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Driving LEDS using L497X, L597X, L692X dc/dc converter families

Posted: 28 May 2004     Print Version  Bookmark and Share

Keywords:power 

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AN1891

APPLICATION NOTE

March 2004

1 INTRODUCTION

Light Emitting Diodes (LEDs) are becoming more and more popular in a variety of different applications:

Mobile appliances

- Mobile phone screen and keyboard backlight

- PDA and digital camera backlight

Signs and displays

- Advertising signs

- Traffic variable messages signs

"Automotive applications

- Interior application: lights for instrumental panel and dashboard

- Exterior lighting stop/turn/tail lights

Signals

- Traffic signals

- Arrows and pedestrian signals

Illumination

- Flashlights

- Architectural lighting

- Emergency lighting

Despite the different final application we can draw a line between 2 major LEDs Drivers topology: powered

by AC source or by DC source.

This paper will focus on the devices suitable to drive LEDs from a DC source.

We will refer mainly to white LEDs, that are characterized by a forward drop ~3-4V and currents ranging

from few tenths mA to nearly 1A.

2 DRIVING LEDS

LEDs have to be current driven, because the forward voltage variance can be greater than +/-100mV giv-

en the same forward current. This can lead to poor current and brightness matching in configuration

where currents are not regulated (Fig. 1)

Figure 1.

by A. Maggioni, M. Merisio

APPLICATION IDEAS: DRIVING LEDS USING L497X, L597X,

L692X DC-DC CONVERTERS FAMILIES

AN1891 APPLICATION NOTE

2/10

LEDs can be driven in series (see Fig. 2) or in parallel (see Fig 3).

Both solutions show advantages and disadvantages:

Figure 2. LEDs driven in series.

Driving in SERIES:

We have uniform brightness since the same current flows through all the diodes

The LEDs forward drops add together causing a high total voltage drop.

Figure 3. LEDs driven in parallel

Driving in PARALLEL:

It requires a lower voltage (one forward drop)

The LED - to - LED voltage drop variation can cause a variation in the brightness between the LEDs.

The so called "ballast resistors" are used to match the current through all the LEDs, but affects the

efficiency.

The choice of the most suitable DC-DC converter depends both on the chosen topology and on the DC

source.

The main difference can be drawn between battery powered application and the applications powered by

a pre-regulated DC bus.

Iled

IN OUT

Rsense

DC/DC

Converter

Feedback

DC

SOURCE

IN OUT

DC/DC

Converter

Feedback

I1 I2 I3

IledRsense

Ballast

resistors

DC

SOURCE

3/10

AN1891 APPLICATION NOTE

2.1 DC BUS SUPPLY.

Most of automotive and lighting application belong to this category.

The most common input voltages are 12V, 18V, 24V and 48V .

For these application we suggest to use L497x and L597x/L6902 families of step down monolithic DC-DC

converters.

Here a table summing up the devices characteristics (devices are grouped by families and sorted by out-

put current):

Table 1.

There are many criteria to select the most suitable device: of course input voltage range, output current

capability, output voltage range (according to the number of serial LEDs) - as in standard DC-DC converter

choice.

But there are characteristics that can be better exploited in LEDs driving: the first one is the presence of

a voltage reference VREF (when available).

As we have to control the LEDs current L6902 with its current loop control could be the best choice - if all

the other IC parameters can fit the application requirements.

Otherwise to regulate the LEDs current we can use a standard Step Down configuration, substituting the

upper resistor of the output resistive divider with the LED (s). The remaining resistor will work as SENSE

resistor, and it will be used to program the LEDs current :

As the application efficiency is defined as:

the smaller the losses on the RSENSE the higher the efficiency. In order to decrease the losses we have

to make the drop across the RSENSE as small as possible.

If a device has no VREF available, the drop across the RSENSE will be VFB.

If there is a reference voltage available, it can be connected to the RSENSE via a resistive divider, and the

drop across RSENSE will be:

We will show in the application ideas using L5970D and L4973 how this characteristic can be really useful

in improving the efficiency of the application.

device

Iout

(A)

Vout (V) Vin (V) Fsw (KHz) Features useful for LEDs packages

L4976 1 0.5 to 50 8 to 55 Up to 300 5.1V Vref Minidip/SO16W

L4971 1.5 3.3 to 50 8 to 55 Up to 300 Minidip/SO16W

L4978 2 3.3 to 50 8 to 55 Up to 300 Minidip/SO16W

L4973x3.3 3.5 0.5 to 50 8 to 55 Up to 300 5.1V Vref DIP18/SO20

L4973x5.1 3.5 5.1 to 50 8 to 55 Up to 300 DIP18/SO20

L6902D 1 0.5 to 35 8 to 36 250 Constant current control, 3.3V

Vref

SO8

L5970D 1 0.5 to 35 4.4 to 36 250 3.3V Vref SO8

L5972D 1.5 1.23 to 35 4.4 to 36 250 SO8

L5973AD 1.5 0.5 to 35 4.4 to 36 500 3.3V Vref HSOP8

L5973D 2 0.5 to 35 4.4 to 36 250 3.3V Vref HSOP8

ILED

VSENSE

RSENSE

----------------------=

VLED IOUT

VIN IIN

-------------------------------=

VSENSE VFB

VREF VFB-

R1

------------------------------- R2-=

AN1891 APPLICATION NOTE

4/10

Here some application ideas with key characteristic highlighted:

2.1.1 L6902D application idea

Figure 4. L6902D application idea

Table 2.

efficiency 1 LED 2 LEDs 3 LEDS 4 LEDs 5LEDS 6LEDs

Vin=12V

Iout=350mA 75% 89%

Iout=700mA 89% 90%

Vin=18V

Iout=350mA 86% 89% 96.5%

Iout=700mA 88% 92.5% 94.5%

Vin=24V

Iout=350mA 84% 88% 90% 94% 97.5%

Iout=700mA 86% 92.5% 93.5% 95% 96.5%

22nF

L6902D

Vcc

VrefGND

FB

COMP

CS+

CS-

OUT

Rsens

e

193K

5.1K

STPS340U

22 5H

10 5F

25v

100 5F

25v

220pF

VIN

12V to 24V

Up to

6 LEDs

10K

The Rsense Value is

Relationated To Iout

Iout=350mA

Rsense=0.28

Iout=700mA

Rsense=0.14

100mV

R1

R2

The resistors divider

On FB is not mandatory

But is used to clamps the

Maximum output voltage

To: 1.23x[1-(R1/R2)]

5/10

AN1891 APPLICATION NOTE

2.1.2 L5970D application idea

Figure 5. L5970D application idea

Table 3.

efficiency 1 LED 2 LEDs 3 LEDS 4 LEDs 5LEDS 6LEDs

Vin=12V

Iout=350mA 76% 89% 93.5%

Iout=700mA 77% 88% 90.5%

Vin=18V

Iout=350mA 83.5% 90% 95% 90%

Vin=24V

Iout=350mA 86.5% 90% 94.5% 97.5%

Iout=700mA 87% 90% 93.5% 96%

OUT

FB

VREF

INHGND

COMP

VCC

VIN

12V to 24V

L5970D

15 5H

15K

40K

STPS2L25U

105F

25V

220pF 4.7K

22nF

Vsense=0.45V

Rsense

Up to

6 LEDs

The resistors divider on

the FB pin is used

to reduce the Vsense

from 1.23V to 0.45V

decreasing the losses

on Rsense

3305F

25V

The Rsense

Value is

Related

To Iout

Iout=350mA

Rsense=1.3

Iout=700mA

Rsense=0.65

Vsense=Vfb-{[(Vref-Vfb)/R1]xR2}

R2

R1Vfb-Vsense

Vled

Vref-Vfb

AN1891 APPLICATION NOTE

6/10

2.1.3 L4973 application idea

Figure 6. L4973 application idea

Table 4.

2.2 BATTERY POWERED APPLICATIONS

Battery powered application are characterized by lower input voltages compared to the applications

shown in the previous pages.

As a consequence buck topology is not the commonest topology - boost or buck/boost topologies are often

required.

We will focus our application examples on L692x family, including a step up converter (L6920D) and step

down converters (L6925D/L6926D) that can be used in both boost and in buck/boost topology.

We will go through application ideas showing how to use this devices in applications with 1 to 10 LEDs,

using external references, true shutdown , and dimming.

Application Ideas using L6926D in boost topology

Application Ideas using L6926D in Positive Buck Boost Topology

Application Ideas using L6920D

IOUT=0.35 A IOUT=0.7 A IOUT=1 A IOUT=2 A

12 LEDs 98% 96.5% 97% 96.5%

10 LEDs 94.3% 96% 96.5% 96%

8 LEDs 95% 96.5% 96.5% 95%

7,8

Vcc

16

5.1V

12

Vfb

9

Boot

17

SS

1

Osc

11

Comp

10

INH

4,5,6

13,14,15

GND

2.3

Out

18

Sync

L4973V3.3

9.1K

20K

78K

50K

Rsense

10005F

63V

220nF

63V

2.7nF

470nF 220nF

22nF

220nF

63V

1505H

3x1005F

40V

D1 Up to

12

LEDs

The resistors divider on

The FB pin is used in

Order to reduce the Vfb

From 3.3V to 0.5V

Vsense

0.5V

Vin=48V

Vsense=Vfb-{[(Vref-Vfb)/R1]xR2}

= (Vled x Iout)/(Vin x Iin)

R2

R1 Vfb-Vsense

Vled

7/10

AN1891 APPLICATION NOTE

2.2.1 L6926D in Boost Topology:

Figure 7. Application Idea : driving one white LED

Figure 8. Application Idea: dimming control using an analog voltage

Figure 9. Application Idea: dimming control using PWM signal

105F

6.3V 15F

L=10 5H

1 K

100 225F

6.3V

15F

D1

30

5K

+

-

Vbatt

I= 20mA

72.873.573.771.368.464.6Eff(%)

3.83.63.43.232.8Vbatt (V)

LX

1

4

6

2 3

7

L6926D

Vcc

5

8

RUN

PGOOD

GNDCOMP

VFB

Sync

+

-

30

Analog Brightness Control

3.3V=LED Off

0V=LED On

5K2

2K 1

K

100

D1

Vbatt

105F

6.3V

15F

225F

6.3V

15F

LX

1

4

6

2 3

7

L6926D

Vcc

5

8

RUN

PGOOD

GNDCOMP

VFB

Sync

L=10 5H

+

-

30

5K

1K

100

D1

Vbatt

105F

6.3V

15F

225F

6.3V

15F

LX

1

4

6

2 3

7

L6926D

Vcc

5

8

RUN

PGOOD

GNDCOMP

VFB

Sync

L=10 5H

3.3V PWM Signal

100%=LED Off

0%=LED On

22K2.2K

1nF

AN1891 APPLICATION NOTE

8/10

2.2.2 L6926D in Positive Buck Boost Topology

Figure 10. Application Idea: Powering 10 LEDs

Figure 11. Application Idea: Two LEDs arrays controlled by an external switch

C1

105F

6.3V

LX

1

4

6

2 3

7

L6926

GND

Vcc

5

8

RUN

Sync

COMP

PGOOD

VFB

C3

220pF

R1

10K

7.5 5H

STT5NF20V

STPS1L30M

Q1

D1

C2

22 5F

25V

20 20

13

84878584.5Eff(%)

4.23.63.33Vbatt(V) Iout=45mA

Vbatt

C1

105F

6.3V

LX

1

4

6

2 3

7

L6926D

GND

Vcc

5

8

RUN

Sync

COMP

PGOOD

VFB

C3

220pF

R1

10K

7.5 5H

STS6NF20V

Q1

D1

C2

22 5F

25V

30

BSS131

Q2

Keypad

LEDs

Backlight

LEDs

Control

OFF

ON

100K1

0nF

Backlig

ht

OFF

Backlig

ht

ON73.568.268.47067.8Eff(%)

4.643.632.6Vbatt(V

66,7676768.468.4Eff(%)

4.643.632.6Vbatt(V

9/10

AN1891 APPLICATION NOTE

2.2.3 L6920D Application Ideas

Figure 12. ApplicationIdea: driving one white LED with external reference

Figure 13. Application Idea: driving one LED

Eff (%)

Vbatt (V)

8179.879.779

32.62.21.8

lx

VOUT

LBI

GND

VREF

FB

______

SHDN

____

LBO

1

3

4 5

6

7

2 8

L6920D 475F

475F

100nF

D1

20

15F

12.5K 5

KE

xternal

Reference

3.3V

Vsense=0.4V

Iout=20mA

L=10 5H

The Ext. Reference is used

In order to reduce the losses

On Rsense

Rsense

Vbatt

Vsense=Vfb-{[(Vext-Vfb)/R1]xR2}

R2

R1

Vfb-Vsense

Vext -Vfb

LX

VOUT

LBI

GND

VREF

FB

______

SHDN

____

LBO

1

3

4 5

6

7

2 8

L6920D

100nF

D1

60.4

Vbatt

Iout=20mA

L=10 5H

475F

475F

68.36866.56665.464.6Eff (%)

3.63.22.82.62.21.8Vin (V)

Vled

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences

of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted

by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject

to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not

authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.

The ST logo is a registered trademark of STMicroelectronics.

All other names are the property of their respective owners

) 2004 STMicroelectronics - All rights reserved

STMicroelectronics GROUP OF COMPANIES

Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan -

Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States

www.st.com

10/10

AN1891 APPLICATION NOTE

Figure 14. Application Idea : Driving one LED with True Shutdown

3 CONCLUSION

The aim of this paper is to show how monolithic DC-DC converters can be used to drive LEDs.

The right choice between all the ICs belonging to L497x, L597x and L692x families depends on the final

application requirements: (power source, number of LEDs, additional functions.... )

This paper shows only few application ideas about LEDs driving: you may find much more details on the

specific ICs in the dedicated Datasheet and Application Notes available on ST web site.

True SHDN

Signal

STT5NF20V

LX

VOUT

LBI

GND

VREF

FB

______

SHDN

____

LBO

1

3

4 5

6

7

2 8

L6920D

100nF

D1

3.5

Vbatt

Iout=350mA

L=10 5H

475F

475F

During the Device's Shutdown ,

The TRUE SHDN signal is used

in order to avoid a direct

connection between the input and

GND





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