Keith Szolusha
Compared to current automotive interior and exterior lighting solutions, LED lighting has many advantages, such as higher performance, longer life, lower cost, etc., which enhances the aesthetics and performance of automotive lighting. However, to drive LEDs directly from a car battery, a DC/DC converter is needed to regulate a constant LED current and protect the LED from the drastic changes in the car battery. Such converters should be optimized based on the number of LEDs included in the series LEDs and the type of LEDs and functions based on applications such as headlights, taillights and signal indicators, in-car reading lights, instrument panels or entertainment display lighting. Need to pay attention to:
1. Architecture - The relationship between LED voltage and battery voltage is determined by a buck, boost or buck-boost architecture. The architecture chosen must maintain control of the LED current over the entire battery voltage range.
2. Dimming - A large proportion of LED dimming must maintain the same color characteristics at the brightness level and there is no undulation or oscillation visible to the naked eye.
3. Efficiency - In the non-operating state, power loss consumes battery power, and in an environment where the heat pressure of the automobile is already large, the consumed power is converted into heat.
Driving a single LED
White and white headlights and makeup lamps may use one or two 3W LEDs, each producing 75 to 100 lumens of brightness. These LEDs typically have a forward voltage range of 3V to 4.5V and a maximum current of 1A to 1.5A, such as Lumileds' Luxeon III Star. The simplest LED driver design uses a buck regulator that drives a single LED directly from a car battery.
As shown in Figure 1, it is a single LED internal lighting circuit with dimming function. Automotive batteries typically operate from 9V to 16V (typically 12V). A battery that consumes power may drop to 9V before the car starts. After the car starts, the alternator charges it to restore its voltage to 14.4V. At cold start, the battery voltage may drop to 4V. Only critical electronic circuits must work.
On the battery and chassis, long cables between different locations and an electronic noise environment always cause high voltage spikes in the car. 36V transients must be considered when choosing a switching regulator for automotive design, usually with a simple protection diode or filter to handle higher voltage spikes. The LT3471 converter IC used in Figure 1 is a high voltage, high current buck LED converter with a wide PWM dimming ratio that can drive one or more LEDs up to 1A. The LT3471 offers a wide input voltage range of 4V to 36V, allowing the LED driver to operate directly from automotive batteries while providing constant LED current, while its buck architecture and adjustable wideband range allow for small, low cost, high temperature A ceramic capacitor with a low chopping LED current.
The efficiency of the LT3474 single LED step-down converter is higher than 80% at 12VIN. When the analog control is performed through the VADJ pin, the efficiency decreases as the LED current and brightness decrease, but the power consumption remains low. Specifically tailored for automotive and battery-powered applications, the LT3474 consumes less than 2uA (typically 10nA) in shutdown. The LED on/off button function is still available when the unit is turned off, just like a button or microcontroller. The LT3475 LED driver is a dual-channel version of the LT3474 that can drive two sets of individual LEDs or multiple strings of LEDs with a current of 1.5A.
Figure 1: The step-down high-voltage 1A LED driver LT3474 with a 250:1 PWM dimming ratio.
PWM dimming and brightness control
The LED brightness can be controlled by the LT3474 of Figure 1, which connects an analog voltage input to the VADJ pin or a digital PWM signal to the gate and PWM pins of the PWM dimming MOSFET. Simple analog brightness control reduces the constant LED current from 1A to a lower value by lowering the voltage of the internal sense resistor, but the color of the LED light changes at low currents. The practical limit of the dimming ratio is approximately 10:1.
Another way to reduce the brightness is digital PWM dimming. The LED current is held at a steady 1A during PWM turn-on. The LED current is zero during PWM shutdown. This maintains the LED light color and true color characteristics while reducing brightness.
Inside the IC, the PWM function allows the LED to return to the set current value for fast response to PWM dimming. The LT3474's maximum digital PWM dimming ratio is 250:1, which is more than enough for internal lighting. The LT3475 can be dimmed with a dimming ratio greater than 1000:1.
LCD display with multiple strings of LED illumination
GPS navigation and in-car entertainment displays require bright tandem LEDs in daylight conditions and wide dimming ratios when working at night. Series LEDs present a different challenge than single LED headlamps. In these displays, for smaller LEDs, multiple strings of 6 to 10 LEDs are typically low in series current (<150mA), but the accumulated voltage is higher than the car battery voltage for these displays. High power boost architecture LED drivers with high efficiency and PWM dimming capability are necessary.
Figure 2 shows an application of the LT3486 dual-group output boost LED driver that drives two strings of LEDs at a constant current of 100mA with an LED voltage of up to 36V. This boost converter uses a small voltage sense resistor in series with the LED and the PWM dimming MOSFET in series with high efficiency. The entire battery voltage range of 9V to 16V is lower than the voltage of the LED series. The dual-channel LED driver drives two LEDs of 20 LEDs in series while maintaining the highest switching voltage below the 42V rating of the IC, and a single string of 20 LEDs in series requires a higher voltage.
Figure 2: In a GPS LCD display, the LT3486 drives 20 white LEDs at 100mA.
The efficiency is approximately 90% over the battery operating voltage range. If the battery voltage drops to 4V, the LT3486 will still operate, but depending on the LED setting current and the number of LEDs, it may enter the current limit state. The converter consumes less than 1uA (typically 100nA) when it is turned off. The LED current is set by selecting the external sense resistor value. When the sense resistor is selected, high efficiency is achieved based on the very low 200mV sense resistor voltage. The current in series with each LED can be adjusted via the analog signal on the CTRL pin, with a maximum dimming ratio of 10:1, or can be adjusted with a PWM signal to achieve a higher dimming ratio.
For an extremely bright display at night, 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 that can return LED current from zero to 100mA for less than 10us for true color PWM dimming. In high-end displays, two LEDs, each representing red, green, green, and blue (RGGB), are connected in series with two LT3486s to achieve a 1000:1 dimming ratio and maintain the display in very dark nighttime environments. True color characteristics.
Signal Indicators, Taillights, and Headlamp Lighting <br> External signal indicators, taillights, and headlamps require the highest power DC/DC LED drivers because they use the highest brightness and the highest number of LEDs. Despite the limitations of heat and steady flow, extremely bright LED headlights are rare, but red and yellow-brown brake lights and signal indicators are becoming more common due to their aesthetics and durability. Driving high-power tan and red LEDs in series brings the same challenges for internal lighting and fine-tuning of lighting, but the difficulty of the challenge is different, high-lighting ratio is not necessary, but simple on/off and high/ Low brightness is useful. The voltage in series with LEDs often exceeds the voltage range of automotive batteries, requiring LED drivers with buck and boost, or buck-boost functions.
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 grounded, and the two terminals that are connected are typically the converter's output and battery input. The LT3477 features two unique, 100mV floating current sense input pins that are connected to multiple current-series resistors in parallel with the LEDs and not grounded. Accurate LED stabilization is achieved at currents up to 1A over the operating voltage range of automotive batteries and below this range. The LT3477's shutdown pin is used to achieve the on/off function of the lamp and reduces the input current to 1uA (typically 100nA) when not in use. The IADJ pin is used for brake and taillight applications, such as rear-end signal indicators or brake lights to achieve an analog dimming ratio higher than 10:1. Such applications do not require true color PWM dimming.
Figure 3: The buck-boost LT3477 drives the 1A brake light and signal indicator multiple strings of LEDs with 80% efficiency.
As shown in Figure 4, the high-power LED driver LTC3783 uses a buck-boost architecture to drive 6 to 10 3W red LEDs for automotive taillight applications. External switching MOSFETs and switch current sense resistors provide maximum design flexibility for high power and high voltage LED driver designs. If the battery voltage drops below 9V, a 9V to 36V input and a current of 1.5A, a series output of up to 25V LED requires a nominal switching voltage of 100V and a peak switching current capability of greater than 8A. The 1.5A constant battery current is stable over the entire vehicle battery voltage range. For brake and taillight dimming, at 100Hz, the LED current can be reduced by a PWM signal directly connected to the LTC3783 PWM pin. A dimming ratio of up to 200:1 is achieved. At 1 kHz, the dimming ratio is reduced to 20:1, which is sufficient for taillight applications. Adjusting the ILIM pin also reduces the LED current.
Figure 4: The LTC3783 Brake Light LED Driver drives eight 1.5A red LEDs with greater than 90% efficiency.
In the most powerful automotive applications, high efficiency is paramount. In this type of application, with up to 36W output, 93% efficiency can reduce battery consumption 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 LEDs in series to GND instead of VIN and turning the architecture into boost, the flexibility of the LTC3783 high-power LED driver makes it a high-power boost regulator that can drive up to 60W of higher voltage LEDs in series This requires the maximum battery voltage of the LED series voltage higher than 36V, and the LED pin is required to be disconnected through the PWM pin when the lamp is turned off. High-brightness LEDs with high-brightness white LEDs will soon be available with this high-power LED driver with boost architecture.
Conclusions <br>Many automotive LED applications require dedicated high-power, simple, and high-efficiency LED drivers that produce different combinations of LEDs depending on the application, but all combinations require low current consumption when disconnected. High PWM and analog dimming ratios and excellent LED current regulation. A variety of different automotive LED drivers from Linear Technology further overcome the challenges. (The author is an application engineer for Linear Technology)
Filter Choke Inductor,power Filter Inductor,PCB mount Filter Inductor,UU Filter Inductor,Common Mode Choke Filter
IHUA INDUSTRIES CO.,LTD. , https://www.ihua-transformer.com