UC2823 capable for fixed off-time current controlled on-time

Kajino-san,

The app note U-111 (slua110) is a general app note for volt age-mode and current-mode PWM's, so most of the topics can be applied to most PWM controllers.

The UC3843 uses a single RT/CT pin, whereas the UC2823 uses 1 separate pins. The UC2823 RT pin uses a resistor to GND to set the osc charging current, this is internally mirrored and then fed to the CT pin to charge the timing cap.

So to adapt the cct of Fig. 35 in U-111 to work with UC2823, the RT & CT pins should not be tied together. Instead, the RT pin uses a timing resistor to GND as usual, and the CT pin uses a timing to cap to GND. Then the OUT pin drives a shorting npn Q1 across CT cap through R1/R2 (same as Fig. 35).

This is how the cct. should look:

I hope this helps answer your question.

Thanks,

Bernard

temp.pdf

The cct. did not paste correctly above, here it is again as an attachment.