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. PWM dimming is widely used to adjust the luminance of lighting emitting diode (LED) due to the advantage of no LED shimming. When the dimming ratio is relatively small, the LED current maybe unable to increase from zero to a desired level and thus the LED may be undersupplied with electricity. This paper proposed a new PWM dimming technique having a minimum switching cycle. The LED driver utilizing this technique comprises a boost converter having a pulse width detect circuit. The pulse width detect circuit compares the pulse width of the PWM dimming signal with a preset time, if the pulse width of the dimming signal is greater than the preset time, the boost converter is controlled by the dimming signal; if the width of the dimming signal is smaller than the pre-set time, the booster converter keeps its on-time equaling to the pre-set time, so the boost converter could transfer enough energy to the output capacitor to supply the LED. The LED driver is fabricated in SMIC with 0.35 um Bi-CMOS technology, silicon tests show that the LED driver could adapt all range of pulse width of dimming signal, even for a micro-second level pulse width.

PWM Dimming with Minimum Switching Cycle