SLUS829G August 2008 – February 2020 UCC2897A
CS is a direct input to the PWM and current-limit comparators of the UCC2897A controller. The CS pin never connects directly across the current-sense resistor (RCS) of the power converter. A small, customary R-C filter between the current-sense resistor and the CS pin is necessary to accommodate the proper operation of the onboard slope-compensation circuit and in order to protect the interna- discharge transistor connected to the CS pin (RF1, CF).
Slope compensation is achieved across RF by a linearly-increasing current flowing out of the CS pin. The slop- compensation current is only present during the on-time of the gate-drive signal of the main-power switch (OUT) of the converter. The internal-pulldown transistor of the CS pin is activated during the discharge time of the timing capacitor. This time interval is (1 – DMAX) × TSW long and represents the specified off-time of the main-power switch.