ANALOG DEVICES Automotive LED Driver Power Conversion Topology User Guide

User Guide for ANALOG DEVICES models including: Automotive LED Driver Power Conversion Topology, Automotive LED, Driver Power Conversion Topology, Power Conversion Topology, Conversion Topology, Topology

Automotive LED Driver Power Conversion Topology Guide

automotive system, LEDs, buck-boost converter, switching topology, power-conversion regulators

Joshua Caldwell

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Automotive LED Driver Power Conversion Topology Guide | Analog Devices

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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 difficult and complex task. This article aims to simplify the selection process by explaining the benefits, trade-offs, and applications for different switching topologies used for LED drivers.
LEDs are unlike traditional electrical light producing filament or gas components. Utilizing specific semiconductor junctions, LED manufacturers can produce specific 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 efficiency can extend battery life in electric vehicles, and multiple LEDs in a single system can eliminate singlecomponent 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 system) 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.
VIN
+
Figure 1. Buck converter. Several critical features to look for in step-down LED drivers are fixed frequency operation, high efficiency 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.

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VIN 20 V
2.2 µF

INTVCC

VIN

BST

274 k

10 µF 2× 1 µF

EN/UVLO

SW VOUT

30.1 k

LT3932 FB

VREF

100 k

CTRL

GND

100 k

PWM

ISP

22 nF 8.2 µH

110 k

10 k

COUT 100 µF

2 A Max

2.2 µF

100 k FAULT

INTVCC

ISN

FAULT SYNC/SPRD

PWMTG ISMON

ISMON

RT RP

45.3 k 2 MHz

28.7 k 7.8 kHz

162 k

9 V

100 nF

10 nF

50 m

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 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
Higher bandwidth (>1/5 of fSW)
Best EMI performance
Smallest inductor sizing

Preboost regulator required in most automotive systems

Turn signals/animation Matrix headlamps Short-safe systems

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
Typically, smallest total solution size
Good EMI performance

Input voltage must be higher than LED voltage
Lower bandwidth (<1/20 of fSW)
Higher inductor current rating

High beam/low beam Heads-up displays
Backlighting

Direct battery to LED conversion

Cannot short output to GND

VIN 6 V to 40 V
(80 V Transient)

2.2 F 2×
499 k
130 k

1 µF

6.04 k

100 k

28.0 k

NTC

100 k Trim
Figure 4. Boost converter example: LT8356-1.

137 k f = 200 kHz

47 µH

VIN EN/UVLO

GATE SENSE

GND

VREF

ISP

PWM LT8356-1

CTRL

ISN PWMTG INTVCC

IADJ RT

FAULT VC

4.7 µF 1 M

15 m ILIM = 7 A (TYP)

11.5 k

625 m 0.4 A

100 k FAULT
36 k

2.2 µF

1 nF

40 W Led String

2 AuTOMOTIVE LED DrIVEr POwEr CONVErSION TOPOLOGy GuIDE

Boost-Buck Using a Boost Converter
Some step-up (or boost) LED drivers may be configured to return the LED cathode to the supply. This configuration is referred to as buck-boost. The total output voltage is VIN (VBATTERY), which is added to the total LED string voltage. The benefit 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.

Buck Mode Using a Boost Converter
Some step-up (or boost) LED drivers may be configured to step-down from the supply (rather than ground referenced, as in a standard buck)--creating a buck-mode configuration. This configuration has the same limitations as a buck, where the total LED string voltage must be less than the input supply.
VIN +

Figure 5. Boost-buck converter.

Table 3. Advantages and Trade-Offs of Using BoostBuck Converters as LED Drivers

Benefits to Boost-Buck LED Drivers

Trade-Offs to Boost-Buck LED Drivers

Applications

Direct battery to LED conversion
LED voltage may be higher or lower than supply
Good EMI performance

Lower efficiency
Lower bandwidth (<1/20 of fSW)
Higher inductor current rating

High beam/low beam Turn signal
Daytime running lights

May use matrix to short entire string

Cannot short output to GND

Multiple strings on the same output

Figure 7. Buck-mode converter.

Table 4. Advantages and Trade-Offs of Using BuckMode 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
May use the same driver for multiple applications

Preboost regulator required in most automotive systems
Cannot short output (LED cathode) to GND

Turn signal Daytime running lights

VIN 6 V to 24 V

L1 22 µH

33 nF

C1 33 µF 50 V

C2 10 µF C3 50 V 1 µF
50 V

1 µF

2.2 µF

100 k

VIN

EN/UVLO

OVLO

BST VOUT GND VOUT

C11 0.1 µF 50 V
C10 0.1 µF 50 V
R3 249 k

VREF LT8386

GND

R6 26.1 k

CTRL SYNC/SPRD PWM INTVCC
FAULT SS RT

ISP
ISN
PWMTG ISMON RP VC

C7 10 nF

R2
249 k 400 kHz

R8 14.7 k
C8 10 nF

C12 10 µF 50 V

R4 20 k

R5 20 k

C9 10 µF, 3×, 50 V

R7 0.2
M1
18 V 500 mA LED

Figure 6. Boost-buck converter: LT8386.

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VIN 24 V to 80 V

1 µF 1 M 2×

VIN

ISP

SHDN/UVLO

61.9 k

VREF

ISN

CTRL

INTVCC

PWM

PWMOUT

LT3756-2

100 k OPEN LED

GATE

0.1 µF

SENSE

GND INTVCC

28.7 k 375 kHz

47 k 0.001 µF

C2 4.7 µF

1.5 k 0.1

200 k 1 A
Q2

200 k 200 k

C3 4.7 µF 5× 25 V

Q1 1 k

20 k

5 White LEDs 20 W

L1 33 µH
D1
M1
0.033

VIN
C4 4.7 µF

Figure 8. Buck-mode example: LT3756-2.
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 flexible. 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 BuckBoost Converters as LED Drivers

Benefits to Buck-Boost LED Trade-Offs to Buck-Boost

Drivers

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 configuration allows the lighting designer to create complete subsystems for the different lighting requirements throughout an automobile. Selecting the correct power conversion switching topology and configuration for the system optimizes requirements such as complexity, efficiency, EMI, and safety.

4 AuTOMOTIVE LED DrIVEr POwEr CONVErSION TOPOLOGy GuIDE

VIN 6 V to 55 V

499 k

+ 33 µF 63 V

VIN

EN/UVLO

BST1

221 k 100 k

INTVCC 4.7 µF

TG1

SW1

FAULT

LSP

VREF

LSN

0.47 µF 100 k
Analog DIM

CTRL2

BG1

LT8391

CTRL1

GND

PWM DIM
100 k 400 kHz

2.2 k 10 nF

PWM RP SYNC/SPRD RT VC SS
0.1 µF

BG2 SW2 TG2 BST2
FB VOUT ISP ISN

PWMTG

Figure 10. Buck-boost example: LT8391.

4.7 µF 100 V 2×

10 µF 50 V
2×

0.1 µF 0.1 µF
0.004 10 µH

5.1

1 M 34.8 k

0.05

25 V 2 A LED

About the Author
Josh Caldwell was with Linear Technology (now part of Analog Devices) for 10 years as a design engineering section leader responsible for the definition, 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.

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