Advanced Automotive LED Lighting Solutions: Challenges and Solutions

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1 Advanced Automotive LED Lighting Solutions: Challenges and Solutions 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 2

2 LED Applications in Automotive 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. 4

3 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. 5 LED for Backlighting in Automotive Applications LED Backlighting for dashboard LED Backlighting for TFT display 6

4 Advantages of LED Lighting in Automotive 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. 8

5 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 9 Technical Requirement & Challenge of Driving LEDs in Automotives

6 Technical Requirement of driving LEDs in Automotive -- Constant Current Regulation 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, V F change ), & control ripple current at acceptable level under all conditions. A LED driving circuit is a type of power conversion circuit but it delivers constant current instead of constant voltage!! 11 Technical Requirement of driving LEDs in Automotive -- Reverse Battery Protection 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 + Battery - LED Driver +Vin GND 12

7 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 13 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: 14

8 Central Load Dump Protection Most modern alternators are equipped with centralized load dump clamp Clamping at typically 35-42V for 12V Bus System 15 Clamping at typically 50-60V for 24V Bus System 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: UB 12v 3-8v 5-9.5v 5 ms 15 ms 50 ms 1-10s 100 ms t/s 16

9 Technical Challenges of driving LEDs in Automotive -- Load Dump & Cold Crank 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 17 LED Driving Techniques

10 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 19 Linear Constant Current Regulator 12V. Advantages: Simple Good current regulation Linear IC with Constant Current Source Disadvantages: Poor efficiency Excessive high generation at linear IC 20

11 LED Driving Methods Pros & Cons Driving Method Resistor Limiting Linear Regulation Switching Regulation Pros Inexpensive Simple Simple Constant Current Regulation High efficiency Constant Current Regulation Cons Current varies as Vin & V F changes Poor efficiency Heat generation at resistor Poor efficiency Heat dissipation at driver IC Relatively expensive Circuit complexity 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 devices because of highly efficiency. 21 Buck Boost LED Drivers for High Brightness Automotive Lighting

12 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 Automotives calls for various types of LED driver topologies Some 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!! 23 LM3423 Floating Buck-Boost Application with Fast Dimming, Robust Protection & Fault Indication Vin range: 4.5V to 75V able to operate within very wide input range VIN Programmable switching frequency up to 2 MHz CIN RT VIN RCT LM3423 IS GATE HSP QSW RHSP COUT 100k RSENSE Low side MOFET driver with High side current sense able to configure boost, floating buck & floating buckboost approach Dimming & Precision UVLO Dual function pin This application diagram shows the buckboost topology, which is the most attractive solution for HB automotive lighting!! PWM/UVLO VCC Enable CT CTIMR CCOMP RCOMP RCSH FLT ndim DPOL EN TIMR COMP CSH HSN VCC DDRV RPD OVP LRDY PGND AGND EP RHSN QDIM QPWM CBYP 10k VIN Vin_min<N*Vf<Vin_max 24

13 LM3423 Floating Buck-Boost Application with Fast Dimming, Robust Protection & Fault Indication V_A VIN CIN Latch & Cut off the input path if any Over-Current, Short Circuit, OVP, or Over- Temperature Occurs RT CT VIN RCT FLT LM3423 IS GATE HSP HSN QSW RHSP RHSN COUT 100k RSENSE QDIM Allow to set very accurate operating input window!! Precision Input ULVO Precision Input OVLO Provide Robust Protection & Fault Indication, which is very important in High Reliability System!! PWM/UVLO VCC Enable CTIMR CCOMP RCOMP RCSH ndim DPOL EN TIMR COMP CSH VCC DDRV RPD OVP LRDY PGND AGND EP CBYP QPWM 10k Send the Fault Signal as LED current not within +30/-20% regulation Band VIN V_A 25 LM3424 Floating Buck-Boost Application with Fast Dimming, Thermal Foldback & Frequency Synchronization Vin range: 4.5V to 75V able to operate within very wide input range Programmable switching frequency up to 2MHz with frequency synchronization by external clock Dimming & Precision UVLO Dual function pin Output OVP Protection Thermal Foldback/ Analog Dimming!! Vin_min<N*Vf<Vin_max 26

14 + - Thermal Foldback Concept I NOM LED Current T CORNER Temperature 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 / safetyrelated Automotive Lighting System!! 27 Thermal Foldback Implementation Vcc Temperature increases, RNTC resistance decreases VSNS RTRIM + - IGM To LED Foldback ckt RGAIN RNTC VREF VCLAMP RMIN R1 Region 3 V SNS = R NTC R NTC + R MIN + R + R MIN TRIM 28

15 Thermal Foldback/Analog Dim with LM3424 As I TF = 0, i.e. No thermal feedback 29 LM3424 Frequency Synchronization 10 MHz RC filter to eliminate unwanted high frequency noise from coupling into the RT pin All LM3424 drivers run at the same frequency which can reduce the EMI, or simplifies the EMI design in the system! 30

16 Boost LED Drivers for LED Backlighting 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. 32

17 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 = t ON T Analog Brightness Control alter the LED continuous current Not preferred because color tone will change 33 LM3423 Boost Application with Fast Dimming & Zero Current Shutdown Vin range: 4.5V to 75V able to operate within very wide input range Low side MOFET driver with High side current sense able to configure boost, floating buck & floating buckboost approach LM3423 Boost LED driver is a popular solution for: - LED backlighting in Automotive Navigation Display/Dashboard Up to 20 pcs of LED!! N*Vf>Vin_max Max N*Vf = or < 75V Can be Configure as Input OVLO, or Output OVP protection Input OVLO prevent Vin > Vo to ensure boost mode operation Output OVP protection avoid any damage of LEDs as Vo > n*vf during line transient 34

18 LM3423 Boost Application with Fast Dimming & Zero Current Shutdown 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!! Allow Fast PWM Dimming for Brightness Control!! 35 LM3423 Dimming Performance Vin =12V, I LED = 100 ma, no of LEDs in series = 10 Dimming Frequency: 230 Hz Dimming Duty Cycle: 0.5% CH1: Dimming signal CH2: Switch Node of Boost CH3: Vout CH4: I LED Able to achieve very low dimming 230 Hz Provide very low light very dark ambient in LED backlighting in automotive display 36

19 Problems with Conventional Boost LED Driver Leakage current path Leakage Current V IN VOUT I leak ~ V in /R load PWM Gate Drive 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. 37 Problems with Conventional Boost LED Driver No Short Circuit Proof V IN VOUT PWM Gate Drive 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. 38

20 How LM3423 Boost minimizes shutdown leakage current I leakage ~0 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 39 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. I sc ~0 Implementation the True LED String Short-Circuit Proof with LM3423 Boost Converter!! LED shorted to GND 40

21 Multi-strings LED Backlight for Car Infotainment LM3431 Boost LED Driver Controller for Multiple Channel Applications 5V to 36V V IN Range Integrated Boost Controller LED open, short and over-temp protection Up to 25 khz PWM dimming & 100:1 contrast ratio Adjustable sw. freq. up to 1 MHz Main Application : Automotive Navigation Displays Available in 28 pin LLP and TSSOP package 42

22 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 43 Additional channels can be added LM 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 400nsec scale NDRV (NFET) NDRV (NPN) DIM 44

23 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) fault 1 LED open 1 LED short All LEDs open mechanism SC > 3.1V SC > 3.1V AFB > 2.0V action DLY charges DLY charges DLY charges result continue to regulate continue to regulate Latch off or restart type 1 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 multiple LED short multiple LED short VIN<6V Cathode short Current limit UVLO IC over TSD No DLY flag stand by 3 temperature THM SC > 6.0V AFB < 0.85V CFB low at startup COMP at max VCC or EN low THM < 1.2V DLY charges DLY charges DLY charges DLY charges No DLY flag No DLY flag latch off or restart Latch off or restart latch off or restart latch off or restart stand by stand by 2 45 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 46

24 NS Automotive LED Drivers Portfolio for Exterior Lighting & Infotainment Backlighting Application Exterior Lighting (Forward Lighting, DTR, Fog, RCL) Infotainment Backlighting Promote LM3424* LM3423* LM3421* LM3429* LM3410* LM3431* Features/ Benefit LM3421 features, plus thermal foldback/ analog dimming, & frequency synchronization LM3421 features, plus zero current shutdown, fault monitoring, & robust fault protection LM3429 features, plus fast PWM dimming Simple buck/boost, with high Vin, adjustable Current Sense, OVP, PWM dimming Simple Boost/ Sepic, small solution size Greater than 100:1 contrast ratio * Q version available 47 More Resources LM LM Webench LED designer i?application=led_driver 48

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