8.2.2.3 Option C: Minimum VOUT Ripple Configuration
In some applications, the VOUT ripple induced by series resistor R1 may not be acceptable. When low VOUT ripple is required, an external ripple circuit, as shown in Figure 24, can be used to provide the required ripple at the FB pin.
- The time constant Rr × Cr should be more than 8 to 10 times the switching period to generate a triangular waveform at the junction of Rr, Cr and Cac.
- The minimum ripple at FB (at minimum VIN) is equal to: VRIPPLE = (VIN(min) – VOUT) × TON(max) / (Rr x Cr).
- The ripple capacitor Cr should much smaller than the ac coupling capacitor Cac. Typically Cac = 100 nF, Cr = 1 nF, and Rr is chosen to satisfy conditions 1 and 2 above.