The bootstrap capacitor must maintain the V_HB-HS voltage above the UVLO threshold for normal operation. Calculate the maximum allowable drop across the bootstrap capacitor, ΔV_HB, with Equation 1.

Equation 1.

where

• V_DD is the supply voltage of gate driver device
• V_DH is the bootstrap diode forward voltage drop
• V_HBL is the HB falling threshold ( V_HBR(max) – V_HBH)

In this example the allowed voltage drop across bootstrap capacitor is 2.4 V.

It is generally recommended that ripple voltage on both the bootstrap capacitor and VDD capacitor should be minimized as much as possible. Many of commercial, industrial, and automotive applications use ripple value of 0.5 V.

Use Equation 2 to estimate the total charge needed per switching cycle from bootstrap capacitor.

Equation 2.

where

• Q_G is the total MOSFET gate charge
• I_HBS is the HB to VSS leakage current from datasheet
• D_Max is the converter maximum duty cycle
• I_HB is the HB quiescent current from the datasheet

The caculated total charge is 53.41 nC.

Next, use Equation 3 to estimate the minimum bootstrap capacitor value.

Equation 3.

The calculated value of minimum bootstrap capacitor is 22.25 nF. It should be noted that, this value of capacitance is needed at full bias voltage. In practice, the value of the bootstrap capacitor must be greater than calculated value to allow for situations where the power stage may skip pulse due to various transient conditions. It is recommended to use a 100-nF bootstrap capacitor in this example. It is also recommenced to include enough margin and place the bootstrap capacitor as close to the HB and HS pins as possible. Also place a small size, 0402, low value, 1000 pF, capacitor to filter high frequency noise, in parallel with main bypass capacitor.

For this application, choose a C_BOOT capacitor that has the following specifications: 0.1 µF, 25 V, X7R

As a general rule the local VDD bypass capacitor must be greater than the value of bootstrap capacitor value (generally 10 times the bootstrap capacitor value). For this application choose a C_VDD capacitor with the following specifications: 1 µF , 25 V, X7R

C_VDD capacitor is placed across VDD and VSS pin of the gate driver. Similar to bootstrap capacitors, place a small size and low value capacitor in parallel with the main bypass capacitor. For this application, choose 0402, 1000 pF, capacitance in parallel with main bypass capacitor to filter high frequency noise.

The bootstrap and bias capacitors must be ceramic types with X7R dielectric or better. Choose a capacitor with a voltage rating at least twice the maximum voltage that it will be exposed to. Choose this value because most ceramic capacitors lose significant capacitance when biased. This value also improves the long term reliability of the system.