A Type 3 ripple generation 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 using capacitor C_B as shown in Figure 8-1. Type 3 ripple injection is suited for applications where low output voltage ripple is crucial.

Use Equation 16 and Equation 17 to calculate R_A and C_A to provide the required ripple amplitude at the FB pin.

Equation 16.

For the feedback resistors R_FBT = 453 kΩ and R_FBB = 49.9 kΩ values shown in Figure 8-1, Equation 16 dictates a minimum C_A of 742 pF. In this design, a 3300-pF capacitance is chosen. This is done to keep R_A within practical limits between 100 kΩ and 1 MΩ when using Equation 17.

Equation 17.

Based on C_A set at 3.3 nF, R_A is calculated to be 453 kΩ to provide a 20-mV ripple voltage at FB. The general recommendation for a Type 3 network is to calculate R_A and C_A to get 20 mV of ripple at typical operating conditions. A smaller R_A can be required to operate below nominal 48-V input.

While the amplitude of the generated ripple does not affect the output voltage ripple, it impacts the output regulation as it reflects as a DC error of approximately half the amplitude of the generated ripple. For example, a converter circuit with Type 3 network that generates a 40-mV ripple voltage at the feedback node has approximately 10-mV worse load regulation scaled up through the FB divider to V_OUT than the same circuit that generates a 20-mV ripple at FB. Use Equation 18 to calculate the coupling capacitance, C_B.

Equation 18.

where

• t_TR-settling is the desired load transient response settling time.

C_B calculates to 56 pF based on a 75-µs settling time. This value avoids excessive coupling capacitor discharge by the feedback resistors during sleep intervals when operating at light loads. To avoid capacitance fall-off with DC bias, use a C0G or NP0 dielectric capacitor for C_B.