"Background information
Although LEDs have been used in many automotive lighting applications for several years, such as daytime running lights (DRL), brake lights, turn signal lights and interior lighting, applications specific to headlights are still relatively new. At present, only a few mass-produced vehicles have LED headlights, including Honda Accord, Acura RLX and mdx, Audi A8 and R8, Lexus ls600h and rx450h, Toyota Prius, Cadillac's Carlyle and Porsche Cayenne. More car platforms have adopted DRL, and its shape often varies with different car brands. Some industry forecast data show that the LED DRL / headlights market will exceed US $4 billion by 2014, and since the market continues to grow rapidly, it is expected to exceed US $8 billion by 2015.
One of the biggest challenges facing automotive lighting system designers is how to make the most of all the advantages of the latest generation of high brightness (HB) LEDs. HB led needs an accurate, efficient and dimmable DC current source, and must include protection function. In addition, such LED driver ICs must be designed to meet these requirements under various environmental and electrical conditions. Therefore, the power solution must be very efficient, low noise, powerful and reliable, compact and affordable. It can be said that in terms of driving HB led, the most demanding applications are automotive forward lighting applications, i.e. DRL and headlamp applications, because these two applications are in the most severe automotive electrical environment, must provide a large power, generally between 15W and 75W, and must be placed in a shell with very limited space to meet all these requirements, And maintain an attractive cost structure.
Design parameters
Automotive LED drivers must be compact, efficient and support flicker free PWM dimming. These drivers cannot generate large conducted EMI in and around the AM radio band. Unfortunately, the high-power switching mode power supply is not low EMI in essence. The constant switching frequency produces a great EMI component at some frequency points, including the basic operating frequency of the power supply and its harmonics. But these bad things always fall into the AM band.
One way to minimize EMI peak is to allow the operating frequency of switching mode power supply (SMPS) to cover a series of frequency values by using spread spectrum switching. It is hoped that the spread spectrum switching will play a role in reducing the EMI peak that may appear on the SMPS basic operating frequency and its harmonics, and expanding the EMI energy to a series of frequency ranges.
The LED driver SPMS has an additional requirement: the frequency expansion should also be synchronized with the PWM dimming (brightness control) signal frequency to ensure that the LED does not flash.
To solve this problem, lt3795 generates an extended spectrum ramp signal by itself and uses a patented technology to make the signal consistent with the PWM dimming input with lower frequency. In this way, even when the PWM dimming ratio is the highest, the possibility of combining the spread spectrum signal with the PWM signal to make the LED produce visible flicker can be eliminated.
High power automobile LED driver
Lt3795 is a high-power LED driver. It adopts the same high-performance PWM dimming method as lt3756 / lt3796 series, but adds the function of internally generating spread spectrum ramp signal to reduce EMI. Lt3795 is a single switch controller IC with input range of 4.5V to 110V and output range of 0V to 110V. It can be configured as boost mode, SEPIC, buck boost mode or buck mode LED driver. The device has a switching frequency range of 100kHz to 1MHz, led open circuit protection and short circuit protection. It can also work as a constant voltage regulator with current limit, or as a constant current SLA Battery or super capacitor charger.
Lt3795 generates its own spread spectrum ramp signal, and uses the patent pending technology to make the signal consistent with the lower frequency PWM dimming input. In this way, even at the highest PWM dimming ratio, the spread spectrum signal cannot be combined with the PWM signal to make the LED flash visible.
Figure 1 shows an automobile LED headlamp driver with efficiency up to 92%, 80V, 400mA, 300kHz to 450kHZ. The driver has spread spectrum frequency modulation and short circuit protection. DRL applications look almost the same, but the maximum LED current requirement is close to 200mA.
Figure 1: 80V, 400mA automotive LED Driver with internal spread spectrum function to reduce EMI
Internal spread spectrum reduces EMI problems
Unlike many high-power LED drivers, the lt3795 generates its own spread spectrum ramp signal to generate switching frequency modulation 30% lower than the set switching frequency. This reduces the conducted EMI peak and reduces 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 change is not synchronized with the PWM cycle. For this reason, in many high-end LED driver applications, achieving spread spectrum is not a trivial task. Without spread spectrum, designers must rely on bulky EMI filters, reduce the gate resistance at the edge of the switch (which will reduce efficiency), and install shock absorbers on the switch and clamping diodes., Read the full text, technical section
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