The capacitor divider on the VCR pin sets two parameters: (1) the divider ratio of the resonant capacitor voltage; (2) the amount of frequency compensation to be added. The first criteria the capacitor divider needs to meet is that under over load condition, the peak-to-peak voltage on the VCR pin is with in 6 V. It is recommended to size the VCR capacitance to give a total peak to peak voltage between 3 V and 4.5 V at full load with the frequency compensation ramp contributing between 1 V and 2 V to the total VCR peak to peak voltage. For this design, the VCR pin capacitance was selected to give a maximum peak to peak voltage of approximately 4.25 V at full load with the internal ramp contributing 1.75 V to the total VCR waveform.
The required VCR capacitance can be calculated directly from the resonant capacitor peak to peak voltage and the minimum expected switching frequency.
Based on the expected peak to peak resonant capacitor voltage, the required capacitor divider ratio can be derived
From the expected minimum switching frequency, the lower VCR capacitance can be derived
A standard value of 8.2 nF is chosen for the lower VCR capacitor. From the selected lower VCR capacitor and calculated capacitor divider ratio, the upper VCR capacitance is given by:
A standard value of 68 pF is selected for the upper VCR capacitor. From the selected upper and lower VCR capacitors, the actual peak to peak voltage on the VCR pin can be calculated