Automotive current LED driver

16 October 2014

One of the biggest challenges for automotive lighting systems designers is how to optimize all the benefits of the latest generation of HB LEDs.

Currently, only a handful of production vehicles are offered with LED headlamps. 
Arguably, the most demanding applications for driving HB LEDs are found in automotive forward lighting applications, in both DRLs and headlamps since they are subjected to the rigors of the automotive electrical environment, must deliver high power, typically between 15W to 75W, and must fit into every space, all while maintaining an attractive cost structure. 

Design Parameters

Automotive LED drivers must be compact, efficient and support flicker-free PWM dimming. They should not produce significant conducted EMI at and around the AM radio band. Unfortunately, low EMI is not in the nature of high power switch mode power supplies—the constant switching frequency produces a significant EMI signature at a number of frequencies, including the power supply’s fundamental operating frequency and its harmonics.

One way to minimise EMI peaks is to allow the switch mode power supply
(SMPS) operating frequency to cover a range of values by incorporating spread spectrum switching. The desired effect of spread spectrum switching is to push down the EMI peaks that would occur at the SMPS fundamental operating frequency and harmonics, spreading the EMI energy over a range of frequencies instead.
The frequency spread¬ing should also be synchronised with the PWM dimming (brightness control) frequency to ensure that there is no resulting LED flicker.

To solve this problem, the LT3795 generates its own spread spectrum ramp signal and aligns it with the lower frequency PWM dim¬ming input with a patent pending tech¬nique. This eliminates the chance that the spread spectrum frequency could combine with the PWM signal to pro¬duce visible flicker in the LEDs—even at the highest PWM dimming ratio.

Automotive LED drivers

The LT3795 is a high power LED driver that uses the same high performance PWM dim¬ming scheme as the LT3756/LT3796 fam¬ily, but with the additional feature of the internal spread spectrum ramp for reduced EMI. It is a 4.5-to-110V input to 0-to-110V output single-switch controller IC that can be configured as a boost, SEPIC, buck-boost mode or buck mode LED driver. It features a 100kHz to 1MHz switching frequency range, open LED protection, short-circuit protection, and can also be operated as a constant voltage regulator with current limit or as a constant current SLA battery or supercapacitor charger.  
Figure 1 shows a 92% high efficiency 80V, 400mA, 300-450kHz auto¬motive LED headlamp driver with spread spectrum frequency modula¬tion and short-circuit protection. A DRL application will looks nearly identical but with a maximum LED current requirement closer to 200mA.

Reducing EMI Issues

The LT3795 produces a 30% switch¬ing frequency modulation below its programmed switching frequency. This lowers its conducted EMI peaks, reducing the need for costly and bulky EMI input filter capacitors and inductors.
Using an external spread spectrum clock to produce the switching frequency in an LED driver it can produce visible flicker during PWM dimming since the spread spectrum frequency pattern is not synchronised with the PWM period. For this reason, implementing spread spectrum is non-trivial. Without spread spectrum, designers must rely upon bulky EMI filters, gate resistors that slow down switching edges and snubbers on the switch and catch diode.

Figure 2 shows a comparison of the conducted EMI measurements of the LT3795 LED driver around the AM band when spread spectrum is enabled and disabled. Normal (non-spread spectrum) operation yields high energy peaks at the switching frequency and its harmon¬ics. These peaks can prevent the design from passing stringent EMI require¬ments in EMI sensitive applications such as automobiles. For reference, the CISPR 25 class 5 automotive con¬ducted EMI limits are shown in Figure 2. Figure 3 shows the effect of spread spec¬trum over a wider frequency band.

Since there is no limit between 300-580kHz, this is an excellent range for the fundamental frequency to be placed. In this application it is placed at 450kHz and spreads down to 300kHz. Spread spectrum can be disabled by simply grounding the RAMP pin.

The 6.8nF capacitor at the RAMP pin sets the spread spectrum frequency modula¬tion rate to a 1kHz triangle—that is, the LT3795’s operating frequency sweeps from 300 to 450kHz and back every millisecond. The addition of the tri¬angular 1kHz spread spectrum signal has a negligible effect on LED ripple current, as shown in Figure 4.

The modulation frequency of 1kHz is cho¬sen because it is low enough to be within the LT3795’s bandwidth, yet high enough to significantly attenuate AM-band conducted EMI peaks. Further reducing the modula¬tion frequency degrades peak attenuation in the AM band, where it may be most important for classification. The choice of spread spectrum modulation frequency does not appear to affect EMI peak attenu¬ation at higher frequencies.

Flicker-free PWM dimming

It is possible to reduce EMI with a spread spectrum source that is not synchro¬nised with the PWM signal, but the beat of the switching frequency and PWM signal can produce visible flicker in the LED. The spread spectrum ramp generated inside the LT3795 synchro¬nises itself with the PWM period when PWM dimming is used. This provides repeatable, flicker-free PWM dimming, even at high dimming ratios of 1000:1.

Figure 5 compares the PWM dimming current waveforms of two spread spec¬trum solutions: one with the LT3795’s spread spectrum to PWM synchronisation technique, and one without. Both scope captures are produced with infinite persist, showing an overlay of a number of cycles of a 1% PWM dim¬ming waveform. Figure 5(a) shows the result of LT3795’s spread spectrum operation on the PWM LED current. The waveform is consistent cycle-to-cycle, which results in flicker-free opera¬tion. Figure 5(b) shows the results of a comparable, non-LT3795, spread spectrum solution. The cycle-to-cycle variation in on-time shape produces variation in average LED current, which can be seen as LED flicker at high dimming ratios.

Note that spread spectrum driver ICs without the LT3795’s patented technique might produce a clean spread spectrum EMI reduction result, the flicker may still be present. One has to observe the LEDs or the LED current waveform to under¬stand if flicker is present. In the case of the LT3795, both the conducted EMI scan and the scope shot of LED current are good.

Short-circuit proof boost

The LT3795 boost LED driver is short-circuit proof. The high side PMOS disconnect is not only used for PWM dimming, but also for short-circuit protection when the LED+ terminal is shorted to ground. Unique internal circuitry monitors when the output current is too high and the LED+ volt¬age is too low, turns off the disconnect PMOS and reports a short LED fault. Similarly, if the LED string is removed or opened, the IC limits its maximum output voltage and reports an open LED fault.

Multitopology Solution

The LT3795 can be used to drive LEDs in a boost setup, or it can be used in buck mode, buck-boost mode, SEPIC and flyback topologies when the relationship of the LED string voltage and input voltage ranges requires it. All topolo¬gies feature the same spread spectrum and short-circuit protection. The LT3795 can even be configured as a constant boost or SEPIC voltage regulator with spread spectrum frequency modulation.

The continual acceleration of HB LED applications, especially those found in automotive DRLs and headlamps is being driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new robust HB LED driver ICs. LED drivers such as the LT3795 offer solutions for applications with inputs up to 110V. It also offers built-in spread spectrum frequency modulation to reduce EMI. This simplifies the design of LED applications that must pass stringent EMI testing. Spread spectrum requires only a single capacitor and produces flicker-free LED opera¬tion during PWM dimming. Short-circuit protection is available in all topologies, making this IC a robust and powerful solution for driving automotive LEDs.

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