SNVS787I January 2012 – August 2021 LM5018
A type II ripple circuit, as described in the Section 7.3.11, section is chosen for this example. For a constant on-time converter to be stable, the injected in-phase ripple must be larger than the capacitive ripple on COUT.
Using type II ripple circuit equations with a minimum FB pin ripple of 25 mV, the values of the series resistor RC and ac coupling capacitor Cac can calculated.
Assuming RFB2 = 6.98 kΩ and RFB1 = 1 kΩ, the calculated minimum value of Cac is 0.013 µF. A standard value of 0.1 µF is selected for Cac = C8. The value of the series output resistor RC is calculated for the minimum input voltage condition when the inductor ripple current as at a minimum. Using Equation 13 and assuming VIN = 12.5 V, the minimum inductor ripple current is 27 mA. The calculated minimum value of RC is 0.93 Ω. A standard value of 1.5 Ω is selected for RC = R2 to provide additional ripple for stable switching at low VIN.