VDD

CT

SNS

RST

MR

GND

UCC28810

UCC28811

Triac Dimming Detection Circuit

The comparator U5 compares the copied input voltage waveform to a fixed threshold, and emits a positive pulse

any time the input waveform is detected to be below the threshold. This results in a small pulse at every zero

crossing in normal operation. When a triac dimmer is used, the pulse width matches the triac dimmer off-time.

This pulse sums in to the current-sense error amplifier and reduces the current regulated in the string

proportional to the off time of the triac. The programmable delay supervisory component, U6, blanks the dimming

at startup and allows the converter to start properly.

Figure 23. Triac Dimming Detection Circuit

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