How cars drive LED lighting

Compared with current automotive interior and exterior lighting solutions, LED lighting has many advantages, such as higher performance, longer life, and lower cost. This lighting method improves the aesthetics and performance of automotive lighting. Driving LEDs directly from the car battery requires a DC/DC converter to regulate a constant LED current and protect the LEDs from the vagaries of the car battery bus. This converter should also be optimized based on the number and type of LEDs contained in a string of LEDs, and also based on the function of the application such as headlights, taillights and signal indicators, interior reading lights, dashboard or entertainment display lighting. The following aspects need to be optimized:

Topology - The relationship between the LED voltage and the battery voltage determines whether a buck, boost, or buck-boost topology is used. The topology selected should be able to maintain control of the LED current over the entire battery voltage range.

Dimming - Large-ratio LED dimming must maintain color characteristics over brightness levels without visible fluctuations or oscillations.

Efficiency – Power losses during non-operational conditions consume battery power, and in an already thermally stressful environment like a car, that power is converted into heat.

Driving a Single LED

White ceiling lights and vanity lights in a car may use one or two 3W LEDs, each producing 75 to 100 lumens. The typical forward voltage range for these LEDs is 3V to 4.5V, with a maximum current of 1A to 1.5A, such as Lumileds' LuxeonIIIStar. The simplest LED driver design uses a buck regulator to drive a single LED directly from the car battery. Figure 1 shows a single LED interior lighting circuit with dimming function. The typical operating voltage range of a car battery is 9V to 16V (typical value is 12V). A depleted battery may drop to 9V before the car is started, and the alternator charges it after the car is started, restoring its voltage to as high as 14.4V. During a cold car start, the battery voltage may drop to 4V, at which time only the critical electronic circuits must work.

Long cables between the battery and various locations on the chassis and the electronically noisy environment mean that high voltage spikes are always present in the car. 36V transients must be considered when selecting a switching regulator for an automotive design. Often simple protection diodes or filters are used to handle higher voltage spikes.

The LT3471 converter IC used in Figure 1 is a high voltage, high current step-down LED converter with a wide PWM dimming ratio that can drive one or more LEDs up to 1 A. The following features of the LT3471 make it ideal for driving LEDs in an automotive environment:

* It is a dedicated LED driver with on-chip high-voltage switches and low-voltage current monitoring resistors to minimize board area and simplify design while maintaining high efficiency.

* A wide input voltage range of 4V to 36V allows this LED driver to operate directly from a car battery while providing a constant LED current.

* Its buck topology and wide adjustable frequency range allow the use of small, low-cost, high-temperature-coefficient ceramic capacitors to provide low-ripple LED current.

The LT3474 single LED buck converter has an efficiency of over 80% at 12VIN. With analog control via the VADJ pin, efficiency drops as LED current and brightness decrease, but power dissipation remains low. Tailored for automotive and battery-powered applications, the LT3474 consumes less than 2uA (10nA typical) when placed in shutdown. Shutdown also acts as an LED on/off button, just like a pushbutton or microcontroller. The LT3475 LED driver is a dual-channel version of the LT3474 that can drive two individual LEDs or LED strings at 1.5A each.

PWM dimming and brightness control

LED brightness can be controlled by the LT3474 in Figure 1 by connecting an analog voltage input to the VADJ pin or by delivering a digital PWM signal to the gate of the PWM dimming MOSFET and the PWM pin. Simple analog brightness control reduces the constant LED current from 1A to lower values ​​by reducing the voltage across the internal sense resistor, but the color of the LED light will change at low currents. The practical limit of the dimming ratio is about 10:1.

Another way to reduce brightness is digital PWM dimming. During the PWM on time, the LED current is very well regulated at 1A. During the PWM off time, the LED current is zero. This reduces brightness while maintaining the LED light color and true color characteristics.

The PWM function inside the IC allows the LED to return to the programmed current value, and the response to PWM dimming is very fast. The maximum digital PWM dimming ratio of the LT3474 is 250:1, which is more than enough for interior lighting. The LT3475 can dim at a ratio greater than 1000:1.

LED string lighting for LCD displays

GPS navigation and in-car entertainment displays require bright LED strings in daylight conditions, yet require wide dimming ratios for nighttime operation. LED strings present different challenges than single LED dome lights. In these displays, multiple strings of 6 to 10 LEDs are typically low current (<150mA) for smaller LEDs, but the accumulated voltage is higher than the car battery voltage. For these displays, high power boost topology LED drivers with high efficiency and PWM dimming capability are required.

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

Efficiency is approximately 90% over the battery operating voltage range. If the battery voltage drops to 4V, the LT3486 will still operate, but may go into current limiting depending on the programmed LED current and the number of LEDs. The converter consumes less than 1uA (100nA typical) when in shutdown. The LED current is set by selecting the external sense resistor value, which is selected based on a very low 200mV sense resistor voltage for high efficiency. The current in each LED string can be adjusted with an analog signal on the CTRL pin for up to 10:1 dimming ratios, or with a PWM signal for much higher dimming ratios.

For viewing extremely bright displays at night, the LT3486 provides a 1000:1 PWM dimming ratio with its unique internal PWM dimming architecture. The internal LED current memory has an ultra-fast PWM response time, which can return the LED current from zero to 100mA in less than 10us to achieve true color PWM dimming. In high-end displays, two LT3486s are used to drive four LED strings representing red, green, green, and blue (RGGB), respectively, to achieve a 1000:1 dimming ratio and maintain the true color characteristics of the display in very dark night working environments.

Signal, tail and headlight lighting

Exterior signal indicators, taillights, and headlights require the highest power DC/DC LED drivers because these lights use the brightest and most numerous LEDs. Although extremely bright LED headlights are not yet common due to heat and current regulation limitations, red and amber brake and signal indicators are becoming more common due to their aesthetics and durability. Driving high-power amber and red LED strings presents similar challenges to interior lighting and lighting trim, but the magnitude of the challenge is different. High dimming ratios are not required, but simple on/off and high/low brightness functions are useful. The voltage of the LED string often exceeds the voltage range of the car battery, requiring an LED driver with buck and boost, or buck-boost, capabilities.

The LT3477 buck-boost LED driver shown in Figure 3 drives two high-power LEDs at 1A. The two LEDs do not need to be ground referenced, and the two terminals connected are typically the converter output and the battery input. The LT3477 has two unique and 100mV floating current sense input pins, which connect to current sense resistors in series with the LED string and are not ground referenced. Accurate LED current regulation is achieved at currents up to 1A within and below the operating voltage range of the automotive battery. The LT3477's shutdown pin is used to implement the light on/off function and reduce the input current to 1uA (typically 100nA) when not in use. The IADJ pin is used to achieve analog dimming ratios greater than 10:1 for brake and tail light applications such as rear signal indicators or brake lights. True color PWM dimming is not required for these applications.

As shown in Figure 4, the high-power LED driver LTC3783 uses a buck-boost topology to drive 6 to 10 3W red LEDs for automotive taillight applications. The external switching MOSFET and switch current sense resistor provide maximum design flexibility for high-power and high-voltage LED driver designs. If the battery voltage drops below 9V, the 9V to 36V input and the LED string output up to 25V at 1.5A require a 100V rated switch voltage and a peak switch current capability of more than 8A. The 1.5A constant battery current is well regulated over the entire automotive battery voltage range. For brake and taillight dimming, at 100Hz, the LED current can be reduced with a PWM signal directly connected to the LTC3783 PWM pin to achieve a dimming ratio of up to 200:1. At 1kHz, the dimming ratio is reduced to ~20:1, which is sufficient for taillight applications. The LED current can also be reduced by adjusting the ILIM pin.

In the highest power automotive applications, high efficiency is of utmost importance. In this application, with up to 36W output, 93% efficiency can reduce the drain on the battery when braking, especially when the car is not running. The RUN pin for brake light on/off control reduces the LED current to 20uA.

The flexibility of the LTC3783 high power LED driver can be turned into a high power boost regulator to drive higher voltage LED strings up to 60W by connecting the LED string to GND instead of VIN and changing the topology to boost. This requires the LED string voltage to be higher than the maximum battery voltage of 36V and the LED disconnection to be achieved through the PWM pin when the lights are turned off. High lumen headlight applications using very bright white LEDs will soon adopt this high power LED driver with boost topology.

in conclusion

Many automotive LED applications require dedicated high-power, yet simple and efficient LED drivers. There are different combinations of LEDs depending on the application, but each combination requires low current consumption when off, high PWM and analog dimming ratios, and excellent LED current regulation capabilities. Linear Technology offers a variety of different automotive LED drivers that can overcome all of these challenges.