JAJSBF8B June 2011 – April 2018 TPS54478
PRODUCTION DATA.
The TPS54478 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT and PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is recommended because of the stable characteristics over temperature and voltage.
To improve drop out, the TPS54478 is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is greater than 2.2 V. The high side MOSFET is turned off using an UVLO circuit, allowing for the low side MOSFET to conduct when the voltage from BOOT to PH drops below 2.2 V. Since the supply current sourced from the BOOT pin is very low, the high side MOSFET can remain on for more switching cycles than are required to refresh the capacitor, thus the effective duty cycle of the switching regulator is very high.