SNVSBZ3 June 2021 LM5168-Q1
A Type-3 ripple generation network uses an RC filter consisting of RA and CA across SW and VOUT1 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 CB. Type-3 ripple injection is suited for applications where low output voltage ripple is crucial, and is chosen for this example.
Equation 29 is used to calculate CA. With the values used in this example, CA > 245 pF. A value of 3300 pF is selected to keep RA 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 30 with nominal input voltage, a value of RA > 117 kΩ was found and a value of 118 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 feed-back ripple.
Finally, Equation 31 is used to calculate the coupling capacitance CB. In the equation, TR is the approximate settling time of the control loop to a load transient disturbance. This was taken as 50 μs.
In this example, a value of > 37 pF was calculated for CB 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.