When the device runs at a low switching frequency (for example, less than 10 kHz with a 100-nF bootstrap capacitor), the bootstrap capacitor voltage might not be able to maintain a proper voltage level for the high-side gate driver. A bootstrap capacitor undervoltage protection circuit (BST_UVP) will prevent potential failure of the high-side MOSFET. When the voltage on the bootstrap capacitors is less than the required value for safe operation, the DRV841x2 will initiate bootstrap capacitor recharge sequences (turn off high side FET for a short period) until the bootstrap capacitors are properly charged for safe operation. This function may also be activated when PWM duty cycle is too high (for example, less than 20 ns off time at 10 kHz). Note that bootstrap capacitor might not be able to be charged if no load or extremely light load is presented at output during BST_UVP operation, so it is recommended to turn on the low side FET for at least 50 ns for each PWM cycle to avoid BST_UVP operation if possible.
For applications with lower than 10-kHz switching frequency and not to trigger BST_UVP protection, a larger bootstrap capacitor can be used (for example, 1-µF capacitor for 800-Hz operation). When using a bootstrap cap larger than 220 nF, it is recommended to add 5-Ω resistors between 12-V GVDD power supply and GVDD_X pins to limit the inrush current on the internal bootstrap circuitry.