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In direct feedback applications, CCG3PA uses an IDAC connected to the Feedback pin of DC-to-DC to control the converter’s output voltage. Whenever a change in VBUS voltage is needed, CCG3PA will either source or sink a proportional current at the feedback node, based on the amount of voltage change needed. When the default value of VBUS upon power up is 5 V, the feedback pin of CCG3PA should be close to 1.2 V.
Figure 1. Two-Resistor Divider
For converters that have the same default feedback target voltage of 1.2 V, a two-resistor divider network is needed. Because the PPS voltage step should be 20 mV, R1 should be 20 mV/0.1 uA= 200 kΩ. When R1 is resolved, R2 could be calculated from the equation VFB=(5V*R2)/(R1+R2). The recommended value for R2 is 68 kΩ.
Figure 2. Three-Resistor Divider
For converters that have a lower feedback target voltage such as TI LM5175, TI LM5176, TI LM34396, a three-resistor divider network is needed. R1 should still be 200 kΩ because of the PPS voltage step requirement. If the feedback target voltage of DCDC is Vref, you should set the correct value for R2 and R3 so that VFB(CCG3PA)=1.2 and VFB(DCDC) = Vref. Solve these two equations and you get:
R2=63157.894 – 52631.833* Vref
Some common used values of resistors based on different reference voltages are listed in Table.1.
Default VBUS value (V)
Table 1. Quick Reference Table
For the more information about CCG3PA power adapter design, please refer to AN218238.