Advanced Automotive LED Lighting Solutions:

Challenges and Solutions

2

Agenda

• LED Applications in Automotive• Advantages of LED Lighting in Automotive• Technical Requirement & Challenge of Powering

LEDs in Automotive• LED Driving Techniques• Buck - Boost LED Drivers for HB LED Exterior Lamp• Boost LED Drivers for Backlighting in Automotive

Display

LED Applications inAutomotive

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Traditional Approach

• Traditionally, Incandescent light bulbs of 21 to 27 W, producing 280 to 570 lm are used for brake, turn, reversing and rear fog lamps.

• Incandescent bulbs of 4 to 10 W, producing 40 to 130 lm are used for tail lamps, parking lamps, sidemarker lamps and side turn signal repeaters.

• Xenon HID (High Intensity Discharge) tubes are used for Headlamps.

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LED in Various Types of Exterior Automotive Lighting Applications

• The initial LED application in Automotive is Centre High Mount Stop Lamp (CHMSL).

• Other exterior lighting, including brake lamp, turn signal lamp, and interior lighting have already adopted LEDs in the market.

• Recent emerging exterior LED applications also include Daylight Running Lamp (DRL) and low beam front lamp.

• High beam front lamp will appear to use HB LEDs in the near future.

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• LED Backlighting for dashboard

• LED Backlighting for TFT display

LED for Backlighting in Automotive Applications

Advantages of LED Lightingin Automotive

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Advantages of LED Lighting in Vehicles (I)

• With Color Temp of approx. 6,000 Kelvin in White LEDs, this is nearly equal to the quality of daylight.– The human eye perceives the road and the edge of the lane

at night in their natural colors and differentiates contrasts considerably better.

• The average lifetime of LEDs is longer than that of Incandescent light bulbs & HID Lamps. – High reliability – increases the resistance to failure at night– eliminates the need for complicated bulb changes in the

future.

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Advantages of LED Lighting in Vehicles (II)

• Potential reduction in energy consumption of LED– More environmentally friendly

• Smaller Lighting Fixtures with LEDs light source– space savings & easy styling

• Faster turn-on of the LEDs stop lamps– reduce the risk of rear-end collisions– Around 250 ms quicker than incandescent bulbs

• Solid State Light Source– More resistance to mechanical shock

Technical Requirement & Challenge of Driving LEDs in

Automotives

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• Light Output of LED mainly depends on its operating current level.

• The LED driving circuit requires to– deliver a constant average current under all conditions (e.g.

input voltage change, temperature change, VF change……), &

– control ripple current at acceptable level under all conditions.

Technical Requirement of driving LEDs in Automotive -- Constant Current Regulation

A LED driving circuit is a type of power conversion circuit but it delivers constant current instead of constant voltage!!

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• Require to add blocking element (such as Diode) for Reverse Battery Protection

• Avoid to damage the LED driver or/and create short circuit path from battery if any accidental reverse connection

Technical Requirement of driving LEDs in Automotive -- Reverse Battery Protection

Battery

+Vin

-

LED Driver

+

GND

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Technical Challenge of driving LEDs in Automotive -- Cold Crank & Load Dump

• Under normal operation of Vehicles, voltage at supply lines (battery) is between 9V and 16V (12V system bus) for passenger car, or between 18V and 32V (24V system bus) for Trucks.

• However, the input range varies significantly during load dump & cold crank condition

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Load Dump

• Load Dump occurs when battery disconnected while engine is still running– Energy stored in the alternator causes a voltage spike

• Various Automotive Standard give somewhat different definitions of load dump test:

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Central Load Dump Protection

Most modern alternators are equipped with centralized load dump clamp

Clamping at typically 35-42V for 12V Bus System

Clamping at typically 50-60V for 24V Bus System

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12v

t/S

5-9.5v3-8v

5 ms 15 ms 50 ms 1-10s 100 ms

UB

Cold Crank

The cold temperature ambient conditions aggravate the supply voltage drop from the battery rail as the starter is activated.Here shows the typical “cold cranking” test wave shape:

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• As the input supply of LED driver are connected to the battery input, exterior safety lighting (for example: Brake Lamps) should be able to survive or/and even work properly during the “load dump” & “cold crank” conditions.

– The automotive LED drivers are required to operate from a wide input supply range

Typically,

– 12V bus system input range: 6V – 42V– 24V bus system input range: 12V – 60V

Technical Challenges of driving LEDs in Automotive-- Load Dump & Cold Crank

LED Driving Techniques

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Resistor Limiting

.

12V

Resistor Limiting

Advantages:• Inexpensive & simple

Disadvantages:• Current varies as VF & Vin changes

Poor current regulation• Poor efficiency• Excessive high generation at

limiting resistor

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Linear Constant Current Regulator

Linear IC withConstant Current Source

.

12V

Advantages:• Simple• Good current regulation

Disadvantages:• Poor efficiency• Excessive high generation at

linear IC

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LED Driving Methods – Pros & Cons

• Relatively expensive• Circuit complexity

High efficiencyConstant Current Regulation

Switching Regulation

• Poor efficiency• Heat dissipation at driver IC

SimpleConstant Current Regulation

Linear Regulation

• Current varies as Vin & VF changes• Poor efficiency• Heat generation at resistor

InexpensiveSimple

Resistor LimitingConsProsDriving Method

Resistor limiting & constant current linear regulator is impractical for driving HB LEDs due to excessive heat. As switching power converters are needed for exterior/interior lighting LED devicesbecause of highly efficiency.

Buck – Boost LED Drivers for High Brightness

Automotive Lighting

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Why employs Buck-Boost LED Driver for HB automotive lighting?

• The buck-boost LED driver able to work either input voltage is lower or higher than output voltage (total forward of LED arrays at the output)

• Certain safety lighting in automotive must able to function properly during both “Cold-Crank” & “Load Dump” condition.

• Large variety of LED fixtures in Automotivescalls for various types of LED driver topologiesSome lighting manufacturers would like to use single LED driver platform for different LED configurations in different lighting applications

• Buck-boost LED driver offers attractive solution for most of HB automotive lighting applications!!

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VIN

EN

RCT

VCC

LM3423

CSH

HSN

nDIM

VIN

GATE

IS

COMP

DDRV

Enable

PGND

HSP

AGND

RCOMPCCOMP

RT

CT

RCSHEP

CIN QSW

DPOL

TIMRCTIMR

FLT

RPD

LRDY

PWM/UVLO

OVP

CBYP

COUT

RSENSE

QPWM

RHSP

RHSN

VIN

100k

10k

QDIM

VCC

Vin range: 4.5V to 75Vable to operate within

very wide input range

Low side MOFET driver with High side current sense

able to configure boost, floating buck & floating buck-boost approach

Vin_min<N*Vf<Vin_max

Programmable switching frequency up to 2 MHz

Dimming & Precision UVLO Dual function pin

This application diagram shows the buck-boost topology, which is the most attractive solution for HB automotive lighting!!

LM3423 Floating Buck-Boost Application with Fast Dimming, Robust Protection & Fault Indication

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VIN

EN

RCT

VCC

LM3423

CSH

HSN

nDIM

VIN

GATE

IS

COMP

DDRV

Enable

PGND

HSP

AGND

RCOMPCCOMP

RT

CT

RCSHEP

CIN QSW

DPOL

TIMRCTIMR

FLT

RPD

LRDY

PWM/UVLO

OVP

CBYP

COUT

RSENSE

QPWM

RHSP

RHSN

VIN

100k

10k

QDIM

VCC

LM3423 Floating Buck-Boost Application with Fast Dimming, Robust Protection & Fault Indication

Provide Robust Protection & Fault Indication, which is very important in High Reliability System!!

Precision Input ULVO

Precision Input OVLO

Allow to set very accurate operating input window!!

Latch & Cut off the input path if any Over-Current, Short Circuit, OVP, or Over-Temperature Occurs

Send the Fault Signal as LED current not within +30/-20% regulation Band

V_A

V_A

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Vin range: 4.5V to 75Vable to operate within very

wide input range

Vin_min<N*Vf<Vin_max

Programmable switching frequency up to 2MHz with frequency synchronization by external clock

Dimming & Precision UVLO Dual function pin

LM3424 Floating Buck-Boost Application with Fast Dimming, Thermal Foldback & Frequency Synchronization

Output OVP Protection

Thermal Foldback/Analog Dimming!!

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Thermal Foldback Concept

One of major design exercise in all High Brightness LED solid state lighting applications Thermal Cooling of the System!

Concept of Thermal Feedback:

Under abnormal ambient/condition, the LED average current level reduces gradually versus LED case temp after hit the preset threshold

Advantage: avoid damage the LED module during abnormal condition, while still maintaining certain brightness in the System!

Important in highly reliable / safety-related Automotive Lighting System!!

Temperature

LED

Cur

rent

INOM

TCORNER

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Thermal Foldback Implementation

+-

-+

R1

Vcc

RGAINRNTC

RMIN

Temperature increases, RNTC resistance

decreases

VREF

VSNSRTRIM

IGM

VCLAMP

To LED Foldback ckt

TRIMMINNTC

MINNTCSNS RRR

RRV++

+=

Region 3

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Thermal Foldback/Analog Dim with LM3424

As ITF = 0, i.e. No thermal feedback

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LM3424 ─ Frequency Synchronization

All LM3424 drivers run at the same frequency which can reduce the EMI, or simplifies the EMI design in the system!

10 MHz RC filter to eliminate unwanted high frequency noise from coupling into the RT pin

Boost LED Drivers forLED Backlighting

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Boost LED Driver for LED Backlighting

• Step up (Boost) constant current LED driver is typically used for driving long strings of LEDs in instrumental LED backlights, for example display of dash board, navigation systems..– Advantage of boost LED driver To maximize the number of

LEDs per string

• In automotive displays , Dimming Control should be required to control the brightness of LED backlights.

– High Brightness in light ambient– Low Brightness in dark ambient, to avoid blink driver’s eyes

for safety purpose

• → Need PWM Dimming Control with high linearity of light intensity, even at low PWM duty ratio.

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PWM Dimming Control

• PWM signal (EN/SD pin, FET, or special PWM pin)– “Strobing” LEDs at frequencies >100 Hz results in reduced

brightness, as perceived by the eye. Eye filters/averages brightness

– “Average” brightness proportional to Duty Cycle (D): D = tON ÷ T

Analog Brightness Control – alter the LED continuous current

Not preferred because color tone will change

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LM3423 Boost Application with Fast Dimming & Zero Current Shutdown

Vin range: 4.5V to 75Vable to operate within very

wide input range

Low side MOFET driver with High side current sense

able to configure boost, floating buck & floating buck-boost approach

N*Vf>Vin_maxMax N*Vf = or < 75V

Can be Configure as Input OVLO, or Output OVP protectionInput OVLO prevent Vin > Vo to ensure boost mode operationOutput OVP protection avoid any damage of LEDs as Vo > n*Vf during line transient

LM3423 Boost LED driver is a popular solution for:

- LED backlighting in Automotive Navigation Display/Dashboard

Up to 20 pcsof LED!!

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LM3423 Boost Application with Fast Dimming & Zero Current Shutdown

Allow Fast PWM Dimming for Brightness Control!!

nDIM feature Dual functions, input UVLO & PWM dimming

As the PWM Dimming signal is applied, the DDRV pin drive the DIM N-FET to chop LED string on & off for Brightness Control

The Dimming Frequency can up to 50 kHz!!

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LM3423 – Dimming Performance

Vin =12V, ILED = 100 mA, no of LEDs in series = 10Dimming Frequency: 230 HzDimming Duty Cycle: 0.5%

CH1: Dimming signalCH2: Switch Node of BoostCH3: VoutCH4: ILED

Able to achieve very low dimming ratio @ 230 HzProvide very low light intensity @ very dark ambient in LED

backlighting in automotive display

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Problems with Conventional Boost LED Driver

• Even the main switch is turned off by disabling the Boost Converter, there is a leaking path from Vin to Vout.

• If there is any load at the output, the leakage current path will shorten the battery life.

PWMGate Drive

VIN VOUT

Leakage Current

Leakage current path

Ileak ~ Vin/Rload

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Problems with Conventional Boost LED Driver

• During Output Short Circuit, there is no any output short circuit proof for boost converter, even the main switch is turned off.

• The short circuit current flowing through Vin, via inductor, rectifier, and GND, is only limited by PCB trace resistance, parasitic resistance of rectifier, & DCR resistance of inductor.

• Input fuse must be added to protect over the short-circuit.

PWMGate Drive

VIN VOUT

No Short Circuit Proof

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How LM3423 Boost minimizes shutdown leakage current

As the Boost Converter is disabled by EN pin, Main Switch, DIM FET, FLT pin are switched off.

I_leakage ~ 0

To prevent any leakage current flowing through LEDs string as the boost converter is disabled !!

save battery life

Ileakage~0

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How LM3423 Boost provides LED string shorted proof

When LED string shorted to GND, the input P-FET driving by FLT pin will be switched off, thus the input path becomes open.

Implementation the True LED String Short-Circuit Proof with LM3423 Boost Converter!!

Isc~0

LED shorted to GND

Multi-strings LED Backlight for Car Infotainment

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LM3431 Boost LED Driver Controller for Multiple Channel Applications

5V to 36V VIN Range

Integrated Boost Controller

Up to 25 kHz PWM dimming & 100:1

contrast ratio

Adjustable sw. freq. up to 1 MHz

LED open, short and over-temp

protection

Available in 28 pin LLP and TSSOP

packageMain Application :Automotive Navigation Displays

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LM3431 Typical Application for Driving Multiple Strings of LEDs

LED off regulated by AFB Anode feedback pin, and the AFB voltage is sampled by SH circuit as the LED is on.

LED on regulates the lowest cathode voltage (1.2V CFB feedback voltage) to minimize the headroom & power loss of linear regulator

3 Independent current regulators –Programmable SNS pin voltage

Additional channels can be added

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LM3431 - High Dimming Linearity

• Typical LED current slew rate = 1.2A/usec– Varies with NPN (or NFET) selection– 100nsec typical DIM to NDRV delay– Very linear relationship between %DIM and %LED current

I LED

DIM

NDRV (NPN)

NDRV (NFET)400nsec scale

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LM3431 – Full Featured Fault Protection

• 3 types of faults– Type 1: DLY flag and no

Shutdown– Type 2: DLY flag and Shutdown– Type 3: Shutdown with no DLY

flag (thermal)

• DLY time set by external cap– DLY charges when LEDs are ON– DLY time depends on dimming

duty– DLY is high impedance when

LEDs off– DLY discharges slowly when

fault is removed

• Shutdown occurs when DLY=2.8V– Except type 1 fault

stand by No DLY flagTHM < 1.2VTHM

stand by No DLY flagIC over temperatureTSD 3

stand by No DLY flagVCC or EN lowUVLO

latch off or restartDLY chargesCOMP at maxCurrent limit

latch off or restartDLY chargesCFB low at

startupCathode

short

Latch off or restartDLY chargesAFB < 0.85V

multiple LED short –VIN<6V

latch off or restartDLY chargesSC > 6.0Vmultiple LED

short

2

Latch off or restartDLY chargesAFB > 2.0VAll LEDs

open

continue to regulateDLY chargesSC > 3.1V1 LED short

1

continue to regulateDLY chargesSC > 3.1V1 LED open

typeresultactionmechanismfault

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LM3431 – Thermal Protection

• Thermal Protection

– TSD for LM3431 at 160degC junction temperature

– THM comparator input with hysteresis current to monitor LED temp

– NTC Thermistor mounted at LED array

– LED temp threshold and hysteresis are adjusted with 2 resistors

– Automatic restart occurs after OTP fault is removed

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NS Automotive LED Drivers Portfolio for Exterior Lighting & Infotainment Backlighting

LM3421 features, plus thermal foldback/ analog dimming, & frequency synchronization

LM3424*

Simple Boost/ Sepic, small solution sizeLM3410*

Greater than 100:1 contrast ratioLM3431*InfotainmentBacklighting

LM3421 features, plus zero current shutdown, fault monitoring, & robust fault protection

LM3423*

Features/ BenefitPromoteApplication

Simple buck/boost, with high Vin, adjustable Current Sense, OVP, PWM dimming

LM3429*

LM3429 features, plus fast PWM dimmingLM3421*

Exterior Lighting(Forward Lighting, DTR,

Fog, RCL)

* Q version available

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More Resources

• LM3432– http://www.national.com/pf/LM/LM3432.html

• LM3431– http://www.national.com/pf/LM/LM3431.html

• Webench® LED designer– http://webench.national.com/webench5/power/webench5.cg

i?application=LED_DRIVER