Switching supplies drive LED lighting in automobiles

• Topology. The relation of LED-to-battery voltage dictates a buck, boost or buck-boost topology that maintains control of the LED current over the full battery voltage range.
• Dimming. Large-ratio LED dimming must preserve chromatic characteristics across brightness levels, without visible-to-the-eye ripple or oscillations.
• Efficiency. Power losses drain the battery during nonoperation and are dissipated as heat in an already thermally stressed environment.

Figure 1 shows a single-LED interior-lighting circuit with dimming. The typical operating-voltage range of the car battery is from 9 to 16 V (12 V typical). A drained battery may drop down to 9 V before the car is started and the alternator charges it back up to 14.4 V. During cold-crank conditions, it may drop down to 4 V, where only the critical electronics must still work.

The long cables between the battery and different locations around the chassis, and the electronically noisy environment, ensure that high-voltage spikes are an ever-present factor. It's necessary to consider the effect of 36-V transients when choosing a switching regulator for automotive design. Higher-voltage spikes are commonly handled with simple protection diodes or filters.

The LT3474 converter IC used in Figure 1 is a high-voltage, high-current buck LED converter with a wide pulse-width modulation (PWM) dimming ratio that can drive one or more LEDs up to 1 A.

Figure 1: LT3474 high-voltage stepdown 1-A LED driver has 250:1 PWM dimming.

It has several features that make it suitable for driving LEDs in an automobile environment:

• It is a dedicated LED driver with an on-board high-voltage switch and low-voltage current sense resistor, to minimize board space and simplify design while maintaining high efficiency.
• The wide 4- to 36-V operating input voltage range allows the LED driver to operate directly from the battery, while regulating constant LED current.
• The buck topology and the adjustable high-frequency range provide low-ripple LED current with small, low-cost, high-temperature-coefficient ceramic capacitors.

PWM dimming and brightness control
LED brightness can be controlled on the LT3474 in Figure 1 with an analog voltage input to the VADJ pin, or a digital PWM signal to the gate of the PWM dimming MOSFET and PWM pin.

Simple analog brightness control reduces the constant LED current from 1 A to a lower value by reducing the internal sense resistor voltage, but the chromaticity of the LED changes at low current. The dimming ratio has a practical limitation of around 10:1. Another way of reducing brightness is digital PWM dimming. During PWM on-time, the LED current is very well regulated at 1 A. During PWM off-time, the current is zero. This maintains the chromaticity and true-color characteristics of any LED while reducing its brightness.

The PWM function inside the IC makes the response to PWM very fast in returning the LED to its programmed LED current. The LT3474 provides a maximum 250:1 digital PWM dimming ratio, more than sufficient for interior lighting. The LT3475 can dim wider than a 1,000:1 ratio.

LCD monitor displays with LED strings
GPS navigation and in-cabin entertainment displays require bright strings of LEDs, which function in daylight conditions but need wide dimming ratios for nighttime operation. LED strings pose a different challenge than the single-LED dome light. Multiple strings of six to 10 LEDs in these displays are usually lower current (under 150 mA) for the smaller LEDs, but stack up to a higher voltage than the car battery. A high-power boost topology LED driver with high efficiency and PWM dimming capability is necessary for these monitors.

The LT3486 dual-output boost LED driver application in Figure 2 drives two strings of LEDs with a constant 100-mA current up to 36 V of LED voltage.

Figure 2: LT3486 drives 20 white LEDs at 100 mA in a GPS LCD monitor.

The boost converter provides high efficiency using a low-voltage sense resistor in series with the LEDs and PWM dimming MOSFET. The full range of battery voltage, 9 to 16 V, is less than the voltage of the LED strings. The dual-channel LED driver drives 20 LEDs in two strings while keeping the maximum switch voltage below the 42-V IC rating. A single 20-LED string requires much higher voltage.

Efficiency is approximately 90 percent over the battery operating range. If the battery drops down to 4 V, the LT3486 will still operate, but possibly in a current-limited state, depending on the programmed LED current and number of LEDs. The converter shutdown current consumption is under 1 microamps (typically 100 nA). The LED current is set by selecting the external sense resistor value based on a very low 200-mV sense resistor voltage for high efficiency. The LED current can be adjusted on either string with an analog signal on the CTRL pin for a maximum 10:1 dimming ratio, or with a PWM signal for a much wider dimming ratio.

For nighttime viewing of the extremely bright displays, the LT3486 has a PWM dimming ratio of 1,000:1 with its unique internal PWM dimming architecture. An ultrafast PWM response time with internal LED current memory returns the LED current to 100 mA from zero in less than 10 microseconds for true-color PWM dimming. Using two LT3486 for four strings of R-G-G-B LEDS in top-end displays provides a 1,000:1 dimming ratio and maintains the true color of the display during very dim nighttime operation.

Signal, tail and headlighting
Exterior signal, tail- and headlights require the highest-power dc/dc LED drivers because they have the brightest and greatest number of LEDs. Although extremely bright LED headlights are not yet common due to thermal and regulatory constraints, red and amber brake and signal lighting are increasingly seen based on their aesthetic properties and durability.

Driving high-power strings of amber and red LEDs poses similar challenges as interior and trim lighting, but on a different scale. High dimming ratios are not necessary, but simple on/off and high/low brightness functions are useful. The string voltage usually crosses over the full voltage range of the car battery, creating the need for an LED driver with both step-up and step-down, or buck-boost, capability.

The LT3477 buck-boost LED driver shown in Figure 3 drives two high-power LEDs at 1 A.

Figure 3: LT3477 buck-boost drives brake and signal 1-A LED strings with 80% efficiency.

The LEDs do not need to be ground-referred and are connected between what would typically be the converter output and the battery input. The LT3477 has two unique, floating 100-mV current sense input pins that are connected to a nonground-referred current sense resistor in series with the string of LEDs. Accurate LED current regulation is provided at up to 1 A over the operating voltage range of the car battery and below.

The LT3477 shutdown pin is used for on/off function of the lights and for reducing the input current to 1 microamp (typically 100 nA) when not in use. The I_ADJ pin is used for greater than 10:1 analog dimming range for brake and taillight applications such as the rear-signal lights. True-color PWM dimming is not necessary for these applications. The LTC3783, high-power LED driver powers six to ten 3-W red LEDs in a buck-boost topology as shown in Figure 4 for automotive taillights.

Figure 4: LTC3783 brake-light LED driver for eight 1.5A red LEDs, with greater than 90% efficiency.

The external switching MOSFET and switching current-sense resistor provide maximum design flexibility for high-power and high-voltage LED-driver designs. The 9- to 36-V input and up to 25-V LED string output at 1.5 A require a 100-V switch rating and greater than 8-A peak switch current capability, if the battery drops below 9 V.
The constant 1.5-A battery current is well-regulated over the entire car battery voltage range. For brake and taillight dimming, the LED current can be reduced to up to 200:1 dimming ratio with a PWM signal tied directly to the PWM pin of the LTC3783 at 100Hz. At 1 kHz, this dimming ratio is reduced to 20:1, sufficient for taillight applications. An adjustment to the ILIM pin can reduce the LED current as well.

High efficiency is most important in the highest-power applications of the vehicle. With up to 36 W output in this application, the 93 percent efficiency reduces the draw on the battery during braking, especially when the car is not running. The RUN pin, used for on/off control of the brake lights, reduces the LED current to 20 microamps.

The flexibility of the LTC3783 high-power LED driver enables it to turn into a high-power boost regulator to drive an even higher-voltage string of LEDs of up to 60 W, by connecting the string of LEDs to GND as opposed to VIN and turning the topology into a boost or step-up. This requires that the LED string voltage is greater than the battery voltage maximum of 36 V, and that LED disconnect is provided via a PWM pin while the light is turned off. High-lumen headlight applications using very bright white LEDs will soon adopt this high-power LED-driving boost topology.

Conclusion
Many automotive LED applications require a dedicated high-power, yet simple and efficient, LED driver. Low current consumption during off-time, high PWM and analog dimming ratios, and excellent LED current regulation are needed in different combinations depending on the application.

About the author
Keith Szolusha is an applications engineer with Linear Technology Corp. in Milpitas, Calif. He holds a BSEE and MSEE from MIT, with a concentration in technical writing.