SLUS829G August 2008 – February 2020 UCC2897A
FB and SS/SD interact. The one with the lower-voltage value takes control on the duty cycle, refer to SS/SD description. This pin is an input for the control voltage of the pulse-width modulator of the UCC2897A.The control voltage is generated by an external-error amplifier by comparing the output voltage of the converter to a voltage reference and employing the compensation for the voltage-regulation loop. Usually, the error amplifier is located on the secondary side of the isolated-power converter and the output voltage is sent across the isolation boundary by an optocoupler. Thus, the FB pin is usually driven by the optocoupler. An external-pullup resistor to the VREF pin (pin 4) is also required for proper operation as part of the feedback circuitry.
The control voltage is internally buffered and connected to the PWM comparator through a voltage divider to make it compatible to the signal level of the current-sense circuit. The useful voltage range of the FB pin is between approximately 2.5 V and 4.5 V. Control voltages below the 2.5-V threshold result in zero-duty cycle (pulse skipping) while voltages above 4.5 V result in full-duty-cycle (DMAX) operation.