COVER STORY FEATURE
These peaks can prevent the design from passing stringent EMI requirements in EMI sensitive applications such as automobiles. For reference, the CISPR 25 class 5 automotive conducted EMI limits are shown in Figure 2. Figure 3 shows the effect of spread spectrum over a wider frequency band. Since there is no limit between 300kHz and 580kHz, that is an excellent place for the fundamental 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. 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 modulation frequency does not appear to affect EMI peak attenuation at higher frequencies. Nothing above 100Hz is perceived by the human eye.
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 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 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.
/ ELECTRONICS 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 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.
Figure 4 (a) & (b)
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 current 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.
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 frequency to be detected by the human eye as flicker
MULTITOPOLOGY 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. The LT3795 is an 110V, versatile LED
driver IC with 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 unlike external- clock-based spread spectrum solutions, produces flicker-free LED operation during PWM dimming. Short-circuit protection is available in all topologies, making this IC a robust and powerful solution for driving automotive LEDs.
Figure 5 (a) & (b)
Linear Technology (UK) Ltd.
www.linear.com 01628 477 066
Enter 202
Figure 5. Comparison of two spread spectrum LED driver solutions and the effect on PWM dimming. The infinite persist scope captures show repeated and overlaid PWM LED current waveforms. In (a), the patent-pending spread spectrum technique of the LT3795 produces consistent cycle-to-cycle LED PWM on-time shape. The result is flicker-free operation at high dimming ratios. The waveforms in (b) show a comparable, non-LT3795, spread spectrum LED driver result. In this case, without the spread-spectrum-to- PWM synchronisation of the LT3795, the LED current waveform is inconsistent cycle-to-cycle, producing perceivable flicker at high PWM dimming ratios
ELECTRONICS | JUNE 2015 13
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