SLVAF45 August   2021 TPS51397A , TPS566231 , TPS566235 , TPS566238 , TPS568230

 

  1.   Trademarks
  2. 1Introduction
  3. 2Why We Need Cff in High Output Voltage D-CAP2/3 Converter for Stability
  4. 3Effects of Feedforward Capacitor on the Loop
  5. 4Cff Choose Method
  6. 5Application Design Example of D-CAP2/D-CAP3 Converter with Cff
  7. 6Experimental Verification
  8. 7Conclusion
  9. 8References
  10. 9Appendix A

5 Application Design Example of D-CAP2/D-CAP3 Converter with Cff

Figure 5-1 is the design flow chart for D-CAP2/D-CAP3 converter with Cff. All the inductance and capacitance used in choosing Cff are effective value considering degrading.

Figure 5-1 Design Flow Chart of D-CAP2/D-CAP3 Converter with Cff

Here TPS568230 is used as an example to illustrate the design method for a 5V Vo application. The condition of example is Vin=12 V, Vo=5 V, Ioutmax=8 A, fsw=600kHz.

First select voltage divider resistors R1(Rtop)=220 kΩ and R2(Rbottom)=30 kΩ to achieve 5 V output with 0.6 V reference voltage. Then, same as the analysis mentioned in section 2, the range of inductance can be got as 1.52 uH-3.04 uH, according to the principle to limit inductor current ripple as 20%-40% of Ioutmax. And we can select 744311220 inductor and its effective inductance with 8 A current is about 1.8 uH.

About 180uF output capacitance are used to meet output ripple requirement. Here the 885012108012 MLCC are used. With about 52.5% degrading at 5 V bias for the 47 uF capacitance, the effective capacitance of each MLCC is about 22.35 uF. Capacitance of 8 parallel MLCCs are 178.8 uF.

Then the method is used to select Cff. As Acp=29.3 and ωRI=270 k for TPS568230, we can get Cff>44 pF according to Equation 14 and Equation 15. Here we can choose 120 pF Cff.