SNVSBK9C November 2019 – September 2020 LM63635-Q1
Every switching regulator has a minimum controllable on-time dictated by the inherent delays and blanking times associated with the control circuits. This imposes a minimum switch duty cycle and, therefore, a minimum conversion ratio. The constraint is encountered at high input voltages and low output voltages. To help extend the minimum controllable duty cycle, the LM63635-Q1 automatically reduces the switching frequency when the minimum on-time limit is reached. This way, the converter can regulate the lowest programmable output voltage at the maximum input voltage. Use Equation 2 to find an estimate for the approximate input voltage for a given output voltage before frequency foldback occurs. The values of tON and ƒSW can be found in Section 7. As the input voltage is increased, the switch on-time (duty-cycle) reduces to regulate the output voltage. When the on-time reaches the limit, the switching frequency drops while the on-time remains fixed. This relationship is highlighted in ƒSW versus VIN curves in Section 9.2.4.