SLUSD49A September 2017 – January 2019 UCC256303
Refer to the PDF data sheet for device specific package drawings
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 VCR pin is with in 6 V.
As derived earlier, the following relationship between VCOMP voltage, ΔVCR, switching period, input average current, and the VCR capacitor divider is shown in Equation 66
In this equation, C1 is the upper capacitor on the capacitor divider; C2 is the lower capacitor on the capacitor divider. VCOMP is contributed by two parts – the divided resonant capacitor voltage, and the voltage generated by the VCR pin internal current sources. Define the contribution of the internal current source to be KVCRRamp.
Select C1 and C2 so that KVCRRamp is within 0.1 ~ 0.6 range, and at over load condition, VCOMP is less than 6 V. In this example C1 = 150 pF and C2 = 15 nF is select.