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LED Applications and Driving Techniques
Chris Richardson
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© 2007 National Semiconductor Corporation
Content
•
LED Basics•
LED Applications•
LED Driving Techniques•
LED Dimming and Contrast Ratios•
Lighting Resources and Tools
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LED Basics
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© 2007 National Semiconductor Corporation
What is an LED?
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© 2007 National Semiconductor Corporation
LED Development
5mm Lamp2-3 lumensIF
= 30mA
1970
SuperFlux4-8 lumensIF
= 70mA
1992
Luxeon20-40 lumensIF
= 350mA
1997
LumiledsOsramCree
Seoul SemiAvago
IF
= 700 to 1.5A
2007
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© 2007 National Semiconductor Corporation
Structure of High Brightness LED
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© 2007 National Semiconductor Corporation
Materials used in color LEDs
White LED:White light is generated by blue LED striking a phosphor coating
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© 2007 National Semiconductor Corporation
Many New Applications Have Emerged Because……
•
Typical spec. of HB LED–
1 Watt LED•
Full intensity 350mA, Maximum current 500mA•
2.8V Volt drop @ 350mA–
3 Watt LED•
Full intensity 700mA, Maximum current 1A•
4.3V Volt drop @ 700mA–
5 Watt LED (multi-die package)•
Full intensity 700mA, Maximum current 1A•
7.1V Volt drop @ 700mA–
5 Watt LED (single-die)•
Full intensity 1.5A
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© 2007 National Semiconductor Corporation
Characteristics of LEDs
Forward Voltage (VF) drop across LEDDiodes are current driven!
Wavelength variationsCrystal and junction growth defects
Brightness variationsCrystal defects resulting formation of phonons and non-radiation energy transfer
Temperature Junction temperature of the device affects each of the parameters above
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© 2007 National Semiconductor Corporation
Temperature effect on LED Parameters
As Temperature increases:•
Light output decreases•
Wavelength gets longer•
Forward voltage decreases
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© 2007 National Semiconductor Corporation
LED Binning
•
Manufacturers bin their devices for color/wavelength, brightness, and forward voltage
•
Binning for all three characteristics is expensive, and forward voltage is often the specification that is allowed to vary the most
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LED Applications
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© 2007 National Semiconductor Corporation
LED Applications
•
Old days–
Signal Indicators–
Numeric and Alpha-numeric displays•
Nowadays–
Automotive–
Backlights–
Flashlights for portable devices–
General illumination–
Projector Light Sources–
Signage–
Torch Lights–
Traffic Lights
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Backlight Applications
•
Possible because of white LED development•
Almost all mobile phone color LCDs
use white LED backlighting.
•
Size of displays from smallest to largest–
1. Mobile phones, PDAs–
2. Automotive, aerospace infotainment–
3. Laptop displays–
4. Desktop PC monitors–
5. LCD televisions
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LED Driving Techniques
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© 2007 National Semiconductor Corporation
Resistor Limiting, Linear Regulation
.
6V
Resistor Limiting Linear IC withConstant Current Source
.
6V
Heat dissipation in resistor or linear IC
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© 2007 National Semiconductor Corporation
LED Driving Circuit ……
•
Delivers a constant average current under all conditions (eg. input voltage change, temperature change, VF
change……)•
Controls ripple current at acceptable level under all conditions
A LED driving circuit is a type of power conversion circuit that delivers constant current instead of
constant voltage
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Constant Voltage to Constant Current Conversion:
1
21
FB
FBFBFBO R
RRVV +=
FB
FBF R
VI =
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© 2007 National Semiconductor Corporation
Average Current and Ripple Current
•
Iav1
> Iav2
, thus Iav1
is brighter than Iav2
but color also changes
•
Human eye cannot detect the high frequency ripple current•
Human eye cannot detect shift in average current of < 20%
ILED
Iav1
Iav2I
Brighter
Dimmer
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Buck LED Driving
•
VO
must be lower than VIN
•
Output capacitor is optional
•
Typical Application: general lighting
TON
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Buck Driving – How it works
•
If R4 = 5Ω, current passing through R4 = 0.5A. Current passing through the LED is also the same because FB is a high impedance pin.
•
LED current setting can be done by R4.
X
X
FB voltage is maintained at 2.5V
Problem: Power dissipation at R4 = 1.25W!
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Dedicated Buck LED Driver
200mV Feedback Voltage
•
FB voltage is reduced to 200 mV
•
Power dissipation at RSNS
= 0.5A ×
0.2V = 0.1W.
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Using Boost Regulator: Series LED Connection
•
Pros:–
Matching Guaranteed–
Most efficient drive method–
Easy to route (Only 1 or 2 connections between driver and LEDs)
•
Cons–
High voltage output is needed
–
Output capacitor typically large due to voltage requirement
GND FB
VSW
VIN
VOUT
Rballast
With external ballastWith internal current sink
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•
Pros–
Workable with low-
voltage semiconductor processes
–
Can work with common anode or common cathode module
•
Cons–
Good matching requires regulated current sources
–
Requires 1 connection per each LED i.e. driver IC requires more pins
Using Boost Regulator Parallel LED Connection
Constant current source connection
. . . .
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Boost: Inductor based vs Charge Pump
Charge Pump Inductor BasedLED Connection
Usually Parallel Usually Series
Efficiency Depends on VIN
, VO
, and gain modeReduced dependence on VIN
and VO
PCB Space Less MoreWide Vin –
Vout SupportNot Practical OK
EMI Generation
Less More, due to presence of inductor
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Efficiency of a Charge Pump
•
Charge pumps are very efficient if VIN
x Gain is close to target VO
•
Efficiency drops off as VIN
increases.•
1.5 x mode is introduced to boost efficiency in conversion from one Li-
ion battery to 5V VO
LM2751 -
2×, 1.5×
charge pump (switched capacitor) white LED driver which can deliver up to 150mA at 725KHz switching frequency.
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Charge Pumps with built-in Current Source
Built-in current source,better current matching in driving several LEDs.
LM2754 -
2×, 1.5×
charge pump (switched capacitor) white LED driver which can deliver up to 800mA at 1MHz switching frequency.
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Inductive vs Charge Pump Efficiency Comparison
LM3508 Inductive Boost Efficiency
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True Shutdown Isolation
•
Method 1–
Add switch in return path.
•
Method 2–
Synchronous rectification.
A switch is added to cut off leakage
path during shutdown
Diode is replaced by MOSFET and it is
switched off during shutdown
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Bucking and Boosting
•
High power LEDs are being adopted into portable lamps (bicycle, mining, flashlight) with varying number and chemistry of batteries
•
Low-voltage AC lighting (garden path) varies due to I2R loss
•
Combine varying VIN
with VF
that changes with process and temperature
•
Requires true buck-boost regulator
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Buck Boost Efficiency < Buck or Boost
Buck: Input direct to output when power switch is on
Boost: Input direct to output when power switch is off
Buck-boost: input is never connected directly to output
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Buck-Boost Driving: SEPIC Regulator
•
Uses standard low-side regulator/controller
•
Low-side or high-side current sensing
•
Requires two inductors or coupled inductor
•
Requires an output capacitor
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Buck-Boost Driving: Cuk Regulator
•
Uses low-side regulator/controller but requires negative FB pin
•
Low-side or high-side current sensing
•
Negative VO
doesn’t matter in current drivers
•
Requires two inductors or coupled inductor
•
Can run without CO
Amplifier doubles as polarity inverter
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‘Floating’ Buck-Boost Regulator
•
Uses only one inductor•
VO
is controlled WRT VIN•
Requires high-side sensing for accurate IF
control
Basic Low-Side MOSFET
Controller
FB
VIN NGATE
GND
IN
VO = VIN + VF
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Floating Buck Boost with High-side Sense
Basic Low-Side MOSFET
Controller
FB
VIN NGATE
GND
VIN
RSNS +-
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LED Dimming and Contrast Ratios
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Adjusting Light Level with LEDs
•
“Analog Dimming”–
Linear adjustment of current through LEDs–
Causes shift in peak and dominant wavelength in monochromatic LEDs
–
Causes shift in Correlated Color Temperature (CCT) in white LEDs
–
Difficult to optimize driver efficiency•
“Digital Dimming”
(PWM Dimming)–
Drive at only one current level–
Turn LEDs on and off at > 120Hz–
Human eye integrated and averages light above this frequency
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PWM Dimming Control
•
PWM signal (EN/SD pin, FET, or special PWM pin)–
“Average”
Brightness proportional to Duty Cycle (D): D = tON
/ T
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Controlling Color
•
Colored LEDs shift their peak/dominant wavelength as IF
changes•
Requires control of IF
and ΔiF•
Accuracy of IF
is highly dependent on the application
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Controlling CCT
•
CCT provides the basis for “cool”
white (more blue) and “warm”
white (more red.)
•
CCT shifts with IF•
Much easier to see than with colored LEDs
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White LED Structure
BROAD RANGE PHOSPHOR
From LED
From phosphor
BLUE InGaN
DIE
Human eye color sensitivity curve
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CCT Shift
1W LED driven at 50 mA continuous
Same 1W LED driven at 300 mA with 1/6th
duty cycle (500Hz)
More Yellow More Blue
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PWM Dimming with Switching Regulators
•
Use buck regulator whenever possible•
Only the buck can eliminate the output capacitor*
•
No RHP zero means fastest control loops (when using clocked regulators)
•
Easy implementation of hysteretic and controlled on-time (COT) control–
Even faster loops!
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Contrast Ratio
•
1 : Wishful Thinking•
Contrast ratio is highly dependent on the external components
•
Therefore, it is highly susceptible to specmanship
•
One definition of contrast ratio is 1/DDIM(MIN)
, where DDIM(MIN)
= 2 / fSW–
Circuit must be on DCM/CCM boundary
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Frequency and Duty Cycle Limits
Rise and fall times where IF
is between 0 and 100% cause further error
TttD SUD
MIN+
= TtT
D SDMAX
−=
PWMfT 1=
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LM3404 Eval Board
Drives a 1W white (InGaN) LED at 1A from 24V
GNDDIM
BOOT SW
CS
RON
LM3404/04HV
VIN
D1
L1CB
RSNS
CF
RONCIN
VIN = 6V to 42V (LM3404)VIN = 6V to 75V (LM3404HV)
VCC
OFF
DIM1
LEDANODE
LEDCATHODE
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LM3404 Delay, tD
tD = 51 ns
Bandgap, analog functions were already powered
DIM
IF
SW
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LM3404 Slew Up, tSU
tSU = 3 μs
ΔiL
= ΔiF
(no CO
)
ΔiL
= (VIN
– VO
) / L
Limited by tOFF-MIN
tOFF-MIN
= 300 nsDIM
IF
SW
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LM3404 Slew Down, tSD
ΔiL
= –
VO
/ L
tSU = 8.4 μs
DIM
IF
SW
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Calculate the Contrast Ratios
•
LM3404•
tD
+ tSU
= 3.05 μs•
DMIN
= 3.05 / 2000 = 0.001525•
CR = 1 / DMIN
= 655 : 1
fDIM = 500 Hz, TDIM = 2 ms
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Low Frequency ( < 1 kHz)
•
General and automotive applications•
More efficient: less transitions•
Duty cycle requirements not as strict: 10% to 90% is typical
•
Usually achievable by using the DIM or EN pins
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High Frequency ( > 10 kHz)
•
Technical requirements force the users to high frequency
•
Generation of white light from RGB in backlights, video projectors
•
Machine vision and industrial inspection–
Fast slew rates for light pulses that sync to sensors and cameras
•
Loss of efficiency due to the transitions•
Usually requires a parallel dimming FET
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Parallel FET Dimming
Continuous inductor current
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Parallel FET Results
FET
GATE
IF
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Parallel FET Results
FET
IF
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CR Vs. fDIM
•
Circuit parameters placed a limit on the minimum dimming on-time, tMIN
= DMIN
x fDIM
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New LED Driving Tools
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LED.National.com
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Step 1 - Enter Design Inputs
Select an LED from the list
Enter input voltage range Enter Number of LEDs
Show Recommended Parts
Narrow LED choice by vendor and/or color
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Step 2 - Choose an LED Driver
Choose part
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Step 3 – Optimize and Customize
Key operating values:Frequency,EfficiencyPeak to Peak ILEDTemperature
Optimization knob:
Customize design for:No output capSpecify peak to peak
LED ripple
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Optimize for Efficiency
Optimization for efficiency:Lowers switching frequency, emphasizes low component power dissipation
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Step 4 - Simulate Electrical Behavior
Spice simulation includes:Steady stateInput transientPWM dimmingStartup
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View Waveforms
Click and drag mouse to zoom in on plot
Add/delete waveforms
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Build It!
•
Latest addition to LED WEBENCH®
for LM3402/02HV and LM3404/04HV
•
Generic evaluation board accepts a wide variety of external components
•
User orders and receives a kit with blank PCB and all external components as selected through LED Webench
•
Simply solder, connect to LEDs, and go•
Note: LEDs are not included
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© 2007 National Semiconductor Corporation
LED Reference Design Library
Modify
Custom datasheet of the resulting design
http://power.national.com
Customize
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