PWM controller drives LEDs from high-voltage lines


Posted on Jun 26, 2012

The circuit forces a current to build up in the L1 coil and the LEDs until the voltage developed across R3 reaches VFB/3.3V. At this point, power switch Q1 turns off, and the magnetizing current keeps circulating in the coil and LEDs, thanks to freewheeling diode D1. To maintain a `clean` current in the LEDs, L1 must be large enough to keep the ripple to an acceptable value and to avoid pushing the controller to the minimum on-time (400 nsec) in high-line conditions. Because of the poor TRR (reverse-recovery time) of the LEDs, you must add an external filter, comprising R2 and C1 to the IC`s internal leading-edge-blanking circuitry. R1 sets the voltage-feedback level; keeping it lower than 3.3V prevents the NCP1200A`s internal short-circuit protection from tripping. In the example, the feedback voltage of 2.5V thus imposes a peak current of 2.5/3.3/4.7=161 mA.






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