8.2.2.10 Bootstrap Capacitor C_BST and Boost Diode D_BST

The bootstrap capacitor between the BST and SW pin supplies the gate current to charge the high-side N-channel MOSFET device gate during each cycle’s turnon and also supplies recovery charge for the bootstrap diode. These current peaks can be several amperes. The recommended value of the bootstrap capacitor is 0.1 μF. C_BST must be a good-quality, low-ESR, ceramic capacitor located at the pins of the device to minimize potentially damaging voltage transients caused by trace inductance. The minimum value for the bootstrap capacitor is calculated as follows:

Equation 35.

where

• Q_G is the high-side N-channel MOSFET gate charge
• ΔV_BST is the tolerable voltage droop on C_BST, which is typically less than 5% of VCC or 0.15 V, conservatively

In this example, the value of the BST capacitor (C_BST) is 0.1 µF.

The voltage rating of D_BST must be greater than the peak SW node voltage plus 16 V. A low leakage diode is mandatory for the bypass operation. The leakage current of D_BST must be low enough for the BST charge pump to maintain a sufficient high-side driver supply voltage at high temperature. A low leakage diode also prevents the possibility of excessive VCC voltage during shutdown, in high output voltage applications. If the leakage is excessive, a zener VCC clamp or bleed resistor may be required. High-side driver supply voltage must be greater than the high-side N-channel MOSFET switch’s gate plateau at the minimum input voltage.