5.10 High-Side Drive and Bootstrap

The MCP16367 features an integrated high-side N-Channel MOSFET for high efficiency step-down power conversion. An N-Channel MOSFET is used for its low resistance and size (instead of a P-Channel MOSFET). The N-Channel MOSFET gate must be driven above its source to fully turn on the transistor. A gate drive voltage above the input supply is necessary to turn on the high-side N-Channel MOSFET. The high-side drive voltage must be between 3.0V and 5.5V.

The N-Channel MOSFET source is connected to the inductor and Schottky diode or switch node. When the switch is off, the inductor current flows through the Schottky diode, providing a path to recharge the bootstrap capacitor from the boost voltage source, typically the output voltage for 3.0V to 5.5V output applications.

Prior to start-up, the bootstrap capacitor has no stored charge to drive the switch. An internal regulator is used to “precharge” the bootstrap capacitor. When precharged, the switch is turned on and the inductor current starts to flow. When the switch turns off, the inductor current free-wheels through the Schottky diode, providing a path to recharge the bootstrap capacitor. Worst case conditions for recharge occur when the switch turns off for a very short time, at light load, limiting the inductor current ramp. In this case, there is a small amount of time for the bootstrap capacitor to recharge. For high-input voltages, there is enough precharge current to replenish the bootstrap capacitor charge. For input voltages above 5.5V typical, the MCP16367/8/9 device will regulate the output voltage with no load. After starting, the MCP16367/8/9 will regulate the output voltage until the input voltage decreases below 4V.