The bootstrap capacitor (C2, Figure 7-1) is charged during the PWM on-time of the low-side FET of the left-hand-side half bridge. During the PWM off-time, C2 rises with the switch pin voltage and serves as the AMC0106M25 power supply. R4 serves as a current-limiting resistor during the charging phase, and D1 prevents reverse current from flowing back to the bootstrap supply during the discharge phase.

The voltage C2 charges up to during the PWM on-time depends on the values of the bootstrap supply and the current limiting resistor R2. Additionally, this voltage depends on the PWM duty cycle and the forward voltage of the diode D1 (V_{F, D1}).

The voltage C2 discharges to during the PWM off-time depends on the reverse recovery time of D1. Additionally, this voltage depends on the PWM duty cycle and the current draw of the AMC0106M25 (I_AVDD) . To minimize switching losses, select a fast switching diode with high forward current capability.

Make sure C2 is sized to support the maximum I_AVDD current for the duration of the maximum PWM off-time. During this time, make sure C2 does not discharge below the minimum recommended AVDD voltage of 3V. Lower capacitance values allow faster charging and therefore support lower PWM duty cycles. However, lower values also generate more voltage ripple and limit the maximum PWM off time. In this example, a ripple voltage (V_RIPPLE) of less than 200mV is targeted. The maximum PWM off-time is 95% × (1 / f_PWM) = 0.95 × 62.5µs, which is approximately 60µs. I_{AVDD, MAX} is specified as 8.8mA. The minimum capacitance value is calculated as C_{2, MIN} = I_{AVDD, MAX} × t_{PWM-OFF, MAX} / V_RIPPLE = 8.8mA × 60µs / 200mV = 2.6µF. A 4.7µF capacitor is selected to allow for component tolerances and adds margin to the design.

Make sure the bootstrap circuit supports recharging C2 within the minimum PWM on-time of 5% × (1 / f_PWM) = 0.05 × 62.5µs, or approximately 3.1µs. The average charging current during this time is C2 × V_RIPPLE / t_{PWM-ON, MIN} = 4.7µF × 200mV / 3.1µs, which is approximately 300mA. This current is the minimum forward current that diode D1 has to support. The maximum allowable voltage drop across diode D1 and current limiting resistor R4 is determined by the minimum capacitor voltage and the V_BS value. The minimum capacitor voltage is 3V and equivalent to AVDD_MIN. V_BS is the bootstrap supply voltage and is equal to 6V. Assume a diode forward voltage of 1V is used. Make sure R4 is < (V_BS – V_{F, D1} – V_{C2, MIN} ) / I_CHARGE = (6V – 1V – 3V) / 300mA = 6Ω. A 2Ω resistor is selected to provide margin to the design.