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VISIT ANALOG.COM
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
dicult and complex task. This article aims to simplify the selection process
by explaining the benets, trade-offs, and applications for different switching
topologies used for LED drivers.
LEDs are unlike traditional electrical light producing lament or gas components.
Utilizing specic semiconductor junctions, LED manufacturers can produce
specic 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 eciency 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.
V
IN
– +
Figure 1. Buck converter.
Several critical features to look for in step-down LED drivers are xed frequency
operation, high eciency 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
100 kΩ
100 kΩ
10 µF
COUT
100 µF
VIN
EN/UVLO
VREF
CTRL
PWM
SYNC/SPRD
INTVCC
SS
RT
RP
VC
ISP
ISN
FB
VOUT
SW
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
V
IN
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
RT
GATE
SENSE
GND
INTVCC
VC
FB
PWMTG
ISN
ISP
LT8356-1
VREF
100 kΩ
NTC
15 mΩ
ILIM = 7 A (TYP)
FAULT
FAULT
VISIT ANALOG.COM 3
Boost-Buck Using a Boost Converter
Some step-up (or boost) LED drivers may be congured to return the LED cathode
to the supply. This conguration is referred to as buck-boost. The total output
voltage is VIN (VBATTERY), which is added to the total LED string voltage. The benet
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 congured to step-down from the
supply (rather than ground referenced, as in a standard buck)—creating a
buck-mode conguration. This conguration has the same limitations as a buck,
where the total LED string voltage must be less than the input supply.
V
IN
–
+
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
VIN
6 V to 24 V
EN/UVLO
OVLO
INTVCC
BSTSW
VOUT
VOUT
FB
RP VCRT
ISP
GND
GND
ISN
PWMTG
ISMON
LT8386
R8
14.7 kΩ
C3
1 µF
50 V
C8
10 nF
L1
22 µH
M1
R7
0.2 Ω
C11
0.1 µF
50 V
C10
0.1 µF
50 V
18 V
500 mA
LED
SS
R2
249 kΩ
400 kHz
C7
10 nF
SYNC/SPRD
VREF
PWM
CTRL
R1
100 kΩ
C2
10 µF
50 V
C1
33 µF
50 V
R6
26.1 kΩ
R3
249 kΩ R4
20 kΩ R5
20 kΩ
Q1
C12
10 µF
50 V
1 µF
33 nF
2.2 µF
C9
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 conguration allows the lighting designer to create complete
subsystems for the different lighting requirements throughout an automobile.
Selecting the correct power conversion switching topology and conguration
for the system optimizes requirements such as complexity, eciency, EMI,
and safety.
Figure 8. Buck-mode example: LT3756-2.
VIN
LT3756-2
GNDVC INTVCC
SHDN/UVLO
FB
VREF
VIN
INTVCC
ISP
0.001 µF
0.1 µF
V
IN
24 V to 80 V
28.7 kΩ
375 kHz 47 kΩ
100 kΩ
C2
4.7 µF
CTRL
0.033 Ω
0.1 Ω
M1
C3
4.7 µF
5×
25 V
C4
4.7 µF
L1
33 µH D1
1 A
OPEN LED
PWM
SS
RT
ISN
GATE
PWMOUT
SENSE
1 MΩ
61.9 kΩ
C1
1 µF
2×
200 kΩ
20 kΩ
5 White LEDs
20 W
200 kΩ 200 kΩ
1 kΩ
1.5 kΩ
Q1
Q2
M2
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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 denition, 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
FB
VOUT
ISP
ISN
PWMTG
SYNC/SPRD
RP
RT
VC
SS
25 V
2 A
LED
PWM DIM
Analog DIM
VIN
6 V to 55 V
+
FAULT
FAULT
/