SLVSDV8 July 2017 TPS54424
The device provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and SW pins provides the gate drive voltage for the high side MOSFET. The BOOT capacitor is refreshed when the low-side MOSFET is on. The recommended value of the BOOT capacitor is 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is recommended for stable performance over temperature and voltage.
When operating with a low voltage difference from input to output, the high side MOSFET of the device will operate at 100% duty cycle as long as the BOOT to SW pin voltage is greater than 2.2 V. The device will begin to transition to 100% duty cycle operation when the high-side MOSFET off-time is less than 200 ns typical. When the voltage from BOOT to SW drops below 2.2 V, the high-side MOSFET is turned off due to BOOT UVLO and the low side MOSFET pulls SW low to recharge the BOOT capacitor. When operating at 100% duty cycle the high-side MOSFET can remain on for many switching cycles before the MOSFET is turned off to refresh the capacitor because the gate drive current sourced by the BOOT capacitor is small. The effective switching frequency reduced and the effective maximum duty cycle of the switching regulator is near 100%. The output voltage of the converter during dropout is mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, and the printed circuit board resistance.