UC3842: Oscillator charging ramp equations

Hi

The oscillator equation in the UC3842 data sheet is correct. The oscillator is a time varying voltage according to a linear ramp resulting from a constant current charging a fixed capacitor, where the constant current is determined from RT and the fixed capacitor is CT.

Regards,

Steve M

You are correct that V is changing, the current is constant. Instead of thinking about V=IR, consider the rate of change of capacitor voltage with respect to time:

I=C(dV/dt) ---> dV/dt=I/C or dV=(I/C)dt

The current is constant, the capacitor is constant and the voltage is time varying. The CT capacitor is charged (positive RTCT slope) by the constant current through RT up to ~1.7V (plus some small offset of ~0.6V). The CT capacitor is then discharged (negative RTCT slope) through the RTCT pin and the charge cycle repeats. This is the basic principle demonstrating how a PWM oscillator works.

Steve M