The TK75003 is a simple primary side controller optimized for off-line switching power supplies including power factor correctors. It is suitable for both voltage-mode and currentmode control and has advanced features not available in controllers with a higher pin count. The key to full functionality in a 5-pin design is that the current signal and the error signal are added together and fed into the feedback pin. A sawtooth current flowing out of the feedback pin provides a slope compensation ramp (in current-mode applications) or a PWM ramp (in voltage-mode applications), in proportion to the resistance terminating that pin. If the sum of the current sense signal, error signal and ramp signal exceeds the Overcurrent Detector threshold indicating that the Current Control Detector has lost control of the switch current, the charging current of the timing capacitor will be reduced to about 25% for the remainder of the clock period. The reduced charging current causes no more than a one-third reduction in switching frequency, effectively preventing short-circuit current runaway.