Switches that drive LED lighting in cars

Introduction

Compared with current automotive interior and exterior lighting solutions, LED lighting has many advantages, such as higher performance, longer life, lower cost, etc. 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 of LEDs contained in the string and the type of LEDs, and also based on the functionality of applications such as headlights, taillights and signal indicators, interior reading lights, dashboard or entertainment display lighting, etc. Optimization, the aspects that need to be optimized are as follows:

Topology - the relationship between LED voltage and battery voltage determines the use of buck, boost or buck-boost topology. The selected topology must be able to maintain alignment within the entire battery voltage range. LED current control.

Dimming - Large-scale dimming of LEDs must maintain color characteristics constant in brightness levels without visible fluctuations or oscillations to the eye.

Efficiency - Power loss in non-operating states consumes battery power, and in an already thermally stressed environment like a car, the consumed power is converted into heat.

Driving a single LED

interior white light dome light and vanity light may use one or two 3W LEDs, each producing 75 to 100 lumens of brightness. These LEDs have a typical forward voltage range of 3V to 4.5V and a maximum current of 1A to 1.5A, such as the Luxeon III Star from Lumileds (www.lumileds.com). 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 internal lighting circuit with dimming capability. The typical operating voltage range of a car battery is 9V to 16V (12V is typical). A drained battery may drop to 9V before the car is started. After the car is started, the alternator charges it back up to 14.4V. During cold cranking, the battery voltage may drop to 4V, at which time only critical electronic circuits must work.

Figure 1: Step-down High Voltage 1ALED Driver LT3474 with 250:1 PWM Dimming Ratio

Long cables between the battery and various locations on the chassis, as well as the electronically noisy environment, mean there are always high voltage spikes in cars. When selecting a switching regulator for an automotive design, 36V transients must be considered. Higher voltage spikes are often handled with simple protection diodes or filters.

The LT3471 converter integrated circuit used in Figure 1 is a high-voltage, high-current step-down LED converter. The device has a wide PWM dimming ratio and can drive one or more LEDs up to 1A. The following features of the LT3471 make it ideal for driving LEDs in automotive environments:

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

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

* Its buck topology and adjustable wide 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 greater than 80% at 12VIN. With analog control via the VADJ pin, efficiency decreases as LED current and brightness decrease, but power consumption remains low. Tailored for automotive and battery-powered applications, the LT3474 consumes less than 2uA (10nA typical) when placed in shutdown. Shutdown can also function as an LED on/off button, just like a push button or microcontroller. The LT3475LED driver is a dual-channel version of the LT3474 and can drive two separate LEDs or LED strings each at 1.5A. 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 feeding 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 a lower value by reducing the voltage across the internal sense resistor, but the color of the LED light changes at low currents. The practical limit for dimming ratio is approximately 10:1. Another way to reduce brightness is digital PWM dimming. During PWM on, the LED current settles very nicely at 1A. During PWM off period, the LED current is zero. This reduces brightness while maintaining LED light color and true color characteristics. The PWM function inside the integrated circuit allows the LED to respond very quickly to PWM dimming when it returns to the programmed current value. The LT3474's maximum digital PWM dimming ratio is 250:1, which is more than sufficient for interior lighting. The LT3475 can dim with dimming ratios greater than 1000:1. LED string lighting for LCD monitors GPS navigation and in-car entertainment displays require bright LED strings in daylight conditions and wide dimming ratios for night operation. LED strings pose different challenges than single LED ceiling lights. In these displays, multiple LED strings of 6 to 10 LEDs typically have lower current (<150mA) for the 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 capabilities are required. Figure 2 is the application of the LT3486 dual-output boost LED driver. It drives two strings of LEDs with a constant current of 100mA, and the LED voltage is as high as 36V. This boost converter uses a small voltage sense resistor in series with the LED string and PWM dimming MOSFET and offers high efficiency. The entire battery voltage range of 9V to 16V is lower than the voltage of the LED string. The dual-channel LED driver drives two LED strings of 20 LEDs while keeping the maximum switching voltage below the 42V rating of the IC. A single LED string of 20 LEDs requires much higher voltages.

Figure 2: In a GPSLCD display, the LT3486 drives 20 white LEDs at 100mA

over the battery operating voltage range with approximately 90% efficiency. If the battery voltage drops to 4V, the LT3486 will still operate, but may enter a current-limited state depending on the LED programming current and number of LEDs. The converter draws less than 1uA (100nA typical) when shut down. The LED current is set by selecting the external sense resistor value based on a very low 200mV sense resistor voltage for high efficiency. The current of each LED string can be adjusted with an analog signal on the CTRL pin for a maximum dimming ratio of 10:1, or with a PWM signal for much higher dimming ratios.

For extremely bright displays for night viewing, the LT3486 offers 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), which can achieve a dimming ratio of 1000:1 and maintain the true color of the display in a very dark night working environment. characteristic. Signal, taillight and headlight lighting Exterior signal lights, taillights and headlights require the highest power DC/DC LED drivers because these lights use the brightest and largest number of LEDs. While ultra-bright LED headlights are not yet common due to thermal and steady-flow limitations, red and amber brake lights and signal indicators are becoming more common due to their beauty and durability. Driving high-power amber and red LED strings poses similar challenges to interior lighting and lighting fine-tuning, but the magnitude of the challenge is different. A high dimming ratio is not necessary, 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 LED drivers 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. These two LEDs do not need to be referenced to ground. The two terminals connected are usually the output of the converter and the input of the battery. The LT3477 has two unique 100mV floating current-sense input pins that connect to current-sense resistors in series with the LED string that are not referenced to ground. Accurate LED current regulation is achieved at currents up to 1A within the operating voltage range of the car battery and below this range. The shutdown pin of the LT3477 is used to implement the on/off function of the car lights and reduce the input current to 1uA (100nA typical) when not in use. The IADJ pin is used to achieve analog dimming ratios greater than 10:1 for brake and taillight applications such as rear signal indicators or brake lights. This type of application does not require true color PWM dimming.

Figure 3: The buck-boost LT3477 drives a 1A brake light and signal indicator LED string with 80% efficiency.

As shown in Figure 4, the high-power LED driver LTC3783 uses a buck-boost topology to drive 6 to 10 3W red LED for automotive taillight applications. The external switching MOSFET and switching current sense resistor provide maximum design flexibility for high power and high voltage LED driver designs. If the battery voltage drops below 9V, then a 9V to 36V input and LED string output up to 25V at 1.5A requires a nominal switching voltage of 100V and a peak switching current capability of greater than 8A. The 1.5A constant battery current is well stable over the entire car battery voltage range. For brake light and taillight dimming, the LED current can be reduced with a PWM signal connected directly to the LTC3783PWM pin at 100Hz to achieve dimming ratios up to 200:1. At 1kHz, the dimming ratio is reduced to ?20:1, which is sufficient for taillight applications. Adjusting the ILIM pin can also reduce LED current.

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

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

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

Conclusion

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