LED Applications and Driving Techniques

1

LED Applications and Driving Techniques

2

© 2007 National Semiconductor Corporation

Content

• LED Basics• LED Applications• LED Driving Techniques• LED Dimming and Contrast Ratios• Lighting Resources and Tools

3

LED Basics

4


What is a LED?

5


LED Development

5mm Lamp2-3 lumensIf = 30mA

1970

SuperFlux4-8 lumensIf = 70mA

1992

Luxeon20-40 lumens

If = 350mA

1997

6


Materials used in color LEDs

White LED:

White light is generated by blue LED strike on a phosphor coating.

7


White LED Structure

BROAD RANGE PHOSPHOR

From LED

From phosphor

BLUE InGaN DIE

8


LED Advantages

9


Characteristics of LEDs

Forward Voltage (VF) drop across LEDSimilar to a diode

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

10


Temperature effect onLED Parameters

As Temperature increases:• Light output decreases• Wavelength gets longer• Forward voltage decreases

11

LED Applications

12


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

13


Many New Applications Have Emerged Because……

• High Brightness LEDs create many new possibilities.

• 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• Full intensity 700mA, Maximum current 1A• 7.1V Volt drop @ 700mA

14


Structure of High Brightness LED

15

LED Driving Techniques

16


Signal Problem Power Problem

• What has changed?– ILED = 20mA 350mA+– VF = 2V 3V+

MCUVcc

+5V

220Ω

ILED = 20mA

2N3904

17


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 butit delivers constant current instead of constant voltage

18


Average Current and Ripple Current

• Iav1 > Iav2, thus Iav1 is brighter than Iav2 but color tone also changes.• Human eye cannot detect I, ripple current changes as long as it is

within 20% of the average current.

ILED

Iav1

Iav2I

19


Driving Methods –Circuit Configuration

• Relatively expensive• Circuit complexity

High efficiencySwitching Regulation

• Poor efficiency• Heat dissipation at driver IC

SimpleLinear Regulation

• Current varies as VF changes• Poor efficiency• Heat generation at resistor

Inexpensive, Simple

Resistor Limiting

ConsProsDriving Method

20


Driving Method –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 voltage requirement

GND FB

VSWVIN

VOUT

Rballast

With external ballast With internal current sinkTypical Backlight Flex print with LED in series

21


• 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

Driving Method –Parallel LED Connection

22


Boost LED Driving

• Vout must be higher than Vin.

• Output capacitor is a must.

• Typical Application: LCD backlight for portable product.

TON

T

23


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

24


Buck LED Driving

• Vout must be lower than Vin.

• Output capacitor is optional.

• Typical Application : general lighting.

TON

25


Bucking and Boosting

• High power LEDs are being adopted into portable lamps (bicycle, mining, flashlight) with varying battery number and chemistry

• 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

26


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

27


‘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

28

LED Dimming and

Contrast Ratios

29


LED Brightness Control Methods

Brightness can be controlled directly by changing the LED current or indirectly by shutting on and off the LED fast enough to create perception of dimming by the human eye.

• Analog Brightness Control – current alternation.– Not preferred because color tone will change.

• Digital Brightness Control – PWM on/off.– Preferred method.

30


PWM Dimming Control

• PWM signal (EN/SD pin, FET, or special PWM pin)– “Strobing” LEDs at frequencies >100Hz results in

reduced brightness, as perceived by the eye. Eye filters/averages brightness

– “Average” Brightness proportional to Duty Cycle (D): D = tON ÷ T

31


Low Power LED Dimming

• Best choice is a linear regulator with an inline switch

• Can expect rise and fall times of < 100 ns

• Dedicated LED driving linear regulators have multiple channels

• Should also include a feedback signal to adjust input voltage for VF tolerance and drops due to rising LED die temp

32


Adjusted automatically to the lowest possible to

maximize efficiency

V

Temperature

VREG

VLED

VRAIL

Power loss

Always kept to a minimum

VLED

VREG

VIN

LM3432

CDHC

VCC

IREF

PWM

EN

M O D E

F A U L T b

O T M b

VDHC

IOUT1

IOUT2

IOUT3

IOUT4

IOUT5

IOUT6

LM3430

VCC VIN

UVLOSSRT/SYN

OUT

CS

GND

FB

COMP

3X3mm0.8mm(H)

PWM SIGNAL

VIN=6~40V SW. FREQ. Up to 2MHz Up to 80V

ON

OFF

5X4mm0.8mm(H)

DIRECT FROM VIN

VDHC

LM3430+LM3432 Solution

33


High Power LED Dimming

• Use buck regulator whenever possible

• Only the buck can eliminate the output capacitor*

• No RHP zero means fastest control loops (when using PWM regulators)

• Easy implementation of hysteretic and constant on-time (COT) control– Even faster loops!

34


Controlling Color

• Colored LEDs shift their peak wavelength as IF changes

• Requires control of IFand ∆iF

• Accuracy of IF is highly dependent on the application

35


CCT(Correlated Color-Temperature) 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

36


Contrast Ratio

• Ideal Contrast Ratio is a Wishful Thinking

• Contrast Ratio is defined as 1/DDIM(MIN) .

• Contrast ratio is a measure of the system response to a dimming signal. The higher the contrast ratio, the more precise the control over light output. This term has been co-opted as a buzzword in the display industry.

• Contrast ratio is highly dependent on the external components

37


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=

38


A Tale of Two Circuits

• LM3489 is an LM3485 derivative

• Adds a logic enable• Not designed for LED

but:– Fast, pure hysteretic

control– Simple design– Needs output cap– VIN = 7.2V

• LM3404 is an LM5007 derivative

• DIM pin is really a fast enable pin

• Designed for LED– COT is almost as fast as

hysteretic– Fixed on-time simplifies

inductor selection– No output cap– VIN = 24V

39


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

40


LM3489 LED Drive Conversion

EN

FB

GND

ADJ

PGND

PGATE

VINISENSE

LM3489

CO

D1

Q1CIN

RSNS2

RCL CCL

L1

VIN = 6.0V to 8.4V

+-

VIN

CFF

R350

Ri

LM321

U2

U1

RFF

CF

To LED Anode

To LED Cathode

PWM DIM SIGNAL

RSNS1

Drives one white LED at 350 mA from 7.2V

41


LM3404 Delay, tD

tD = 51 ns

Bandgap, analog functions were already powered

DIM

IF

SW

42


LM3489 Delay, tD

tD = 16 µs

Low power shutdown: lots of circuits had to ‘wake up’

Note the overshoot, due to the limited bandwidth of the LM321 used to amplify Vsnsfor the LM3489. If this circuit did not have the amplifier or an output capacitor there would be no overshoot at all.DIM

IF

SW

43


LM3404 Slew Up, tSU

tSU = 3 µs

∆iL = ∆iF (no CO)

∆iL = (VIN – VO) / L

Limited by tOFF-MIN

tOFF-MIN = 300 ns

DIM

IF

SW

44


LM3489 Slew Up, tSU

tSU = 2 µs

Shaped and limited by CO charging

ΔiL = (VIN – VO) / L

DIM

IF

SW

45


LM3404 Slew Down, tSD

∆iL = – VO / L

tSD = 8.4 µs

DIM

IF

SW

46


LM3489 Slew Down, tSD

Shaped and limited by CO discharging

ΔiL = – VO / L

tSD ≈ 8 µs

DIM

IF

SW

47


Compare the Contrast Ratios

• LM3489• tD + tSU = 18 µs• DMIN = 18 / 2000 = 0.009• CR = 1 / DMIN = 111 : 1

• LM3404• tD + tSU = 3.05 µs• DMIN = 3.05 / 2000

= 0.001525• CR = 1 / DMIN = 655 : 1

fDIM = 500 Hz, TDIM = 2 ms

48


Frequency and CR Trade-off

49


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

50


High Frequency ( > 10 kHz)

• Technical requirements force the users to high frequency

• Generation of white light from RGB in backlights, video projectors

• Loss of efficiency due to the transitions

• Usually requires a parallel dimming FET

51


Parallel FET Dimming

Continuous inductor current

52


Parallel FET Results

FET

GATE

IF

53


Parallel FET Results

FET

IF

54

New LED Driving Tools

55


WEBENCH On Line Design Environment for LED Drivers

56


WEBENCH On Line Design Environment for LED Drivers

57

Date post:	03-Apr-2015
Category:	Documents
Upload:	wong-chi-ho
View:	120 times
Download:	2 times

LED Applications and Driving Techniques

Documents