Although LEDs have been used in many automotive lighting applications for several years, such as daytime running lights (DRL), brake lights, turn indicators, and interior lighting, headlamp-specific applications are still relatively new. Currently, only a small number of cars have LED headlights, including Honda Accord, Acura RLX and MDX, Audi A8 and R8, Lexus LS600h and RX450h, Toyota Prius, Cadillac's Escalade and Porsche Cayenne. More automotive platforms have adopted DRL, and their shapes often vary with different car brands. Some industry forecast data show that by 2014, the LED DRL / headlamp market will exceed $4 billion, and since the market continues to grow rapidly since then, it is expected to exceed $8 billion by 2015.
One of the biggest challenges for automotive lighting system designers is how to make the most of all the benefits of the latest generation of high-brightness (HB) LEDs. HB LEDs require an accurate, efficient, dimmable DC current source and must include protection. In addition, these LED driver ICs must be designed to meet these requirements in a variety of environmental and electrical conditions. Therefore, power solutions must be very efficient, low noise, powerful, and reliable, yet compact and affordable. It can be said that the most demanding application for driving HB LEDs is automotive forward lighting applications, namely DRL and headlamp applications, because these two applications are in the harshest automotive electrical environment and must provide a lot of power. Typically between 15W and 75W, and must be placed in a very limited space enclosure to meet all of these requirements while maintaining an attractive cost structure.
Design ParametersAutomotive LED drivers must be compact, efficient, and support flicker-free PWM dimming. These drivers do not produce significant conducted EMI around the AM radio band and around this band. Unfortunately, high-power switch-mode power supplies are not inherently low-EMI, and the constant switching frequency produces significant EMI components at some frequencies, including the fundamental operating frequency of the power supply and its harmonics. But these bad things always fall into the AM band.
One way to minimize EMI peaks is to allow the switching mode power supply (SMPS) operating frequency to cover a range of frequency values ​​by using spread spectrum switching. It is hoped that the spread spectrum switching will reduce the EMI peaks that may occur at the basic operating frequency of the SMPS and its harmonics, extending the EMI energy over a range of frequencies.
The LED driver SPMS has an additional requirement: the frequency extension is also synchronized with the PWM dimming (brightness control) signal frequency to ensure that no LED flicker is produced.
To solve this problem, the LT3795 generates its own spread spectrum ramp signal and uses a patent-pending technique to match the signal to the lower frequency PWM dimming input. This eliminates the possibility of combining the spread spectrum signal with the PWM signal to produce visible flicker of the LED, even at the highest PWM dimming ratio.
High-power automotive LED driverThe LT3795 is a high-power LED driver that uses the same high-performance PWM dimming method as the LT3756 / LT3796 series, but adds the ability to internally generate spread-spectrum ramp signals to reduce EMI. The LT3795 is a single-switch controller IC with an input range of 4.5V to 110V and an output range of 0V to 110V. It can be configured as a boost mode, SEPIC, buck-boost mode or buck mode LED driver. The device features a 100kHz to 1MHz switching frequency range, LED open-circuit protection, and short-circuit protection. It can also operate as a constant voltage regulator with current limit or as a constant current SLA battery or supercapacitor charger.
The LT3795 generates its own spread spectrum ramp signal and uses patent-pending technology to match the signal to the lower frequency PWM dimming input. This way, even at the highest PWM dimming ratio, the spread spectrum signal cannot be combined with the PWM signal, causing the LED to produce visible flicker.
Figure 1 shows an automotive LED headlamp driver with up to 92%, 80V, 400mA, 300kHz to 450kHz efficiency with spread spectrum frequency modulation and short circuit protection. DRL applications look almost identical, but the maximum LED current requirement is close to 200mA.
Figure 1: 80V, 400mA Automotive LED Driver with Internal Spread Spectrum to Reduce EMI
Internal spread spectrum reduces EMI issuesUnlike many high power LED drivers, the LT3795 generates its own spread spectrum ramp signal to produce a switching frequency modulation that is 30% lower than the set switching frequency. This reduces the EMI peaks that it conducts, reducing the need for expensive and bulky EMI input filter capacitors and inductors.
Using an internal or separate spread-spectrum clock to generate the switching frequency of the LED driver may produce visible flicker during PWM dimming because the spread spectrum frequency variation is not synchronized with the PWM period. For this reason, implementing spread spectrum is not a trivial task in many high-end LED driver applications. Without spread spectrum, designers must rely on bulky EMI filters, gate resistors that reduce the edge of the switch (which reduces efficiency), and shock absorbers on the switch and clamp diodes.
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