Trends and Topologies
for Automotive Rear
Lighting Systems
Arun T. Vemuri
General manager
Automotive Body Electronics and Lighting
Texas Instruments

In the early 1900s, driving at night
was precarious.
Lit by kerosene lamps, drivers announced their maneuvers with hand signals andshouts while employing guesswork to predict the actions of the other cars on the road.
With fewer cars moving at slower speeds andAs a result of modern rear-lighting solutions,dodging the occasional horse and wagon, they got

the actions of other drivers are becoming moreby alright, but we’ve come a long way:

predictable in all environments and making•

Advancements from kerosene lamps todriving safer.

Incandescent bulbs to light-emitting diodes

In this white paper, we will focus on electrical(LEDs) and organic LEDs (OLEDs) havechallenges and investigate possible solutions toprovided more reliable, efficient light sources.

overcome them. These challenges include:

The number of light sources installed in a•

Higher power demand.

vehicle for specific light functions has also•

Thermal management.

•

Electromagnetic interference (EMI) compatibility.

•

Fault detection and protection.

•

increased, from a single bulb to multiple
pixelated designs.
•

Variable lighting has allowed drivers to better
communicate their intentions, making driving
safer overall.

•

Exterior lighting serves function and form forstyling and personalization purposes.

•

Electronic lighting control modules enable
signaling functions as required by law, butalso enable dynamic rather than static signal
functions, including personalized welcome
messages for drivers, such as a logo on thepuddle light.

•

As rear-lighting systems become more complex,
engineers from the optical, mechanical,
electrical and manufacturing disciplines all face
new challenges when designing new systems.

Trends and Topologies for Automotive Rear Lighting Systems

2

October 2019

Center high mounted stop light (CHMSL)
Signals that the driver is slowing down. The CHMSL turns on at
the same time as the brake light and is installed in all vehicles.
Brake light

Parking light

Signals that the driver

Signals that the vehicle

is slowing down.

is parked. This light

Brake lights are redfunction is optional and not

and are installed

commonly installed.

in all vehicles.
Turn light
Signals that the driverintends to either turn left orSide-marker lightright. While other rear-light

Indicates the vehicle’s

signaling functions remaincontours and turns

on continuously, the turnwhile serving as

light blinks on and off at aanother tail light.

certain frequency, typically
60 times a minute.

Tail light

Rear fog light

Signals the presence of

The red light is brighter

a vehicle, especially in

License-plate lightthan the tail light anddark conditions. The tailIlluminates the registration

signals the vehicle’s

light is red and installed

plate on the rear side ofpresence in poor-visibility

in all vehicles.

the vehicle.

conditions such as fog.

Figure 1. Components of a rear-lighting system.

Rear lighting on the road

As illustrated in Figure 1, there are a variety ofnecessary rear-signaling functions for automobiles,

Before discussing LEDs and the electricalall of which require a control module. Figure 2 is

topologies, let’s first look at the different signalinga block diagram of a typical rear-light control

functions legally mandated by regulatory agencies

module, showing all rear-light functions as well assuch as the U.S. National Highway Traffic Safety

the power supply, communication interface andAdministration and Europe’s Economic Commission,

LED-driver subsystems.

as shown in Figure 1.

Trends and Topologies for Automotive Rear Lighting Systems

3

October 2019

LED Board

LED Driving Module

Car
Battery
Reverse Voltage
Protection

Voltage Regulator

Constant Current
Regulator

LED Power Supply
LED Driver

Tail
Brake
Reverse
Fog
Turn
Side

Voltage Regulator
Watchdog
Voltage Supervisor
Power Supply
Temp Sensor
Body
Control
ModuleCAN / LIN

Thermal Protection

CAN / LIN ESD
Communication Interface
MCU
OP AmpComparator
Reference
Diagnostic Inputs

MCU

Reverse Voltage
Protection

Constant Current
Regulator

CHMSL

LED Driver

Reverse Voltage
Protection

Constant Current
Regulator

License plate

LED Driver

Figure 2. Block diagram of a rear light control module.

Trends and Topologies for Automotive Rear Lighting Systems

4

October 2019

The basics of LED in rear lighting

Typically, an LED source is six times more efficientthan an incandescent bulb (Figure 4).

An LED is a P-N junction diode that emits lightwhenactivated. Like any P-N junction diode, anElectrical Use by Bulb TypeLED is a unidirectional device with an exponential

140

Electrical Consumption (W)forward-current-forward-voltage (I-V) curve, as
shown in Figure 3.
Forward Current (lf)versus Forward Voltage (Vf)IR, Red, Green, Yellow andWhite Light Emitting Diodes

IR

Red

Yellow

25

White

Incandescent
Halogen
CFL
LED

120

100

80

60

40

Green

20

20
0

lf (mA)

0

500

1000

1500

2000

2500

Initial Luminous Flux (lm)15

Figure 4. Efficiency of an LED light source vs. an incandescentbulb. (Source: https://commons.wikimedia.org/wiki/
File: Electricity_use_by_lightbulb_type.svg)

10

5

0

Not only are LEDs more efficient than traditional
0

1

2

3

incandescent light bulbs, but they also last nearly 42

4

Vf (V)

times longer and enable easy adoption for different

Figure 3. An LED’s I-V curve. (Source:
http://lednique.com/current-voltage-relationships/iv-curves/)

stylistic designs. Traditional incandescent lamps
require replacement every couple of years; their
lifetime is usually around 1,200 hours. The lifetime

When a positive voltage is applied between theof LEDs extend to more than 50,000 hours, so an

LED’s anode and cathode terminals, current flows

automotive LED lamp could last the vehicle’s entire

from anode to cathode, causing the LED to emit

lifetime without needing replacement.

light. The voltage applied between anode andcathode is referred to as the LED forward voltage,

Transition from dot-type tosurface-typewhich is usually measured at a nominal current
condition. The higher the forward current though the

Traditional incandescent lamps have a fixed form

LED, the brighter the LED light and the higher thefactor because of their round housing. In contrast,

LED forward voltage.

LED lighting sources have small form factors, which

LEDs with different colors have different forward

means that automotive lamps no longer have to bevoltages. The forward voltage of most red andthe traditional round shape. By using multiple LEDs,

amber LEDs is around 2 V, while the forward voltage

the lamps can be designed with more flexibility to fitof white LEDs is around 3 V.

different physical design requirements; however, avisual effect of using LEDs in automotive rear lights

Benefits of LED lighting sources

is that the LEDs appear as dots.

A key benefit of LEDs compared to incandescentBecause both the LED light output and the direction

bulbs is that an LED is a more efficient light source.

of light are easily controllable, the latest automotive

The efficiency of a light source is measured inrear lamps use multiple optical methods to transition

lumens per watt, which indicates the amount of lightfrom dot-type lighting sources to surface-type

emitted per unit of power consumed.
Trends and Topologies for Automotive Rear Lighting Systems

lighting sources.
5

October 2019

Using light guides and diffusers are the mostLamp functions must be able to withstand any

common methods to accomplish this transition, butdamage and recover when the voltage returns

they decrease optical efficiency, necessitating an

to normal.

increase in LED brightness, either by using higher-

During load dump, the battery is disconnected whilebrightness LEDs or more of them.

the alternator is generating charging current, with

Design targets and challenges

other loads remaining on the alternator circuit. In

While LEDs have many merits, there are alsohigh as 36 V for 400 ms in the case of suppressed

this scenario, the supply voltage can shoot as

challenges in driving them to fit automotive

alternator scenarios. In the event of a load dump,

specifications. A typical list of automotive LED

LED drivers must be able to recover when thedriver requirements includes:•

voltage returns to normal.

A wide voltage range, with 9- to 16-V, 24-V
and 40-V scenarios.

Thermal considerations

An operating ambient temperature range

Automotive applications need to withstand a wideof –40°C to 85°C.

temperature range. Automotive lamp circuitry needs•

Animating a lamp feature.

to work at up to an 85°C ambient temperature. This•

LED fault diagnostics.

maximum temperature includes the temperature rise•Electromagnetic compatibility (EMC).

•

in the housing caused by self-heating within
an enclosed lamp.

Automotive wide-voltage ranges

High ambient temperatures present two challenges

An LED driver design must be able to operate

in LED rear lamps: controlling the junction

within the wide-voltage range of automotive

temperature of the LED and the LED driver.

12-V batteries. The International Organization

An LED’s lifetime will drop significantly if its junction

for Standardization (ISO) 7637 and ISO 16750

temperature exceeds its upper limit. If the LED

standards present the most common requirements

driver is a constant-current driver, Equation 1 givesin the automotive industry for 12-V battery voltage

an approximate estimation of the temperaturetransients that LED drivers must withstand.

rise on the LED’s junction temperature, based on

An automotive car battery typically ranges fromthe ambient temperature, thermal resistance and

9 to 16 V in normal operation. In this range, theamount of power dissipation:light output must meet the required regulatory

TJUNCTION = TAMBIENT + θ ja P	requirements under all temperature conditions.

(1)More recently, certain lamp functions such aswhere θ ja is the package thermal resistance andturn indicator is operated when the automotive

P is the power dissipated.

battery is at 6 V, which is the typical voltage during

Thermal considerations should also address LED

start-stop. Note that the starting voltage profile fordrivers. Since constant-current drivers used in rearstart-stop may vary with battery condition as welllamps are mostly linear LED driver integrated circuits

as temperature. It is usually not necessary for the(ICs), Equation 2 estimates the power dissipated inrear lamp to remain functional when the supply

an LED driver IC using the voltage drop across thevoltage is below 6 V. On the higher side, the battery

driver times the total current:

voltage could surge to 24 V for one minute at roomP = ∆V * Itemperature in jump-start scenarios.
Trends and Topologies for Automotive Rear Lighting Systems

6(2)

October 2019

where ∆V is the voltage across the LED driver andLinear LED drivers such as TPS92630-Q1 or

I is the LED-forward current.

TPS92638-Q1 from Texas Instruments meet thesepower dissipation requirements.

When the input voltage is at a normal maximum
operating voltage, such as 16 V, and the output LED

Animation trends

voltage is minimal, such as two LEDs with 1.9-VAnimation in rear lights enables style flexibility as wellforward voltages each, the maximum ambientas personalization such as a welcome message.

temperature would be 85°C. A typical linear

As new lamp designs adopt complex animation,

LED driver would need a maximum 2 W of power

rear-lamp designs are transitioning from controllingdissipation over the device in order to fit automotive

strings of LEDs to driving LED pixels independently,

applications. Using Equations 1 and 2, youwhich can be accomplished using a device suchcan estimate the maximum LED current

as TPS929120-Q1. Figure 5 displays the transition

using the θ ja.

Vinfrom string LED control to independent pixel control.

Reverse Battery
Protection

Wide Input Voltage
Linear LED Driver

ILED

Single LED Driver Architecture
String 1

Vin

String N

ILED

Reverse Battery
Protection

LED Pixel 1

.
.
.
.
.

Single LED Pixel
DriverILED
LED Pixel M
Single LED Pixel Driver Architecture

Figure 5. Controlling strings of LEDs with one LED driver vs. controlling each LED independently.

Trends and Topologies for Automotive Rear Lighting Systems

7

October 2019

Single-pixel control architectures require a more

lamp when the lamp design runs across the entire

sophisticated LED driving and control methodology.

rear of the car. Pixelated LED control may require

While most existing rear lamps either use

more complex digital interface architectures, as

microcontroller general-purpose input/outputs or

shown in Figure 6. The communication interfaces ina simple interface such as the Serial Peripheral

these systems have to comply with EMC standards

Interface or I C, these simple interface architectures

while introducing no communication errors.

2

may not be able to satisfy the needs of a larger rear

Light Control Module

Car
Battery

Reverse Voltage
Protection

LED Driving Modules

Voltage
Regulator
LED Power Supply

Voltage Regulator

Constant Current
Regulator

Watchdog

LED Driver

Voltage Supervisor
Power Supply.........
CAN

MCU
Body
Control
ModuleLED Driver

LDM Communication
Transceiver

CAN / LIN

LDM Communication
Transceiver

CAN / LIN ESD
MCUCommunication
Interface

LDM_1

LDM_N

CAN
LCM Communication
Transceiver

Figure 6. A digital interface LED driving module architecture for pixelated rear lights.

A note about DLP® technology for rear lighting
Texas Instruments DLP® technology is already being used in headlights to provide high-resolution adaptive headlight solutions.
DLP products for automotive lighting solutions can also provide animation for rear lighting while enabling the vehicle tocommunicate with the driver and with other vehicles, as well as with pedestrians.
The intent of rear lighting is communication and visibility; dynamic light control functions projected down onto the pavement
provide a new, important layer of communication and visibility. Dynamic lighting control becomes especially important
as vehicle intelligence becomes more sophisticated. Visual communication from vehicles will be crucial for illustrating
autonomous functions to drivers and pedestrians. For example, a car can project its path backwards and its eventual parking
position as it autonomously parallel parks, communicating its intentions to both passing cyclists and approaching vehicles forsafer interaction.

Trends and Topologies for Automotive Rear Lighting Systems

8

October 2019

LED fault diagnostics

EMC

It is mandatory that an automotive lamp fulfill its

Many vehicles today reuse the rear windowlighting-output regulations during its lifetime.

defogger as an antenna, while others use a

However, LEDs are also subject to failure. Althoughseparate antenna over the roof. In order to avoid

the lifetime expectation of an LED is much longer

interfering with the antenna, EMC requirements arethan the maximum operation hours of a vehicle, LEDs

exceptionally stringent for automotive rear lamps.

are still vulnerable to random failures. As many as

Thus, LED drivers must have low emissions and

80% of random LED failures are open-circuit failures,

high immunity. Using linear LED drivers in rear

while less than 20% are short-circuit failures.

lights simplifies rear lighting designs for EMC. The

It’s easy to detect a traditional incandescent bulb-

rear-lamp system is often tested against emission
standards such as Comité International Spécial

based lamp failure because a failed bulb will

des Perturbations Radioélectriques (CISPR) 25 andcompletely turn off. But because an LED-based

immunity standards such as the ISO 11452-5 bulk

lamp uses multiple LEDs in multiple strings, a single
LED open-circuit fault or short circuit may not be

current injection standard.

easily detectable because the rest of the LEDs will

LED driver topologies

continue operating.

Rear light LED driver topologies are either single

While detecting an LED open circuit in a single

or dual stage. The single-stage topology shown instring of LEDs is relatively straightforward, detecting

Figure 7 uses a single linear LED driver, while thea single LED short circuit may not be as easy todual-stage topology shown in Figure 8 uses

identify. Consider a string of three LEDs. A simple

a voltage regulator followed by an LED driver.

LED driver will not be able to distinguish a threeLED string from a two-LED string when a single
LED short circuit occurs. Detecting such faults isVinimperative in an animated turn indicator because

Reverse Battery
Protection

Wide Input Voltage
Linear LED Driver

ILED1

regulations require that the first 200 ms of light
output meet a certain brightness level. If one LED
Wide Input Voltage
Linear LED Driver

short-circuits, the lamp will not be able to meet the

ILED2brightness requirements, therefore, sophisticatedfault-detection techniques are required to achieve
one-fail-all-fail. In other words, a single LED failure turns

Single stage LED driver

off the entire lamp.

Vin

Figure 7. Single-stage LED driver.

Wide Input Voltage
Linear LED Driver

Reverse Battery
Protection

EMC Filter

ILED1

Wide Input Voltage
Boost Controller

SEPIC

Wide Input Voltage
Linear LED Driver

ILED2

SEPIC Power Architecture

Figure 8. Dual-stage LED driver.
Trends and Topologies for Automotive Rear Lighting Systems

9

October 2019

One possible circuit topology for a linear LED driverin junction temperature (see Equation 1), thus

is a discrete circuit with a resistor and a transistor,

preventing damage to the LED.

as shown in Figure 9. Equation 3 calculates the

While rear lights commonly use linear LED

current in the LED as:

drivers, high-current applications sometimes use
a single-stage switching LED driver. In such an

ILED = Vin –Vf (LED)
R		

(3)

implementation, EMC challenges increase, requiring
innovative techniques such as spread spectrum toreduce emissions.

While this circuit is simple to implement, it has many
weaknesses. The LED current is not constant,Dimming the lightgiven variations in the LED’s I-V characteristics

For scenarios that require dimming of the tailand temperature. Plus, the circuit does not havelight and brake light, LED designers use two

diagnostics. Finally, managing thermals in the circuit

methods: analog dimming and digital dimming.

requires the implementation of parallel resistors

Analog dimming reduces the current in the LED,and transistors to divide the power dissipation

which in turn decreases the light output, resulting

among the components and thus prevent a single

in dimming. Digital dimming (also called pulse-

component from failing due to excessive heat.

width modulation [PWM] dimming) employs PWM
to modulate the current between the LED driverVinoutput current and zero current. Thus, the average
light output reduces, which once again results in adimmed output.

R

ILED

Analog dimming is used as a dimming method anda way to improve design homogeneity by calibratingLED

the current in each LED to achieve uniform luminosity.
However, because LEDs are dimmed at a nominal

Transistor

current, reducing the driving current increases theluminosity differences between LEDs, leading tohomogeneity concerns. Current accuracy at low

Figure 9. Discrete LED driver.

currents is crucial for homogeneity designs. Most
analog dimming ratios are limited to below 20-to-1.

In contrast to discrete LED drivers, LED driver

For digital dimming, PWM is precise even at

ICs offer many advantages. Driver ICs not only

low duty cycles. The luminosity mismatches are

generate a constant current regardless of the input

almost negligible; thus, it is suitable for high-voltage, but also implement diagnostics inside toprecision dimming. Digital dimming requires a PWM

achieve one-fail-all-fail. Driver ICs also implement

generator, either discretely with a 555 timer orthermal foldback features that reduce the current

through microcontrollers. In animated lamps, digitalin the LEDs if the temperature of the LED exceeds

dimming is more common, since it is easy tocertain limit. Reducing current in the LEDs reduces

control with firmware.

the power dissipation, which decelerates the rise

Trends and Topologies for Automotive Rear Lighting Systems

10

October 2019

Rear lighting in the future
Automotive rear lighting systems have evolved
significantly to accommodate for greater signaling,stylization and personalization needs in the

If a car is independently operating through radar,
LIDAR, cameras and vehicle-to-everything (V2X)
technology without human intervention, one mightassume lighting is just a drain on the battery.

marketplace. These changes have led to more

But as discussed in this white paper, lighting

complex systems that require an LED driver andappeals to aesthetics as well as serving signaling

present a range of electrical design challenges tofunctions. Consumers have grown accustomed

engineers, including:to the stylistic design of lighting and future
customization will increase the appeal. Also, in•Higher power demand.

•

Thermal management.

•

EMI compatibility.

traditional vehicles as well as pedestrians.

•

Fault detection and protection.

My estimation is that rear lighting is here to stay.

the near future, automotive lighting will be crucial
for signaling in traffic mixed with automated andLights out?

Related websites

As the automotive industry moves toward greater•

Exterior rear light interactive block diagram

autonomy, some may ask: will autonomous vehicles•

TI’s automotive LED driver solutions•

DLP® Products automotive chipsetsneed exterior lighting? It’s an interesting question.

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