Application Note 125

AN125-1

an125f

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

February 2010

Using the LTM2881 as an Isolated 5V Power SupplyBrian Jadus

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

Figure 1. LTM2881 Confi gured as a Dedicated Isolated 5V Power Supply

AN125 F01

A

VCC2 REGULATED 5V

RO

ONOFF ON

VL

TE

DE

DI

DOUT

GND GND2

VCC

VCC

B

IOUT

Z

DIN

PWR

LTM2881

ISO

LA

TIO

N B

AR

RIE

R

2500VRMS Isolated 5V 1W Power With No External Components

The LTM®2881 is an isolated RS485 transceiver that guards against large ground-to-ground differentials. An onboard DC/DC converter provides isolated 5V power to the output. Figure 1 shows the LTM2881 confi gured as a dedicated isolated power supply with the RS485 transceiver disabled, providing a simple powerful general purpose isolated power supply capable of delivering 1W at 5V. Figure 2 shows a logically-controlled switched output by taking advantage of the isolated RS485 driver to control a discrete PMOSFET. The controlling signal DI operates relative to the VL sup-ply, which supports voltages from 1.62V to 5.5V. In both confi gurations, observation of a VCC2 fault is accomplished by observing a high impedance condition on RO or DOUT . When VCC2 is greater than 2.6V, RO and DOUT are driven to the appropriate logic level based on the inputs A and B and DIN. When VCC2 is less than 2.4V, RO and DOUT are high impedance.

Features:

n 5V (LTM2881-5) or 3.3V (LTM2881-3) Input Supply Voltage

n 5V DC Output Delivers 200mA at 62% Effi ciency with a 5V Input (LTM2881-5) or 120mA at 52% Effi ciency with a 3.3V Input (LTM2881-3)

n Zero Current (Typical) Shutdown Mode When ON Pin is Low.

n Overcurrent and Overtemperature Protectionn No External Components Required. Decoupling

Capacitors are Integrated.n Small Package: 15mm × 11.25mm × 2.8mm

Figure 2. Switched 5V Power with Isolated CMOS Logic Connection to VL Voltage Interface

AN125 F02

A

VCC2

IOUT

RO

ON

VL

TE

RE

DE

DI

DOUT

GND GND2

VCC

VCC

CMOSINPUT

CMOSOUTPUT

B

Z

1.8V TO 5.5V

DIN

PWR

ONOFF

REGULATED 5V

SWITCHED 5V

IRLML6402

330k

LTM2881

ISO

LA

TIO

N B

AR

RIE

R

Application Note 125

AN125-2

an125f

Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2010

LT 0210 • PRINTED IN USA

Typical Performance Characteristics

VCC Supply Current vs Temperature at IOUT = 100mA on VCC2

TEMPERATURE (°C)

I CC C

UR

REN

T (

mA

)

350

250

300

200

150

100

50

0

AN125 G01

100755025–25 0–50

LTM2881-3

LTM2881-5

TEMPERATURE (°C)

–500

I OU

T (

mA

)

50

100

150

200

300

–25 0 25 50

AN125 G02

75 100

250LTM2881-5

LTM2881-3

IOUT LOAD CURRENT (mA)

0

VC

C2 V

OLT

AG

E (

V)

5.2

5.0

4.6

4.7

4.9

5.1

4.8

4.515050 250

AN125 G03

300

3.0V

100 200

VCC = 3.0V TO 3.6V, LTM2881-3VCC = 4.5V TO 5.5V, LTM2881-5

5.5V

5.0V

4.5V3.6V

3.3V

IOUT Current vs Temperature; RS485 Disabled: DE = TE = 0, VCC2 = 4.75V

VCC2 Output Voltagevs IOUT Load Current

VCC2 Power Effi ciency VCC2 Load Step (100mA) VCC2 Noise

IOUT OUTPUT CURRENT (mA)

010

EFF

ICIE

NC

Y (

%)

60

50

40

30

20

70

20050 100

AN125 G04

150

LTM2881-5

LTM2881-3

100μs/DIV AN125 G05

VCC2100mV/DIV

IOUT50mA/DIV

200μs/DIV AN125 G06

10mV/DIV

TA = 25°C, LTM2881-3 VCC = 3.3V, LTM2881-5 VCC = 5V, VL = 3.3V, GND = GND2 = 0V, ON = VL unless otherwise noted.