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How to reduce EMI without adding flicker

Posted: 16 Jan 2014     Print Version  Bookmark and Share

Keywords:LED drivers  flicker-free  PWM dimming  SMPS  EMI 

Since there is no limit between 300kHz and 580kHz, that is an excellent place for the funda-mental frequency to be placed. In this application it is placed at 450kHz and spread 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 modulation rate to a 1kHz triangle, that is, the LT3795's operating frequency sweeps from 300kHz to 450kHz and back every millisecond. The addition of the triangular 1kHz spread spectrum signal has a negligible effect on LED ripple current, as shown in figure 4.

Figure 4: Spread spectrum as implemented in the LT3795 has no discernable effect on LED brightness. The 1kHz spread spectrum sweep set in figure 1 has a negligible effect on LED ripple current (b) when compared to no spread spectrum (a) and is much too high a frequen-cy to be detected by the human eye as flicker.

The modulation frequency of 1kHz is chosen 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 modulation frequency degrades peak attenuation in the AM band, where it may be most important for classification. The choice of spread spectrum mod-ulation frequency does not appear to affect EMI peak attenuation at higher frequencies. Noth-ing above 100Hz is perceived by the human eye.

Figure 5: Comparison of two spread spectrum LED driver solutions and the effect on PWM dimming. In (a), the patentpending spread spectrum technique of the LT3795 produces consistent cycle-to-cycle LED PWM on-time shape. The waveforms in (b) show a comparable, non-LT3795, spread spectrum LED driver result.

Flicker-free PWM dimming
It is possible to reduce EMI with a spread spectrum source that is not synchronised 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 synchronises it-self 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 spectrum solutions: one with the LT3795's patent-pending spread-spectrum-to-PWM synchronisation technique, and one without. Both captures are produced with infinite persist, showing an overlay of a number of cycles of a 1% PWM dimming waveform. Figure 5a 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 operation. Figure 5b 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 but the flicker may still be present. One has to observe the LEDs or the LED current waveform to understand 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 shown in figure 1 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 cur-rent is too high and the LED+ voltage 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.

Multi topology solution
The LT3795 can be used to drive LEDs in a boost setup as shown here, 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 topologies 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.

About the author
Keith Szolusha is Applications Engineering Section Lead for Power Products at Linear Technology.

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