A type 3 ripple injection network is selected for this example. This network uses an RC filter consisting of R_A and C_A across SW and V_OUT to generate a triangular ramp that is in phase with the inductor current. This triangular ramp is then AC-coupled into the feedback node through capacitor C_B. Type 3 ripple injection is suited for applications where low output voltage ripple is crucial, and allows the use of low ESR ceramic output capacitors.

Equation 30 is used to calculate C_A. With the values used in this example, C_A > 184 pF. A value of 3300 pF is selected to keep R_A within practical limits. In general, the user needs 20 mV of ripple at the feedback pin for reliable operation, calculated at nominal input voltage. The minimum value of ripple should not be less than 12 mV at minimum input voltage. Using Equation 31 with nominal input voltage, a value of R_A > 120 kΩ was found and a value of 121 kΩ is selected.

While the magnitude of the generated ripple does not affect the output voltage ripple, it produces a DC error of approximately half the amplitude of the generated ripple, scaled by the feedback divider ratio. Therefore, the amount of DC offset, tolerable in the output voltage, imposes an upper bound on the feedback ripple.

Finally, Equation 32 is used to calculate the coupling capacitance C_B. In the equation, T_R is the approximate settling time of the control loop to a load transient disturbance. This was taken as 50 μs.

where

• T_R = 50 μs (typical)

In this example, a value of > 37 pF was calculated for C_B and a value of 56 pF is selected. This value avoids excessive coupling capacitor discharge by the feedback resistors during sleep intervals when operating at light loads. Note that the minimum value of C_B is 47 pF.