Automotive LED applications bring opportunities and challenges to power management ICs

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 was the Central Overhead Parking Light (CHMSL); these applications used red LEDs to provide a very flat, light-emitting array that was easy to install and never needed to be replaced.

Traditionally, incandescent bulbs are the most economical source of light and are still used by many cars. However, as the available lighting space shrinks and the life expectancy of lighting sources continues to increase, the color and design of the lights provided by LEDs is rapidly replacing incandescent bulb applications. Even traditional CCFL TFT-LCD backlight applications are gradually being replaced by white LED arrays.

What's more, people are still using an electric "steerable" high-current LED array to develop headlights, which has been dominated by halogen/æ°™ filament design. Almost all automotive lighting applications, including body interior/external lighting and backlighting applications, will gradually transition to LED adoption. The benefits of using LEDs have many positive implications. First (and perhaps the most important), it never needs to be replaced because its 100,000-hour solid-state life (service life: 11 and a half years) is longer than the car's life. This allows automakers to permanently embed them in the lighting system in the cabin without the need to have an inlet for replacing the filament bulb as in the past. Since the LED lighting system does not require the installation depth or area required for incandescent bulbs, it can also cause significant changes in the shape of the car. Another advantage of LEDs is their low power consumption, which reduces fuel consumption.

Automotive LED lighting design parameters

To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. These special drive circuits must be able to draw power from a fairly demanding automotive power bus, and should also be cost and space "efficient." In order to maintain their long working life, the current and temperature limits of the LED must not be exceeded. value. Table 1 lists the relationship between typical forward voltage and drive current for a high current white LED.

In applications with a single LED to three (series) LEDs, a step-down LED driver (eg, Linear Technology's LT3475) will be required to reduce the automotive bus voltage (nominally 12V) to a further The appropriate LED voltage can vary from 2.68V to 4.88V (per LED) depending on 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 eight LEDs in series, the required output voltage is 21V to 39V, so a boost LED driver must be used (eg: Linear Technology's LT3496). All of the LED drivers offered by Linear Technology use a current mode architecture designed to deliver a constant current.

To produce a constant LED brightness with irregular input voltages, a constant current source must be obtained from these driver ICs. An internal sense resistor is used to monitor the output current for accurate current regulation. A wide dimming range is achieved by maintaining high output current accuracy over a wide current range (35 mA to 1 A). Because Linear Technology's high current LED drivers are current mode regulators, they do not directly adjust the duty cycle of the power switch. Instead, the feedback loop controls the peak current flowing through the switch during each cycle. Current mode control improves loop dynamics compared to voltage mode control and provides cycle-by-cycle current limiting.

In many applications (especially for backlighting and interior lighting), dimming control may be required, thus requiring the driver IC to provide a simple method for adjusting the output current/LED brightness. With a suitable driver IC, dimming can be done with a PWM signal, DC voltage or an external NMOS transistor with a dimming range of up to 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, the user can set the switching frequency from 200kHz to 2MHz to keep the switching noise away from critical bands (eg AM radio band). The high switching frequency also allows the use of small inductors and ceramic capacitors, minimizing the size and cost of the solution.

Dual LED application

Many embedded high current LED applications will include single or two high current (ILED ranges from 1A to 1.5A) LEDs. These applications include interior lighting (eg, roof lights, map lights, storage box lights) and vehicles. External lighting (eg door sill lights or "ground lighting" lights). Depending on the application, they can be colored LEDs (for backlighting of in-vehicle instruments) or white LEDs (for general lighting). Since these LEDs typically have a forward voltage of 3V to 4V and are powered by a 12V to 14V automotive bus, a buck converter (eg LT3475) is required.

The LT3475 is a dual, 36V, 2MHz step-down DC/DC converter designed for use as a constant current dual LED driver. Each channel has an internal sense resistor and dimming control, making it ideal for driving LEDs that require up to 1.5A. The switching operation of one channel is 180° out of phase with the other, thus making two channels The output ripple is reduced. Each channel independently maintains high output current accuracy over a wide current range of 50mA to 1.5A, while the unique TrueColorPWMTM circuit provides a 3000:1 dimming range without any color cast Shift (this phenomenon is common in LED current dimming).

With its wide input voltage range of 4V to 36V (transient voltages up to 40V), the LT3475 is ideal for automotive power systems. Its switching frequency can be set between 200kHz and 2MHz, allowing the use of tiny inductors and ceramic capacitors and keeping switching noise away from the AM radio band. Coupled with a thermally enhanced TSSOP-20 package, the device offers a compact solution for driving high current LEDs.

The LT3475 uses high-side side sensing to achieve a ground connection to the LED's negative terminal, eliminating the need for a ground wire in most applications. It also has an integrated boost diode for each channel, further reducing the footprint and cost of the solution. Additional features include LED open and short circuit protection.

brake light

To date, the most common application of LEDs in automobiles has been the Central Overhead Parking Light (CHMSL). By the end of 2006, at least 60% of the cars had LED CHMSL installed. The benefits include faster lighting speed, higher efficiency, longer working life, and very thin red LED arrays. / Easy to install. LEDs can achieve full illumination in less than 1ms (compared to traditional bulbs can take up to 200ms to produce their maximum brightness), so that the driver of the rear vehicle will greatly reduce the time to identify the brake lights, thereby reducing The probability of a rear-end collision accident.

Moreover, compared with incandescent bulbs, power consumption has also dropped by as much as 80%, ultimately contributing to fuel consumption. Its effective service life will easily exceed the life of the vehicle, thus eliminating the need for replacement. In addition to CHMSL, some cars and motorcycles also replace incandescent brake lights with LEDs in the main brake lights.

In order to maximize the performance and operational life of these LED brake lights, it is essential to have a suitable LED driver that can drive the red LED strings required for these brake systems. Linear Technology's LT3486 was developed specifically for this type of automotive application. The LT3486 is a dual step-up DC/DC converter designed to drive up to 16 LEDs (each converter drives 8 series LEDs) from a single 12V to 14V automotive bus at a constant current. The use of LEDs in series provides equal LED current for uniform LED brightness. Two separate converters can also drive asymmetric LED strings when needed.

Dimming of the two LED strings can also be individually controlled by their respective CTRL pins. By feeding a PWM signal to the respective PWM pins, an internal PWM dimming system extends the dimming range up to 1000:1. The operating frequency of the LT3486 can be set from 200kHz to 2MHz with an external resistor. A low feedback voltage of 200mV (2% accuracy) minimizes power loss in the current setting resistor and is designed to increase efficiency. Additional features include an output voltage limit when the LED is disconnected. The LT3486 offers a very compact footprint and is available in a space-saving 16-pin DFN (5mm x 3mm x 0.75mm) package or a 16-lead thermally enhanced TSSOP package.

Summary of this article

LED lighting has generated many very specific performance requirements for LED driver ICs in high current LED automotive applications due to the unprecedented popularity of LED lighting in today's and tomorrow's automotive markets. The LED driver must provide a constant current to maintain uniform brightness (without input voltage or LED forward voltage variations) and must operate efficiently. They must also be able to withstand the rather demanding electrical characteristics of automotive power buses. In addition, these applications require a solution that is very compact and thermally efficient. Faced with these automotive design requirements, Linear Technology has developed a complete line of high current LED drivers designed to address these automotive challenges.