1-1/2-R040JR SENSE2,: KRL SL-C1-1/2-R050J L1: COILTRONICS CTX20-4 L2: COILTRONICS CTX33-4

KRL (603) 668-3210COILTRONICS (407) 241-7876

Figure 16. LTC1267 Dual Adjustable High Efficiency Regulator Circuit. Output Voltages Set at 3.6V and 5V

HIGH EFFICIENCY 5V TO 3.3V/1.25A CONVERTERIN 0.6 SQUARE INCHESby Randy G. Flatness

The next generation of notebook and desktop computerswill incorporate a growing number of 3.3V ICs along with5V devices. As the number of 3.3V devices increases, thecurrent requirements increase. Typically, a high current

5V supply is already available. Thus, the problem isreduced to deriving 3.3V from 5V at high efficiency in asmall amount of board space.

High efficiency is mandatory in these applications sinceconverting 5V to 3.3V at 1.25A using a linear regulatorwould require dissipating over 2W. This is an unnecessarywaste of power and board space for heat sinking.

Figure 17. High Efficiency Controller Converts 5V to 3.3V in Minimum Board Area

AN66 F17

KRL/BANTRY (603) 668-3210SUMIDA (708) 956-0666

SHDN

ITH

CT

LTC1147-3.3

VIN

SENSE+

SENSE

+

0.01F +

0.1F

RC1K

CC3300pF

CT120pF

6

3

2

8

5

4

CIN47F16V

L110H

RSENSE0.068

VOUT3.3V1.5A

COUT100F10V

1

7

PDRIVE

GND

P-CHSi9433DY

VIN4V TO 10V

0V = >1.5V =

NORMALSHUTDOWN

D1MBRS130LT3

KRL SP-1/2-A1-0R068J SUMIDA CDR74 (ALT: CD54)

RS:L:

+

Application Note 66

AN66-14

The LTC1147 SO-8 switching regulator controller accom-plishes the 5V to 3.3V conversion with high efficienciesover a wide load current range. The circuit shown in Figure17 provides 3.3V at efficiencies greater than 90% from50mA to 1.25A. Using all surface mount components anda low value of inductance (10H) for L1, the circuit ofFigure 17 occupies only 0.6 square inches of PC boardarea. The efficiency of the circuit in Figure 17 is plotted inFigure 18.

At an output current of 1.25A the efficiency is 90.4%; thismeans only 0.4W are lost. This lost power is distributedamong RSENSE, L1 and the power MOSFETs; thus heatsinking is not required.

The LTC1147 series of controllers use constant off-timecurrent mode architecture to provide clean start-up, accu-rate current limit and excellent line and load regulation. Tomaximize the operating efficiency at low output currents,Burst Mode operation is used to reduce switching losses.

The P-channel MOSFET in the circuit of Figure 17 will beon 2/3 of the time with an input voltage of 5V. Hence, thisdevice should be carefully selected to obtain the bestperformance. This design uses an Si9433DY for optimum

Figure 19. Adjustable LT1074/LT1076 0V to 5V Power Supply

LT1074/LT1076 ADJUSTABLE 0V TO 5VPOWER SUPPLYby Kevin Vasconcelos

Linear regulator ICs are commonly used in variable powersupplies. Common types such as the 317 can be adjustedas low as 1.25V in single-supply applications. At low

output voltages power losses in these regulators can be aproblem. For example, if an output current of 1.5A isrequired at 1.25V from an input of 8V, the regulatordissipates more than 10W. Figure 19 shows a DC/DCconverter that functionally replaces a linear regulator inthis application. The converter not only eliminates power

AN66 F19

VIN = 10VTO 20V

C1330F35V

GND

VIN

VC

VSW

FB

R12.7k

C20.01F

LT1076

5

3 2

1

4

D1MBR340P

L1 = COILTRONICS (407) 241-7876

L1CTX100-5A-52

R23.65k1%

R52201/4W5%

R43.01k1%

C3470F50V

R310.65k1%

VOUT

6

+

U1LT1006

+

+2

7

3R55k25T

LT1029

4

R62.2k5%

C40.1F

efficiency; for lower cost an Si9340DY can be used at aslight reduction in performance.

The circuit in Figure 17 has a no load current of only160A. In shutdown, with Pin 6 held high (above 2V), thequiescent current is reduced to less than 20A with theMOSFET held off. Although the circuit in Figure 17 isspecified at a 5V input voltage the circuit will functionfrom 4V to 10V.

OUTPUT CURRENT (A)1mA

60EF

FICI

ENCY

(%)

95

10mA 1A

AN66 F18

100mA 2A

90

85

80

75

70

65

LTC1147-3.3SUMIDA CD54VIN = 5V

LTC1147-3.3SUMIDA CDR74VIN = 5V

Figure 18. 5V to 3.3V Conversion Efficiency

Application Note 66

AN66-15

loss as a concern, but can be adjusted for output voltagesas low as 25mV while still delivering an output current of1.5A.

The circuit of Figure 19 employs a basic positive bucktopology with one exception: a control voltage is appliedthrough R4 to the feedback summing node at Pin 1 of theLT1076 switching regulator IC, allowing the output to beadjusted from 0V to approximately 6V. This encompassesthe 3.3V and 5V logic supply ranges as well as battery packcombinations of one to four D cells.

As R4 is driven from 0V to 5V by the buffer (U1) more orless current is required from R2 to satisfy the loops desireto hold the feedback summing point at 2.21V. This forcesthe converters output to swing over the range of 0V to 6V.

Figure 20 shows a comparison of power losses for a linearregulator and the circuit of Figure 19. The load current is1.5A in both cases although the LT1076 is capable of1.75A guaranteed output current in this application and 2Atypical. If more current is required the LT1074 can be

substituted for the LT1076. This change accommodatesoutputs up to 5A but at the expense of a heftier diode andcoil (D1, L1). An MBR735 and Coiltronics CTX50-2-52 arerecommended for 5A service.

OUTPUT VOLTAGE (V)0

0

POW

ER L

OSS

(W)

10

5

AN66 F20

4321

2

4

6

8

LT1076

LT317

Figure 20. Power Loss Comparison: Linear Regulatorvs Figure 19s Power Supply

TRIPLE OUTPUT 3.3V, 5V AND 12VHIGH EFFICIENCY NOTEBOOK POWER SUPPLYby Randy G. Flatness

LTC1142 Circuit Operation

The application circuit in Figure 22 is configured to provideoutput voltages of 3.3V, 5V and 12V. The current capabilityof both the 3.3V and 5V outputs is 2A (2.5A peak). Thelogic-controlled 12V output can provide 150mA (200mApeak), which is ideal for flash memory applications. Theoperating efficiency shown in Figure 21 exceeds 90% forboth the 3.3V and 5V sections.

The 3.3V section of the circuit in Figure 22 comprises themain switch Q4, synchronous switch Q5, inductor L1 andcurrent shunt RSENSE3. The current sense resistor RSENSEmonitors the inductor current and is used to set the outputcurrent according to the formula IOUT = 100mV/RSENSE.Advantages of current control include excellent line andload transient rejection, inherent short-circuit protectionand controlled start-up currents. Peak inductor currentsfor L1 and T1 of the circuit in Figure 22 are limited to150mV/RSENSE or 3.0A and 3.75A respectively.

OUTPUT CURRENT (A)0.001

60

EFFI

CIEN

CY (%

)

100

0.01 2.5

AN66 F21

0.1 1

65

70

75

80

85

90

95

LTC1142VIN = 8V3.3V SECTION

LTC1142VIN = 8V5V SECTION

Figure 21. LTC1142 Efficiency

When the output current for either regulator section dropsbelow approximately 15mV/RSENSE, that section auto-matically enters Burst Mode operation to reduce switchinglosses. In this mode the LTC1142 holds both MOSFETs offand sleeps at 160A supply current while the outputcapacitor supports the load. When the output capacitorfalls 50mV below its specified voltage (3.3V or 5V) theLTC1142 briefly turns this section back on, or bursts, torecharge the output capacitor. The timing capacitor pins,

Application Note 66

AN66-16

Figure 22. LTC1142 High Efficiency Power Supply Schematic Diagram

which go to 0V during the sleep interval, can be monitoredwith an oscilloscope to observe burst action. As the loadcurrent is decreased the circuit will burst less and lessfrequently.

The timing capacitors CT3 and CT5 set the off-time ac-cording to the formula tOFF = 1.3 (104)(CT). The constantoff-time architecture maintains a constant ripple currentwhile the operating frequency varies only with inputvoltage. The 3.3V section has an off-time of approxi-mately 5s, resulting in a operating frequency of 120kHzwith an 8V input. The 5V section has an off-time of 2.6sand a switching frequency of 140kHz with an 8V input.

Auxiliary 12V Output

The operation of the 5V section is identical to the 3.3Vsection with inductor L1 replaced by transformer T1. The12V output is derived from an auxiliary winding on the 5V

inductor. The output from this additional winding is recti-fied by diode D3 and applied to the input of an LT1121regulator. The output voltage is set by resistors R3 and R4.A turns ratio of 1:1.8 is used for T1 to ensure that the inputvoltage to the LT1121 is high enough to keep the regulatorout of dropout mode while maximizing efficiency.

The LTC1142 synchronous switch removes the normallimitation that power must be drawn from the primary 5Vinductor winding in order to extract power from theauxiliary winding. With synchronous switching, the auxil-iary 12V output may be loaded without regard to the 5Vprimary output load, provided that the loop remains incontinuous mode operation.

When the 12V output is activated by a TTL high (6Vmaximum) on the 12V enable line, the 5V section of theLTC1142 is forced into continuous mode. A resistor

AN66 F22

+

++

1000pF

P-DRIVE 3

SENSE+ 3

SENSE 3

N-DRIVE 3

PGND3 SGND3 CT3 ITH3 ITH5 CT5 SGND5 PGND5

N-DRIVE 5

SENSE 5

SENSE+ 5

P-DRIVE 5

VIN3 SHDN3 SHDN5 VIN5

LTC1142

CT5200pF

4 3 25 27 13 11 17 18

510

3300pF 3300pFCT3390pF

510

1F

224 16 10

9

15

14

20

23

1

28

6

VOUT55V2A

220F10V 2

RSENSE 50.0430H

D2MBRS140

Q3Si9410DY

0V = NORMAL>1.5V = SHUTDOWN

1F22F25V 2

VIN6.5V TO 14V

Q4Si9430DY

Q5Si9410DY

D1MBRS140

100F10V 2

L133H

RSENSE 30.05

VOUT33.3V2A

+22F25V 2

+

Q2Si9430DY

0.01F

COILTRONICS CTX33-4 DALE LPE-6562-A026 PRIMARY: SECONDARY = 1:1.8 KRL SL-1R050J KRL SL-1R040J

COILTRONICS (407) 241-7876 DALE (605) 665-9301 KRL/BANTRY (603) 668-3210

L1:T1:

RSENSE 3:RSENSE 5:

22

R1100

T1

12V ENABLE0V = 12V OFF>3V = 12V ON

(6V MAX)

1000pF

D3MBRS140

R3649k1%

R4294k1%

20pF+

22F25V

+

C922F35V

12V150mA

LT1121

VOUTSHDN

VIN

ADJ

100

Q1VN2222LL

R518k

+

GND

5

8

3

2

1

Application Note 66

AN66-17

divider composed of R1, R5 and switch Q1 forces anoffset, subtracting from the internal offset at Pin 14. Whenthis external offset cancels the built-in 25mV offset, BurstMode operation is inhibited.

Auxiliary 12V Output Options

The circuit of Figure 22 can be modified for operation inlow-battery count (6-cell) applications. For applicationswhere heavy 12V load currents exist in conjunction withlow input voltages (1.5V = SHUTDOWN

CT560pF

LTC1147-3.3

P-CHSi943ODY RSENSE

0.1

VOUT3.3V1A

COUT220F

6.3V

AN66 F23

Application Note 66

AN66-18

Figure 24. The LTC1147 5V to 3.3V Converter Provides BetterThan 90% Efficiency from 20mA to 500mA of Output Current

LOAD CURRENT (A)0.001

60

EFFI

CIEN

CY (%

)

80

90

100

0.01 0.1 1

AN66 F24

70

VIN = 5V

LTC1147-3.3

Giving Up the Synchronous Switch?

The decision whether to use a nonsynchronous LTC1147design or a fully synchronous LTC1148 design requires acareful analysis of where losses occur. The LTC1147switching regulator controller uses the same loss reduc-ing techniques as the other members of the LTC1148/LTC1149 family. The nonsynchronous design saves theN-channel MOSFET gate drive current at the expense ofincreased loss due to the Schottky diode.

Figure 25 shows how the losses in a typical LTC1147application are apportioned. The gate-charge loss(P-channel MOSFET) is responsible for the majority of theefficiency lost in the midcurrent region. If Burst Modeoperation was not employed, the gate charge loss alonewould cause the efficiency to drop to unacceptable levelsat low output currents. With Burst Mode operation, the DCsupply current represents the only loss component thatincreases almost linearly as output current is reduced. Asexpected, the I2R loss and Schottky diode loss dominateat high load currents.

In addition to board space, output current and inputvoltage are the two primary variables to consider whendeciding whether to use the LTC1147. At low input-to-output voltage ratios, the top P-channel switch is on mostof the time, leaving the Schottky diode conducting only asmall percentage of the total period. Hence, the power lostin the Schottky diode is small at low output currents. This

is the ideal application for the LTC1147. As the outputcurrent increases the diode loss increases. At high input-to-output voltage ratios, the Schottky diode conductsmost of the time. In this situation, any loss in the diode willhave a more significant effect on efficiency and an LTC1148might therefore be chosen.

Figure 26 compares the efficiencies of LTC1147-5 andLTC1148-5 circuits with the same inductor, timing capaci-tor and P-channel MOSFET. At low input voltages and 1Aoutput current the efficiency of the LTC1147 differs fromthat of the LTC1148 by less than two percent. At lower

INPUT VOLTAGE (V)4

60

EFFI

CIEN

CY (%

)

80

90

100

6 12 14

AN66 F26

70

8 10

ILOAD = 100mA

ILOAD = 1A

LTC1147-5LTC1148-5

Figure 26. At High Input Voltages Combined with Low OutputCurrents, the Efficiency of the LTC1147 Exceeds That of theLTC1148

Figure 25. Low Current Efficiency is Enhanced by Burst ModeOperation. Schottky Diode Loss Dominates at High OutputCurrents

10.03

OUTPUT CURRENT (A)0.01

80

EFFI

CIEN

CY/L

OSS

(%)

100

3

AN66 F25

95

90

85

LTC1147 IQ

GATE CHARGE I2R

SCHOTTKY DIODE

0.1 0.3

Application Note 66

AN66-19

output currents and high input voltages the LTC1147sefficiency can actually exceed that of the LTC1148.

Low Dropout 5V Output Applications

Because the LTC1147 is so well-suited for low input-to-output voltage ratio applications it is an ideal choice forlow dropout designs. All members of the LTC1148/LTC1149family (including the LTC1147) have outstandingly lowdropout performance. As the input voltage on the LTC1147drops, the feedback loop extends the on-time for the

P-channel switch (off-time is constant) thereby keepingthe inductor ripple current constant. Eventually the on-time extends so far that the P-channel MOSFET is on at DCor at a 100% duty cycle.

With the switch turned on at a 100% duty cycle, thedropout is limited by the load current multiplied by thesum of the resistances of the MOSFET, the current shuntand the inductor. For example, the low dropout 5V regu-lator shown in Figure 27 has a total resistance of less than0.2. This gives it a dropout voltage of 200mV at 1Aoutput current. At input voltages below dropout the outputvoltage follows the input. This is the circuit whose effi-ciency is plotted in Figure 28.

Figure 27. The LTC1147 Architecture Provides Inherent LowDropout Operation. This LTC1147-5 Circuit Supports a 1A Loadwith the Input Voltage Only 200mV Above the Output

AN66 F27KRL SL-1-C1-0R050JCOILTRONICS CTX50-4 COILTRONICS (407) 241-7876KRL/BANTRY (603) 668-3210

RS = L =

ITH

CT

GND

VIN

SENSE +

SENSE

+

1000pF

+

VIN(5.5VTO 12V)

RC1k

CC3300pF

2

8

5

4

D1MBRD330

CIN15F25V 3

L50H

1

7

SHDN6

0.1F

3

PDRIVE

+

0V = NORMAL>1.5V = SHUTDOWN

CT470pF

LTC1147-5

P-CHSi943ODY RSENSE

0.05

VOUT5V2A

COUT220F

10V 2

LOAD CURRENT (mA)1

70

EFFI

CIEN

CY (%

)85

90

95

100

10 100

AN66 F28

80

1000

75

VIN = 10V

LTC1147-5

VIN = 6V

Figure 28. Greater Than 90% Efficiency is Obtained for LoadCurrents of 20mA to 2A (VIN = 10V)

Application Note 66

AN66-20

THE LT1432: 5V REGULATORACHIEVES 90% EFFICIENCYby Carl Nelson

Power supply efficiency has become a highly visible issuein many portable battery-powered applications. Higherefficiency translates directly to longer useful operatingtimea potent selling point for products such as note-book computers, cellular phones, data acquisition units,sales terminals and word processors. The holy grail ofefficiency for 5V outputs is 90%.

For a number of reasons, older designs were limited toefficiencies of 80 to 85%. High quiescent current in thecontrol circuitry limited efficiency at lower output cur-rents. Losses in the power switch, inductor and catchdiode all added up to limit efficiency at moderate-to-highoutput currents. Each of these areas must be addressed ina design that is to have high efficiency over a wide outputcurrent range.

Some portable equipment has the additional requirementof high efficiency at extremely light loads (1mA to 5mA).These applications have a sleep mode in which RAM iskept alive to retain information. The instrument may spenddays or even weeks in this mode, so battery drain is

critical. Ordinary 5V switchers draw quiescent currents of5mA to 15mA for these light loads. The efficiency of a 12Vto 5V converter with 10mA supply current and 1mA loadis only 4%. Clearly, some method must be provided toeliminate the quiescent current of the switching regulatorcontrol section.

An additional requirement for some systems is full shut-down of the regulator. It would be ideal if a simple logicsignal could cause the converter to turn off and draw onlya few microamperes of current.

The combination of battery form factors, their discretevoltage steps and the use of higher voltage wall adaptersrequires a switching regulator that operates with inputsfrom 6V to 30V. Both of these voltages present problemsfor a MOS design because of minimum and maximum gatevoltage requirements of power MOS switches.

The LT1432 was designed to address all the requirementsdescribed above. It is a bipolar control chip that interfacesdirectly to the LT1070 family of switching regulators andis capable of operating with 6V to 30V inputs. These ICshave a very efficient, quasisaturating NPN switch thatmimics the resistive nature of MOS transistors with muchsmaller die areas. The NPN is a high frequency device with

Figure 29. High Efficiency 5V Buck Converter

+

+

AN66 F29

C1330F35V

C60.02F R1

680C4

0.1F

D1MBR330P

C50.03F

VIN

LT1271

VSW

FBVC GND

VIN

C34.7FTANT

D21N4148

L150H R2*

0.013

C2390F16V

VOUT5V3A

DIODEVC V+

VIN

MODE

VLIM

VOUTGND200pF

< 0.3V = NORMAL MODE> 2.5V = SHUTDOWN OPEN = BURST MODE

LT1432

+

MODE INPUT

*R2 IS MADE FROM PC BOARD COPPER TRACESL1 = COILTRONICS CTX 50-3-MP (3A) (407) 241-7876

Application Note 66

AN66-21

an equivalent voltage and current overlap time of only10ns. Drive to the switch is automatically scaled withswitch current, so drive losses are also low. Switch anddriver losses using an LT1271 with a 12V input and a 5V,500mA load are only about 2%.

To reduce quiescent current losses, the LT1271 is pow-ered from the 5V output rather than from the input voltage.This is done by pumping the supply capacitor C3 from theoutput via D2. Quick minded designers will observe thatthis arrangement does not self-start; accordingly, a paral-lel path was included inside the LT1432 to provide powerto the IC switcher directly from the input during start-up.Equivalent quiescent supply current is reduced to about3.5mA with this technique.

Catch diode losses cannot be reduced with IC tricksunless the diode is replaced with a synchronously drivenMOS switch. This is more expensive and still requires thediode to avoid voltage spikes during switch nonoverlaptimes. The question is, is it worth it?

The following formula was developed to calculate theimprovement in efficiency when adding a synchronousswitch.

Efficiency change = (VIN VOUT)(Vf RFET IOUT)(E)

2

(VIN)(VOUT)

With VIN = 10V, VOUT = 5V, Vf (diode forward voltage) =0.45V, RFET = 0.1 and IOUT = 1A the improvement inefficiency is only 2.8%. This does not take into accountthe losses associated with MOS gate drive, so realimprovement would probably be closer to 2%. Theavailability of low forward voltage Schottky diodes suchas the MBR330P makes synchronous switches lessattractive than they used to be.

To achieve higher efficiency during sleep, the LT1432 hasBurst Mode operation. In this mode the LT1271 is eitherdriven full on, or completely shut down to its micropowerstate. The LT1432 acts as a comparator with hysteresisinstead of a linear amplifier. This mode reduces equivalentinput supply current to 1.3mA with a 12V battery. Batterylife with NiCd AA cells is over 300 hours with a 1mA 5Vload. Burst Mode operation increases output ripple, espe-cially with higher output currents, so maximum load in thismode is 100mA.

The LT1271 normally draws about 50A to100A in itsshutdown state. A shutdown command to the LT1432opens all connections to the LT1271 VIN pin so its currentdrain is eliminated. This leaves only the shutdown currentof the LT1432 and the switch leakage of the LT1271, whichtypically add up to less than 20Aless than the self-discharge rate of NiCd batteries. For many applications theon/off function is under keystroke control. Digital chipswhich draw only a few microamps are available for key-stroke recognition and power control.

There is no way to design around inductor losses. Theselosses are minimized by using low loss cores such asmolypermalloy or ferrite, and by sizing the core to use wirewith sufficient diameter to keep resistive losses low. The50H inductor shown has a core loss of 200mW with type-52 powdered iron material and 28mW with molypermalloy.For a 1A load this represents efficiency losses of 4% and0.56% respectivelya major difference. Ferrite coreswould have even lower losses than molypermalloy, but themoly has such low losses that ferrites should be chosenfor other reasons, such as height, cost, mounting and thelike. DC resistance of the inductor shown is 0.02. Thisrepresents an efficiency loss of 0.4% at 1A load and 0.8%at 2A. Significant reduction in these resistance losseswould require a somewhat larger inductor. The choice isyours.

The LT1432 has a high efficiency current limit with a sensevoltage of only 60mV. This has a side benefit in that printedcircuit board trace material can be used for the senseresistor. A 3A limit requires a 0.02 sense resistor andthis is easily made from a small section of serpentine trace.The 60mV sense voltage has a positive temperature coef-ficient that tracks that of copper so that the current limit isflat with temperature. Foldback current limiting can beeasily implemented.

The LT1432 represents a significant improvement in highefficiency 5V supplies that must operate over a wide rangeof load currents and input voltages. Its efficiency has avery broad peak that exceeds 90%, requiring a newdefinition of the holy grail. Logic controlled shutdown,millipower Burst Mode operation and efficient, accurate,current limiting make this regulator extremely attractivefor battery-powered applications.

Application Note 66

AN66-22

RegulatorsSwitching (Buck)Low Power (

Application Note 66

AN66-23

+ L1*47H

1k

VIN5V

PWR VINPWR VIN

LTC1265-3.3

SW

PGND

SGND

SHDN SHUTDOWN

NC

SENSE+

2

3

5

6

7

14

1 13

12D1MBRS130LT1

11

10

9

8

VIN

4LBIN

LBOUT

CT

ITHR

SENSE

AN66 F32

3900pF

270pF

0.1FCIN100F10V 0.1**

+ COUT220F10V

VOUT3.3V1A

1000pF

*COILCRAFT D03316-473**KRL SL-C1-OR100J AVX TAJD100K010 AVX TAJD226K010

COILCRAFT 708-639-6400KRL/BANTRY 603-668-3210

LOAD CURRENT (mA)

70

75

90

85

80

95

100

EFFI

CIEN

CY (%

)

1000

AN66 F33

1 10 100

L1 = 47HVOUT = 3.3VRSENSE = 0.1CT = 270pF

Figure 32. High Efficiency 5V to 3.3V Converter Figure 33. Efficiency vs Load Current

+ L1*18H

1k

VIN5V

PWR VINPWR VIN

LTC1265-3.3

SW

PGND

SGND

SHUTDOWNSHDN

NC

SENSE+

2

3

5

6

7

14

1 13

12D1MBRS0520LT1

11

10

9

8

VIN

4LBIN

LBOUT

CT

ITHR

SENSE

AN66 F34

3300pF

51pF

0.1FCIN15F10V 2 0.20**

+ COUT

22F6.3V 2

VOUT3.3V

500mA

1000pF

*SUMIDA CLS62-180**KRL SL-C1-OR200J AVX TAJB155K010 AVX TAJB225K06

SUMIDA 708-956-0666KRL/BANTRY 603-668-3210

Figure 34. 2.5mm High 5V to 3.3V Converter (500mA Output Current) Figure 35. Efficiency vs Load Current

LOAD CURRENT (mA)

70

75

90

85

80

95EF

FICI

ENCY

(%)

500

AN66 F35

1 10 100

L1 = 18HVOUT = 3.3VRSENSE = 0.20CT = 50pF

Application Note 66

AN66-24

+

+ L1*47H

1k

VIN3.5V TO 7.5V

PWR VINPWR VIN

LTC1265-5

SW

PGND

SGND

SHDN

NC

SENSE+

2

3

5

6

7

14

1 13

12D1MBRS130LT3

TP0610L

SHUTDOWN

11

10

9

8

VIN

4LBIN

LBOUT

CT

ITHR

SENSE

AN66 F36

2200pF

220pF

COUT100F/10V

0.1FCIN22F25V 2 VOUT

5V

RSENSE** 0.1

100k

1000pF

AVX TPSD226K025 AVX TPSD106K010 *L1 SELECTION MANUFACTURER PART NO. COILTRONICS CTX50-4 COILCRAFT D03316-473 DALE LPT4545-500LA SUMIDA CD75-470**KRL SL-C1-OR100J

VIN (V) I OUT(MAX) (mA) 3.5 360 4.0 430 5.0 540 6.0 630 7.0 720 7.5 740

Figure 36. Positive (3.5 to 7.5V) to Negative (5V) Converter

RegulatorsSwitching (Boost)Medium Power (1A to 4A)

HIGH OUTPUT CURRENT BOOST REGULATORby Dimitry Goder

Low voltage switching regulators are often implementedwith self-contained power integrated circuits featuring aPWM controller and an onboard power switch. Maximum

switch currents of up to 10A are available, providing aconvenient means for power conversion over wide inputand output voltage ranges. If higher switch currents arerequired, a controller with an external power MOSFET is abetter choice.

Figure 37 shows an LTC1147-based 5V to 12V converterwith 3.5A peak output current capability. The LTC1147 isa micropower controller that uses a constant off-time

Figure 37. LTC1147-Based 5V to 12V Converter

+

+

+

AN66 F37

U1LTC1147

C5, C6 SANYO 0S-CONEFFICIENCY AT 3A 90%

VOUT12V/3A

3.5A PEAK

VIN

CT

ITHC13300pF

C2180pF

R1510

R70.012%

R8100k1%

L115H

C30.01F

C4100pF

R4100

R211.5k1%

R3100

C73.3F

D2BAT54

C6220F10V 2

C5150F16V 2

VIN5V

SENSE

1

2

3

4

8

7

6

5

PDRIVE

R656k

Q1VN2222LL

Q3TP0610L

R5100

GND

VFB

SENSE +

Q2IRL2203

D1MBR735

Application Note 66

AN66-25

architecture, eliminating the need for external slope com-pensation. Current mode control allows fast transientresponse and cycle-by-cycle current limiting. A maximumvoltage of only 150mV across the current-sense resistorR7 optimizes performance for low input voltages.

When Q2 turns on, current starts building up in inductorL1. This provides a ramping voltage across R7. Whenthis voltage reaches a threshold value set internally in theLTC1147, Q2 turns off and the energy stored in L1 is

transferred to the output capacitor C5. Timing capacitorC2 sets the operating frequency. The controller is pow-ered from the output through R5 providing 10V of gatedrive for Q2. This reduces the MOSFETs ON resistanceand allows efficiency to exceed 90% even at full load. Thefeedback network comprising R2 and R8 sets the outputvoltage. Current sense resistor R7 sets the maximumoutput current; it can be changed to meet different circuitrequirements.

RegulatorsSwitching (Boost)Low Power (

Application Note 66

AN66-26

Dual Output Flyback with Overvoltage Protection

Multiple-output flyback converters offer an economicalmeans of producing multiple output voltages, but thepower supply designer must be aware of cross regulationissues, which can cause electrical overstress on the sup-ply and loads. Figure 41 is a dual-output flyback converterwith overvoltage protection. Typically, in multiple-outputflyback designs only one output is voltage sensed andregulated. The remaining outputs are quasi-regulated bythe turns ratios of the transformer secondary. Crossregulation is a function of the transformer used and is ameasure of how well the quasi-regulated outputs maintain

regulation under varying load conditions. For evenly loadedoutputs, as shown in Figure 42, cross regulation can bequite good, but when the loads differ greatly, as in the caseof a load disconnect, there may be trouble. Figure 43shows that when only the 15V output is voltage sensed,the 15V quasi-regulated output exceeds 25V whenunloaded. This can cause electrical overstress on theoutput capacitor, output diode and the load when recon-nected. Adding output voltage clamps is one way to fix theproblem but the circuit in Figure 41 eliminates this require-ment. This circuit senses both the 15V and 15V outputsand prevents either from going beyond its regulatingvalue. Figure 44 shows the unloaded 15V output beingheld constant. The circuits efficiency, which can reach79% on a 5V input, is shown in Figure 45.

++

IOUT0.3A0.5A0.75A

VIN3V5V9V AN66 F40

S/S

FBNC

5

C122F

VIN2.7V TO 16V

D1MBRS130LT3

8

2

1 3

4

3

C20.047F

C30.0047F

4

2

R12k

R22.49k1%R32.49k1%

T1 = COILTRONICS CTX10-2 COILTRONICS (407) 241-7876

C447F

VOUT5VVSW

D2P6KE-15A

D31N4148

NFB

LT1372

T1

VIN

VC1 6, 7

GND

OFFON

Figure 40. LT1372s Positive-to-Negative Converterwith Direct Feedback

+

++

AN66 F41

S/S

5

C122F

VIN2.7V TO 13V

R21.21k1%

R113k1%

MBRS140T3

MBRS140T3

8

2, 3

6, 7

1

84

5

3

C20.047F

C30.0047F

4OFF

ON

R32k

R412.1k1%

R52.49k1%

T1 = DALE LPE-4841-100MB DALE (605) 665-9301

C547F

C447F

VOUT15V

VOUT15V

VSW

P6KE-20A

1N4148

NFB

LT1372

T1

VIN

2

FB

VC1 6, 7

GND

Figure 41. LT1372 Dual Output Flyback Converterwith Overvoltage Protection

OUTPUT CURRENT (mA)

30

30

25

20

15

10

5

0

5

10

15

20

25

V OUT

(V)

100

AN66 F42

1 10

VIN = 5V

VOUT

VOUT

Figure 42. Cross Regulation of Figure 41s Circuit.VOUT and VOUT Evenly Loaded

OUTPUT CURRENT (mA)

30

30

25

20

15

10

5

0

5

10

15

20

25

V OUT

(V)

100

AN66 F43

1 10

VIN = 5V

VOUT

VOUT

Figure 43. Cross Regulation of Figure 41s Circuit.VOUT Unloaded; Only VOUT Voltage Sensed

Application Note 66

AN66-27

OUTPUT CURRENT (mA)

30

30

25

20

15

10

5

0

5

10

15

20

25

V OUT

(V)

100

AN66 F44

1 10

VIN = 5V

VOUT

VOUT

Figure 44. Cross Regulation of Figure 41s Circuit. VOUT Unloaded; Both VOUT and VOUT Sensed

Figure 46. 5V Buck Converter with 5V Overwinding

1:1:1:1 sections. In the application of Figure 46, threesections are paralleled for the main 5V winding and theremaining section is used for the 5V output. This concen-trates the copper where it is needed moston the highcurrent output.

Efficiency with the outputs loaded at 500mA and 50mAis over 80%. Minimum recommended load on the 5Voutput is 1mA to 2mA, and the 5V load current mustalways be less than the 5V load current.1 JUMBO-PAC is a trademark of Coiltronics Inc. (407) 241-7876.

OUTPUT CURRENT (mA)

60

85

80

75

70

65

EFFI

CIEN

CY (%

)

200

AN66 F45

5 10 100

VOUT = 15V VIN = 9V

VIN = 5V

VIN = 3V

Figure 45. Efficiency of Dual Output Flyback Converterin Figure 41

RegulatorsSwitching(Buck/Boost)5V CONVERTER USES OFF-THE-SHELFSURFACE MOUNT COILBy Mitchell Lee and Kevin Vasconcelos

Single-output switching regulator circuits can often beadapted to multiple output configurations with a minimumof changes, but these transformations usually call forcustom wound inductors. A new series of standard induc-tors,1 featuring quadrifilar windings, allows power supplydesigners to take advantage of these modified circuits butwithout the risks of a custom magnetics developmentprogram.

The circuit shown in Figure 46 fulfills a recent customerrequirement for a 9V to 12V input, 5V/800mA and5V/100mA output converter. It employs a 1:1 overwind-ing on what is ostensibly a buck converter to provide a5V output. The optimum solution would be a bifilarwound coil with heavy gauge wire for the main 5V outputand smaller wire for the overwinding. To avoid a customcoil design, an off-the-shelf JUMBO-PACTM quadrifilarwound coil is used. This family of coils is wound with

+470F

VIN = 9V

LT1176CS-5FB

VSWVIN

+100F

VC GND

2k

10nF 1N5818

5V800mA

+470F

1

2

3

4

8

7

6

5

1N58185V100mA

CTX100-5P

AN66 F46

Application Note 66

AN66-28

SWITCHING REGULATOR PROVIDESCONSTANT 5V OUTPUT FROM 3.5V TO 40VINPUT WITHOUT A TRANSFORMERby Brian Huffman

A common switching regulator requirement is to producea constant output voltage from an input voltage that variesabove or below the output voltage. This is particularlyimportant for extending battery life in battery-poweredapplications. Figure 47 shows how an LT1171 switchingregulator IC, two inductors and a flying capacitor cangenerate a constant output voltage that is independent ofinput voltage variations. This is accomplished without theuse of a transformer. Inductors are preferred over trans-formers because they are readily available and moreeconomical.

The circuit in Figure 47 uses the LT1171 to control theoutput voltage. A fully self-contained switching regulatorIC, the LT1171 contains a power switch as well as thecontrol circuitry (pulse-width modulator, oscillator, refer-ence voltage, error amplifier and protection circuitry). Thepower switch is an NPN transistor in a common-emitterconfiguration; when the switch turns on, the LT1171sVSW pin is connected to ground. This power switch canhandle peak switch currents of up to 2.5A.

Figure 48 shows the operating waveforms for the circuit.In this architecture the capacitor C2 serves as the singleenergy transfer device between the input voltage andoutput voltage of the circuit. While the LT1171 powerswitch is off, diode D1 is forward biased, providing a pathfor the currents from inductors L1 and L2. Trace A showsinductor L1s current waveform and trace B is L2s currentwaveform. Observe that the inductor current waveformsoccur on top of a DC level. The waveforms are virtuallyidentical because the inductors have identical inductancevalues and the same voltages are applied across them. Thecurrent flowing through inductor L1 is not only deliveredto the load but is also used to charge C2. C2 is charged toa potential equal to the input voltage.

When the LT1171 power switch turns on, the VSW pin ispulled to ground and the input voltage is applied across theinductor L1. At the same time, capacitor C2 is connectedacross inductor L2. Current flows from the input voltagesource through inductor L1 and into the LT1171. Trace Cshows the voltage at the VSW pin and Trace D is the currentflowing through the power switch. The catch diode (D1) isreverse biased and capacitor C2s current also flowsthrough the switch, through ground and into inductor L2.During this interval C2 transfers its stored energy intoinductor L2. After the switch turns off the cycle is repeated.

Another advantage of this circuit is that it draws its inputcurrent in a triangular waveshape (see Trace A in Figure48). The current waveshape of the input capacitor isidentical to the current waveshape of inductor L1 exceptthat the capacitors current has no DC component. Thistype of ripple injects only a modest amount of noise intothe input lines because the ripple does not contain anysharp edges.

Figure 48. LT1171 Switching Waveforms

A = 1A/DIVIL1, IC1

B = 1A/DIVIL2

C = 10V/DIVVSW

D = 1A/DIVISW

5s/DIVAN66 F48

Figure 47. LT1171 Provides Constant 5V Output from3.5V to 40V Input. No Transformer Is Required

AN66 F47

= NICHICON (AL) UPL1H560MEH, ESR = 0.250, IRMS = 360mA= NICHICON (AL) UPL1H151MPH, ESR = 0.100, IRMS = 820mA= NICHICON (AL) UPL1C471MPH, ESR = 0.090, IRMS = 770mA= COILTRONICS CTX50-4, DCR = 0.090, COILTRONICS (407) 241-7876

C1 C2 C3

L1, L2

VIN

FBVIN(3.5VTO 40V)

R11k

4

2

D1MBR350

C2150F

50V

5

VOUT5V0.5A

VSW

VCGND

3

L150H

L250H

LT1171

1

C41F

+ C156F 50V

+ C3470F 16V

R23.01k1%

R31.00k1%

+

EQUATION 1: VOUT = 1.25V (1 + R2/R3)

Application Note 66

AN66-29

Figure 49 shows the efficiency of this circuit for a 0.5A loadand maximum output current for various input voltages.The two main loss elements are the output diode (D1) andthe LT1171 power switch. A Schottky diode is chosen forits low forward voltage drop; it introduces a 10% loss,which is relatively constant with input voltage variations.At low input voltages the efficiency drops because theLT1171 power switchs saturation voltage becomes ahigher percentage of the available input supply.

This circuit can deliver an output current of 0.5A at a 3.5Vinput voltage. This rises to 1A as input voltage is in-creased. Above 20V, higher output currents can be achievedby increasing the values of inductors L1 and L2. Largerinductances store more energy, providing additional cur-rent to the load. If 0.5A of output current is insufficient, usea higher current part, such as the LT1170.

The output voltage is controlled by the LT1171 internalerror amplifier. This error amplifier compares a fractionof the output voltage, via the R1 to R2 divider networkshown in Figure 47, with an internal 1.25V referencevoltage, and varies the duty cycle until the two values are

equal. (The duty cycle is determined by multiplying theswitch ON time by the switching frequency.) The RCnetwork (R1 and C4 in Figure 47) connected to the VC pinprovides sufficient compensation to stabilize this controlloop. Equation 1 (see Figure 47) can be used to determinethe output voltage.

INPUT VOLTAGE (V)0

0.0

I OUT

(MAX

) (A)

0.6

1.2

20 40

AN66 F49

0.2

0.4

0.8

1.0

5 10 15 25 30 35

IOUT(MAX) EFFICIENCY

50

65

80

55

60

70

75

EFFICIENCY (%)

Figure 49. Efficiency and Load Characteristicsfor Various Input Voltages

SWITCHING REGULATOR PROVIDES15V OUTPUT FROM AN 8V TO 40V INPUTWITHOUT A TRANSFORMERby Brian Huffman

Many systems derive 15V supplies for analog circuitryfrom an input voltage that may be above or below the 15Voutput. The split supply requirement is usually fulfilled bya switcher with a multiple-secondary transformer or bymultiple switchers. An alternative approach, shown inFigure 50, uses an LT1074 switching regulator IC, twoinductors and a flying capacitor to generate a dual-output supply that accepts a wide range of input voltages.This solution is particularly noteworthy because it usesonly one switching regulator IC and does not require atransformer. Inductors are preferred over transformersbecause they are readily available and more economical.

The operating waveforms for the circuit are shown inFigure 51. During the switching cycle, the LT1074s VSW

pin swings between the input voltage (VIN) and the nega-tive output voltage (VOUT). (The ability of the LT1074sVSW pin to swing below ground is unusualmost other5-pin buck switching regulator ICs cannot do this.) TraceA shows the waveform of the VSW pin voltage and Trace Bis the current flowing through the power switch.

While the LT1074 power switch is on, current flows fromthe input voltage source through the switch, throughcapacitor C2 and inductor L1 (Trace C), and into the load.A portion of the switch current also flows into inductor L2(Trace D). This current is used to recharge C2 and C4during the switch OFF time to a potential equal to thepositive output voltage (VOUT). The current waveforms forboth inductors occur on top of a DC level.

The waveforms are virtually identical because the induc-tors have identical values and because the same voltagepotentials are applied across them during the switchingcycles.

Application Note 66

AN66-30

Figure 50. Schematic Diagram for 15V VersionAN66 F50

L250H

VSWVIN

VCFB

VR1LT1074

GND

+

C2470F25V

D1MUR410

C60.01F

R420k

R520k

C70.01F

C11000F

50V

L150H

R27.50k

1%

R31.30k

1%

C3470F

25V

+

C4470F

25V

+

VOUT15V0.5A

D2MUR410

R13.3k

C50.01F

+

VIN8V TO 40V

5

3 2

1

4

VOUT15V 0.5A

2.21V* (1 + R2/R3)VOUTNICHICON UPL1H102MRHNICHICON UPL1E471MPHMOTOROLA MUR410COILTRONICS CTX50-2-52 (407) 241-7876

EQUATION 1: VOUT = VOUT =

C1 = C2, C3, C4 =

D1, D2 = L1, L2 =

When the switch turns off, the current in L1 and L2 beginsto ramp downward, causing the voltages across them toreverse polarity and forcing the voltage at the VSW pinbelow ground. The VSW pin voltage falls until diodes D1(Trace E) and D2 (Trace F) are forward biased. During this

A = 20V/DIVVSW

B = 2A/DIVISW, IC1

C = 1A/DIVIL1, IC3

D = 1A/DIVIL2

E = 1A/DIVID1, IC3

F = 1A/DIVID2, IC4

G = 1A/DIVIC2

5s/DIV

Figure 51. LT1074 Switching Waveforms

AN66 F51

interval the voltage on the VSW pin is equal to a diode dropbelow the negative output voltage (VOUT). L2s currentthen circulates between both D1 and D2, charging C2 andC4. The energy stored in L1 is used to replace the energylost by C2 and C4 during the switch ON time. Trace G iscapacitor C2s current waveform. Capacitor C4s currentwaveform (Trace F) is the same as diode D2s current lessthe DC component. Assuming that the forward voltagedrops of diodes D1 and D2 are equal, the negative outputvoltage ( VOUT) will be equal to the positive outputvoltage (VOUT). After the switch turns on again the cycleis repeated.

Figure 52 shows the excellent regulation of the negativeoutput voltage for various output currents. The negative

0

VOU

T (V

)

15.3

AN66 F52

0.5

IOUT (A)

IOUT = 0.5A

IOUT = IOUT

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

15.2

15.1

15.0

14.9

14.8

14.7

14.6

Figure 52. 15V Output Regulation Characteristics

Application Note 66

AN66-31

output voltage tracks the positive supply (VOUT) within200mV for load variations from 50mA to 500mA. Negativeoutput load current should not exceed the positive outputload by more than a factor of 4; the imbalance causes loopinstabilities. For common load conditions the two outputvoltages track each other perfectly.

Another advantage of this circuit is that inductor L1 acts asboth an energy storage element and as a smoothing filterfor the positive output (VOUT). The output ripple voltagehas a triangular waveshape whose amplitude is deter-mined by the inductor ripple current (see trace C of Figure51) and the ESR (effective series resistance) of the outputcapacitor (C3). This type of ripple is usually small so a postfilter is not necessary.

Figure 53 shows the efficiency for a 0.5A common load atvarious input voltages. The two main loss elements are theoutput diodes (D1 and D2) and the LT1074 power switch.At low input voltages, the efficiency drops because theswitchs saturation voltage becomes a higher percentageof the available input supply.

The output voltage is controlled by the LT1074 internalerror amplifier. This error amplifier compares a fraction ofthe output voltage, via the R2 to R3 divider network shown

in Figure 50, with an internal 2.21V reference voltage andthen varies the duty cycle until the two values are equal.The RC network (R1 and C5 in Figure 50) connected to theVC pin along with the R4/R5 and C6/C7 network providessufficient compensation to stabilize the control loop. Equa-tion 1 can be used to determine the output voltage.

Figure 54 shows the circuits 5V load regulation charac-teristics and Figure 55 shows its efficiency.

Refer to the schematic diagram in Figure 56 for modifiedcomponent values to provide 5V at 1A.

INPUT VOLTAGE (V)

050

EFF

ICIE

NCY

(%)

55

60

65

70

75

15 40

AN66 F55

5 10 20 3525 30

Figure 55. 5V Efficiency Characteristics with 1A Common Load

IOUT (A)

04.8

VOU

T (V

)5.0

5.1

5.3

5.4

5.6

5.7

0.2 0.5 0.7 1.0

AN66 F54

0.1 0.3 0.4 0.6 0.8 0.9

4.9

5.2

5.5

IOUT = IOUT

IOUT = 1A

0

EFFI

CIEN

CY (%

)

75

AN66 F53

40

INPUT VOLTAGE (V)

5 25 30 35

70

65

60

55

5010 15 20

Figure 53. 15V Efficiency Characteristics with0.5A Common Load

Figure 54. 5V Output Regulation Characteristics

Application Note 66

AN66-32

AN66 F56

L250H

VSWVIN

VCFB

VR1LT1074

GND

+

C2680F16V

D1MBR360

C60.01F

R420k

R520k

C70.01F

C11000F

50V

L150H

R22.80k

1%

R32.21k

1%

C3680F

16V

+

C4680F

16V

+

VOUT5V1A

D2MBR360

R12k

C50.033F

+

VIN8V TO 40V

5

3 2

1

4

VOUT5V1A

2.21V* (1 + R2/R3)VOUTNICHICON UPL1H102MRHNICHICON UPL1C681MPHMOTOROLA MBR360COILTRONICS CTX50-2-52 (407) 241-7876

EQUATION 1: VOUT = VOUT =

C1 = C2, C3, C4 =

D1, D2 = L1, L2 =

Figure 56. Schematic Diagram for 5V Version

RegulatorsSwitching(Inverting)HIGH EFFICIENCY 12V TO 12V CONVERTERby Milton Wilcox and Christophe Franklin

It is difficult to obtain high efficiencies from invertingswitching regulators because the peak switch and induc-tor currents must be roughly twice the output current.Furthermore, the switch node must swing twice the inputvoltage (24V for a 12V inverting converter). The adjustableversion of the LTC1159 synchronous stepdown controlleris ideally suited for this application, producing a combina-tion of better than 80% efficiency, low quiescent currentand 20A shutdown current.The 1A circuit shown in Figure 57 exploits the high input-voltage capability of the LTC1159 by connecting the con-troller ground pins to the 12V output. This allows thesimple feedback divider between ground and the output(comprising R1 and R2) to set the regulated voltage, sincethe internal 1.25V reference rides on the negative output.The inductor connects to ground via the 0.05 current-sense resistor.

A unique EXT VCC pin on the LTC1159 allows the MOSFETdrivers and control circuitry to be powered from the outputof the regulator. In Figure 57 this is accomplished bygrounding EXT VCC, placing the entire 12V output voltageacross the driver and control circuits (remember theground pins are at 12V). This is permissible with theLTC1159, which allows a maximum of 13V between theSense and Ground pins. During start-up or short-circuitconditions, operating power is supplied by an internal4.5V low dropout linear regulator. This start-up regulatorautomatically turns off when the output falls below 4.5V.

A cycle of operation begins when Q1 turns on, placing the12V input across the inductor. This causes the inductorcurrent to ramp to a level set by the error amplifier in theLTC1159. Q1 then turns off and Q2 turns on, causing thecurrent stored in the inductor to flow to the 12V output.At the end of the 5s off-time (set by capacitor CT), Q2turns off and Q1 resumes conduction. With a 12V input theduty cycle is 50%, resulting in a 100kHz operating fre-quency.

Application Note 66

AN66-33

Figure 57. LTC1159 Converts 12V to 12V at 1A

+

+

+

AN66 F57

Q3TP0610L

20k100 100

1000pF

0.051k

6800pF

CT390pF

1N5818

SENSE SENSE+

ITH VFB

CT SGND

VCC3.3F

PWR GND

PDRIVE NGATELTC1159

0.15F

VCC EXT VCC

VIN SHDN2

PGATE

1N4148

CAP

8

7

6

5

4

3

2