Analog Devices Automotive LED User guide

Brand: Analog Devices

Model: Automotive LED

Type: User guide

Analog Devices Automotive LED Driver ICs offer a comprehensive range of solutions for various automotive lighting applications. These ICs provide precise current regulation, high efficiency, and protection features, making them ideal for driving LEDs in headlamps, taillights, interior lighting, and more. With integrated safety mechanisms, these ICs ensure reliable operation in harsh automotive environments. The compact size and ease of use make them suitable for space-constrained applications, simplifying the design process.

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Vol 56, No 3—June 2022

Automotive LED Driver

Power Conversion

Topology Guide

Joshua Caldwell , Design Director

Introduction

In many systems—including the myriad of regulators deployed in automotive

power delivery systems—the design of power conversion regulators is often a

dicult and complex task. This article aims to simplify the selection process

by explaining the benets, trade-offs, and applications for different switching

topologies used for LED drivers.

LEDs are unlike traditional electrical light producing lament or gas components.

Utilizing specic semiconductor junctions, LED manufacturers can produce

specic colors of light spanning the entire visible range—as well as IR and UV.

In automotive applications, LEDs can increase the safety in both daylight and

nighttime driving scenarios. Increased eciency can extend battery life in

electric vehicles, and multiple LEDs in a single system can eliminate single-

component failures.

Due to their versatility, LEDs offer the capability of being driven in many different

ways. Since the output from LEDs is well-controlled light, LED loads are unlike

traditional loads to a power system. LEDs only rely upon accurately regulated

current, through the semiconductor junction, to produce light, where the relative

voltages at the terminals to the system ground (or chassis in an automotive sys-

tem) are unrelated. As a result, LED systems can take advantage of the different

topologies offered by switching technologies.

How to Select the Correct Switching Topology

for Automotive LED Systems

The choice of a particular switching topology in an automotive system is related

to the complete system design; considerations should be taken into account

for minimum input voltage, maximum string voltage, chassis return capability,

shorted output capability, maximum input current, output/LED current, and

PWM dimming.

Step-Down (Buck) Converters

Step-down (or buck) LED drivers regulate the current in an LED string from a

voltage that is higher than the total LED string voltage. Buck LED drivers can

be safely shorted to the system ground, making them both intrinsically safe.

They can have the capability of chassis return (one wire for power), and they

can easily be adapted to matrix or animation applications. Figure 1 and an

example schematic in Figure 2 show basic system diagrams with the controller

modulating the high-side switch for current control.

– +

Figure 1. Buck converter.

Several critical features to look for in step-down LED drivers are xed frequency

operation, high eciency through excellent switching control and low resistance

switches, high accuracy throughout the analog dimming range, and, for excellent

EMI, a properly designed spread spectrum frequency modulation.

2 AUTOMOTIVE LED DRIVER POWER CONVERSION TOPOLOGY GUIDE

30.1 kΩ

274 kΩ

110 kΩ

10 kΩ

50 mΩ

22 nF

INTVCC

2.2 µF

100 nF

10 nF

2× 1 µF

8.2 µH

28.7 kΩ

7.8 kHz

162 kΩ

100 kΩ

10 µF

COUT

100 µF

VIN

EN/UVLO

VREF

CTRL

PWM

SYNC/SPRD

INTVCC

ISP

ISN

VOUT

BST

PWMTG

VIN

20 V

ISMON

FAULT

ISMON

GND

2 A

Max

9 V

45.3 kΩ

2 MHz

LT3932

2.2 µF

FAULT

Figure 2. Buck converter example: LT3932.

Table 1. Advantages and Trade-Offs of Using Buck

Converters as LED Drivers

Benefits to Buck

LED Drivers

Trade-Offs to Step-Down

LED Drivers Applications

Grounded string—

chassis return

Input voltage must be

higher than LED voltage High beam/low beam

Matrix switches can shunt

entire string

Preboost regulator

required in most

automotive systems

Turn signals/animation

Higher bandwidth (>1/5

of fSW)Matrix headlamps

Best EMI performance Short-safe systems

Smallest inductor sizing

Step-Up (Boost) Converters

Step-up (or boost) LED drivers regulate the current in an LED string from a

voltage that is lower than the total LED string voltage. This is useful in many

automotive systems, where many LEDs need to conduct in a single string. Typical

12 V automotive systems have operational ranges from 6 V to 18 V—requiring

that the LED driver runs down to 6 V, providing large step-up ratios for the

LEDs to remain illuminated. Figure 3 and an example schematic in Figure 4 show

basic system diagrams with the controller modulating the low-side switch

for current control.

– +

Figure 3. Boost converter.

Table 2. Advantages and Trade-Offs of Using Boost

Converters as LED Drivers

Benefits to Boost

LED Drivers

Trade-Offs to Step-Up LED

Drivers Applications

Grounded—chassis return Input voltage must be

higher than LED voltage High beam/low beam

Typically, smallest total

solution size

Lower bandwidth (<1/20

of fSW)Heads-up displays

Good EMI performance Higher inductor

current rating Backlighting

Direct battery to

LED conversion Cannot short output to GND

Figure 4. Boost converter example: LT8356-1.

1 µF

28.0 kΩ

100 kΩ

Trim

499 kΩ

130 kΩ

6.04 kΩ

6 V to 40 V

(80 V Transient) 2.2 μF

2×

100 kΩ

36 kΩ

1 nF

2.2 µF

47 µH

137 kΩ

f = 200 kHz

4.7 µF 1 MΩ

11.5 kΩ

625 mΩ

0.4 A

40 W Led String

VIN

EN/UVLO

CTRL

IADJ

PWM

GATE

SENSE

GND

INTVCC

PWMTG

ISN

ISP

LT8356-1

VREF

100 kΩ

NTC

15 mΩ

ILIM = 7 A (TYP)

FAULT

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Boost-Buck Using a Boost Converter

Some step-up (or boost) LED drivers may be congured to return the LED cathode

to the supply. This conguration is referred to as buck-boost. The total output

voltage is VIN (VBATTERY), which is added to the total LED string voltage. The benet

of this topology is being able to drive an LED string that is higher, lower, or equal

to the supply voltage. The limitations of this topology are only bounded by the

converter—on the low end by the minimum supply voltage of the controller IC

and on the high end by the controller IC’s maximum output voltage.

– +

Figure 5. Boost-buck converter.

Table 3. Advantages and Trade-Offs of Using Boost-

Buck Converters as LED Drivers

Benefits to Boost-Buck

LED Drivers

Trade-Offs to Boost-Buck

LED Drivers Applications

Direct battery to

LED conversion Lower efficiency High beam/low beam

LED voltage may be higher

or lower than supply

Lower bandwidth (<1/20

of fSW)Turn signal

Good EMI performance Higher inductor current

rating Daytime running lights

May use matrix to short

entire string Cannot short output to GND Multiple strings on the

same output

Buck Mode Using a Boost Converter

Some step-up (or boost) LED drivers may be congured to step-down from the

supply (rather than ground referenced, as in a standard buck)—creating a

buck-mode conguration. This conguration has the same limitations as a buck,

where the total LED string voltage must be less than the input supply.

–

Figure 7. Buck-mode converter.

Table 4. Advantages and Trade-Offs of Using Buck-

Mode Converters as LED Drivers

Benefits to Buck-Mode

LED Drivers

Trade-Offs to Buck-Mode

LED Drivers Applications

Good EMI performance Input voltage must be

higher than LED voltage High beam/low beams

May use matrix to short

entire string

Preboost regulator

required in most

automotive systems

Turn signal

May use the same driver

for multiple applications

Cannot short output (LED

cathode) to GND Daytime running lights

Figure 6. Boost-buck converter: LT8386.

VIN

6 V to 24 V

EN/UVLO

OVLO

INTVCC

BSTSW

VOUT

RP VCRT

ISP

GND

ISN

PWMTG

ISMON

LT8386

14.7 kΩ

1 µF

50 V

10 nF

22 µH

0.2 Ω

C11

0.1 µF

50 V

C10

0.1 µF

50 V

18 V

500 mA

LED

249 kΩ

400 kHz

10 nF

SYNC/SPRD

VREF

PWM

CTRL

100 kΩ

10 µF

50 V

33 µF

50 V

26.1 kΩ

249 kΩ R4

20 kΩ R5

20 kΩ

C12

10 µF

50 V

1 µF

33 nF

2.2 µF

10 µF, 3×, 50 V

FAULT

4 AUTOMOTIVE LED DRIVER POWER CONVERSION TOPOLOGY GUIDE

Buck-Boost Converter

Buck-boost LED drivers regulate LED current from a supply that is higher or lower

than the total LED string voltage. The converter modulates the high-side switch

connected to the input voltage in the step-down mode and the low-side on the

output-side in step-up mode. This topology is the most complex but also the most

exible. VIN and VOUT ranges are only limited by the controller IC. This is a good

choice for matrix applications.

– +

Figure 9. Buck-boost converter.

Table 5. Advantages and Trade-Offs of Using Buck-

Boost Converters as LED Drivers

Benefits to Buck-Boost LED

Drivers

Trade-Offs to Buck-Boost

LED Drivers Applications

Most versatile topology

A minimum of two switches

and two freewheeling

diodes is required

High beam/low beams

May use matrix to short

entire string

Typically, the lowest

conversion efficiency Turn signal

May use the same driver

for multiple applications

Typically, the lowest (worst)

EMI performance Daytime running lights

Short safe systems

Conclusion

Automotive LED lighting systems can be driven with switching regulators

in many different ways. Depending on the application, the selection of switching

topology and conguration allows the lighting designer to create complete

subsystems for the different lighting requirements throughout an automobile.

Selecting the correct power conversion switching topology and conguration

for the system optimizes requirements such as complexity, eciency, EMI,

and safety.

Figure 8. Buck-mode example: LT3756-2.

VIN

LT3756-2

GNDVC INTVCC

SHDN/UVLO

VREF

VIN

INTVCC

ISP

0.001 µF

0.1 µF

24 V to 80 V

28.7 kΩ

375 kHz 47 kΩ

100 kΩ

4.7 µF

CTRL

0.033 Ω

0.1 Ω

4.7 µF

5×

25 V

4.7 µF

33 µH D1

1 A

OPEN LED

PWM

ISN

GATE

PWMOUT

SENSE

1 MΩ

61.9 kΩ

1 µF

2×

200 kΩ

20 kΩ

5 White LEDs

20 W

200 kΩ 200 kΩ

1 kΩ

1.5 kΩ

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the property of their respective owners.

About the Author

Josh Caldwell was with Linear Technology (now part of Analog Devices) for 10 years as a design engineering section leader respon-

sible for the denition, design, and development of monolithic buck, boost, and controller LED drivers. He holds a bachelor’s degree

in electrical engineering from the University of Colorado. In his spare time, he enjoys bicycling and drawing.

Figure 10. Buck-boost example: LT8391.

221 kΩ

100 kΩ

0.1 µF

4.7 µF

100 V

2×

0.1 µF

0.004 Ω

0.1 µF

10 µH

5.1 Ω

33 µF

63 V

499 kΩ

4.7 µF

100 kΩ

0.47 µF

100 kΩ

400 kHz

10 nF

2.2 kΩ

1 MΩ

34.8 kΩ

0.05 Ω

10 µF

50 V

2×

EN/UVLO

VREF

CTRL2

CTRL1

GND

BG1

TG1

LT8391

LSN

LSP

SW1

BG2

SW2

TG2

BST1

BST2

VIN

INTVCC

PWM

VOUT

ISP

ISN

PWMTG

SYNC/SPRD

25 V

2 A

LED

PWM DIM

Analog DIM

VIN

6 V to 55 V

FAULT

Analog Devices Automotive LED User guide

Analog Devices Automotive LED User guide

Analog Devices Automotive LED User guide

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