4.5 MOSFET Gate Drive

The Functional Block Diagram shows a bootstrap circuit, consisting of D1 (a Schottky diode is recommended) and C_BST. This circuit supplies energy to the high-side drive circuit. Capacitor C_BST is charged, while the low-side MOSFET is on, and the voltage on the SW pin is approximately 0V. When the high-side MOSFET driver is turned on, energy from C_BST is used to turn the MOSFET on. As the high-side MOSFET turns on, the voltage on the SW pin increases to approximately V_IN. Diode D1 is reverse biased and C_BST floats high while continuing to keep the high-side MOSFET on. The bias current of the high-side driver is less than 10mA, so a 0.1μF to 1μF capacitor is sufficient to hold the gate voltage with minimal droop for the power stroke (high-side switching) cycle, (i.e. ΔBST = 10mA x 3.33μs/0.1μF = 333mV). When the low-side MOSFET is turned back on, C_BST is recharged through D1. A small resistor R_G, which is in series with C_BST, can be used to slow down the turn-on time of the high-side N-channel MOSFET.

The drive voltage is derived from the V_DD supply voltage. The nominal low-side gate drive voltage is V_DD and the nominal high-side gate drive voltage is approximately V_DD – V_DIODE, where V_DIODE is the voltage drop across D1. An approximate 30ns delay between the high-side and low-side driver transitions is used to prevent current from simultaneously flowing unimpeded through both MOSFETs.