In the past ten years, automotive electronic products have developed by leaps and bounds, and in-vehicle electronic control, in-vehicle information services, and entertainment systems have improved significantly in terms of quantity and precision. This article will focus on one of the main components of this growth, namely: the rapid increase in the use of LED lighting in current and next-generation cars. This new lighting field has brought new challenges to designers and manufacturers of automotive electronics. Understanding these challenges and finding feasible solutions is the most important, because the developments associated with these lighting systems seem to be endless.
LED lighting
Advantages such as small form factor, low power consumption, and fast turn-on time have created a situation where high-brightness LEDs are widely used in today's automobiles. The initial application of LEDs in automobiles is central overhead parking lights (CHMSL); these applications use red LEDs to provide a very thin lighting array that is easy to install and never needs to be replaced.
Traditionally, incandescent bulbs are the most economical light source, and they are still used by many cars. However, with the shrinking of the available lighting space and the continuous improvement of the service life requirements of the lighting source, the color and design of the light provided by the LED are rapidly replacing the application of incandescent bulbs. Even the traditional CCFL TFT-LCD backlight application is gradually being replaced by white LED arrays.
What's more, people are still using an electric "manipulable" type high-current LED array to develop headlights, and this field has been controlled by halogen / xenon filament design. Almost all automotive lighting applications (including vehicle interior / exterior lighting and backlighting applications) will gradually transition to adopt LEDs. The benefits of using LEDs have many positive implications. First (perhaps the most important point), it never needs to be replaced, because its solid-state life of 100,000 hours (service life: 11 and a half years) is longer than the life of the car. This allows automakers to permanently embed them in the lighting system in the cabin without having to leave an entrance for replacing the filament bulb as in the past. Since the LED lighting system does not require the installation depth or area required by the incandescent light bulb, it can also significantly change the shape of the car. Another advantage of LED is that it has low power consumption, which can reduce fuel consumption.
Design parameters of automotive LED lighting
In order to ensure the best performance and long working life, LED needs an effective driving circuit. These special drive circuits must be able to obtain operating power from a rather demanding automotive power bus, and they should also have both cost and space "benefit". In order to maintain its long working life, the current and temperature limits of the LED must not be exceeded. Table 1 lists the relationship between the typical forward voltage and drive current for a high-current white LED.
In the application of a single LED to three (series) LEDs, a step-down LED driver (for example: Linear Technology's LT3475) will be needed to reduce the automotive bus voltage (nominal value is 12V) to a more The appropriate LED voltage can vary from 2.68V to 4.88V (each LED) according to the LED color and brightness requirements of the application. In contrast, in applications such as brake lights that require multiple LED strings consisting of up to 8 LEDs in series, the required output voltage is 21V to 39V, so a boost LED driver (for example: Ling Force LT3496). All LED drivers provided by Linear Technology use a current mode architecture designed to deliver a constant current.
If you want to produce a constant LED brightness under an irregular input voltage, you must obtain a constant current source from these driver ICs. An internal sense resistor is used to monitor the output current to achieve accurate current regulation. High output current accuracy is maintained over a wide current range (35mA to 1A), thus achieving a wide dimming range. Because Linear Technology's high-current LED drivers are current-mode regulators, they do not directly adjust the duty cycle of the power switch, but the feedback loop controls the peak current flowing through the switch in each cycle. Compared with voltage mode control, current mode control improves the dynamic performance of the loop and provides a cycle-by-cycle current limit function.
In many applications (especially back lighting and interior lighting), dimming control may be required, so the driver IC is required to provide a simple method for adjusting the output current / LED brightness. With a suitable driver IC, the dimming operation can be completed by a PWM signal, DC voltage or external NMOS transistor, and the dimming range can be as high as 3000: 1.
Finally, in-vehicle electronics may be sensitive to noise, especially navigation systems, wireless circuits, and AM radio band receivers. To minimize the possibility of noise interference, Linear Technology uses a constant frequency switching topology in its LED driver IC. In addition, users can set the switching frequency in the range of 200kHz to 2MHz to keep the switching noise away from key frequency bands (such as AM radio band). The high switching frequency also allows the use of small inductors and ceramic capacitors, thereby minimizing the size and cost of the solution.
Dual LED applications
Many embedded high current LED applications will include single or two high current (ILED range from 1A to 1.5A) LED. These applications include interior lighting (for example: roof lights, map lights, storage box lights) and exterior lighting (for example: door sill lights or "ground lighting" lights). Depending on the application, they can use color LEDs (for back lighting of in-vehicle instruments) or white LEDs (for general lighting). Since these LEDs usually have a forward voltage of 3V to 4V and are powered by a 12V to 14V automotive bus, a step-down converter (eg LT3475) is required.
The LT3475 is a dual-channel, 36V, 2MHz step-down DC / DC converter designed for use as a constant current dual LED driver (see Figure 1). Each channel has an internal detection resistor and dimming control function, making it ideal for driving LEDs that require up to 1.5A current. The switching operation of one channel is 180 ° out of phase with the other channel, thus reducing the output ripple of both channels. Each channel independently maintains a high output current accuracy in a wide current range of 50mA to 1.5A, and the unique True Color PWMTM circuit provides a 3000: 1 dimming range without any occurrence. Color shift (this phenomenon is very common in LED current dimming).
With its wide input voltage range of 4V to 36V (transient voltage up to 40V), the LT3475 is ideal for automotive power systems. The switching frequency can be set between 200kHz and 2MHz, allowing the use of tiny inductors and ceramic capacitors, and the switching noise away from the AM radio band. Coupled with a thermally enhanced TSSOP-20 package, the device provides a compact solution suitable for driving high-current LEDs.
The LT3475 uses high-voltage side detection to realize the ground connection of the negative electrode of the LED, thereby eliminating the need for a ground wire in most applications. It also has an integrated boost diode for each channel, thus further reducing the solution footprint and cost. Additional features include LED open circuit and short circuit protection.
brake light
To date, the most common application of LEDs in automobiles is central elevated parking lights (CHMSL). By the end of 2006, at least 60% of cars had LED-type CHMSL installed. The benefits include faster lighting speed, higher efficiency, longer working life, and the very thin red LED array also has the ease of design / installation. LEDs can reach full illuminance in less than 1ms (whereas traditional light bulbs require up to 200ms to produce their maximum brightness), this way, the time for the driver of the rear vehicle to recognize the brake lights will be greatly reduced The probability of rear-end collision accident occurred.
Moreover, compared with incandescent light bulbs, power consumption has dropped by as much as 80%, which ultimately has the effect of saving fuel consumption. Its effective life span will easily exceed the life of the vehicle, thus eliminating the need for replacement. In addition to CHMSL, some cars and motorcycles have replaced incandescent brake lights with LEDs in the main brake lights.
In order to maximize the performance and working life of these LED brake lights, a suitable LED driver that can drive the red LED strings required by these brake systems should be used, which is essential. Linear Technology's LT3486 was developed specifically for this type of automotive application. LT3486 is a dual-channel boost DC / DC converter (as shown in Figure 2), designed to drive up to 16 LEDs with a constant current from a 12V to 14V automotive bus (each converter drives 8 LEDs in series). The use of LEDs in series can provide equal LED currents to achieve uniform LED brightness. When needed, two independent converters can also drive asymmetric LED strings.
The dimming of the two LED strings can also be controlled individually through their respective CTRL pins. By feeding a PWM signal to the respective PWM pin, an internal PWM dimming system can extend the dimming range up to 1000: 1. The operating frequency of the LT3486 can be set in the range of 200kHz to 2MHz by an external resistor. A low feedback voltage of 200mV (2% accuracy) minimizes the power loss in the current setting resistor and is designed to improve efficiency. Additional features include output voltage limitation when the LED is disconnected. The LT3486 provides a very compact board footprint solution and can be packaged in a space-saving 16-lead DFN (5mm x 3mm x 0.75mm) or 16-lead thermally enhanced TSSOP package.
Summary of this article
Due to the unprecedented increase in the popularity of LED lighting in today's and future automobiles, many very special performance requirements have been created for LED driver ICs in high-current LED automotive applications. The LED driver must provide a constant current to maintain a uniform brightness (independent of the input voltage or the LED forward voltage change), and must achieve efficient operation. They must also be able to withstand the rather demanding electrical characteristics of the automotive power bus. In addition, these applications also require solutions that occupy a very compact board area and have high heat dissipation efficiency. Faced with these automotive design requirements, Linear Technology has developed a complete high-current LED driver product series aimed at solving the aforementioned automotive problems.
Industry control printed circuit boards (PCB)
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