The maximum value supplied by the bootstrap can be approximated by taking the input supply to the bootstrap and subtracting the voltage drop of the diode. However, the bootstrap can reach steady state before the bootstrap reaches the maximum value, depending on the value of the RC circuit and the PWM signal. The RC time constant is defined as:

The capacitor and resistor determine the RC time constant for charging and discharging the capacitor. There is a trade-off between start-up time and steady-state ripple. A smaller time constant means that the capacitor can charge and discharge more quickly, reaching steady-state sooner. However, once the capacitor reaches steady-state, the capacitor can charge or discharge more voltage per PWM duty cycle than the capacitor can with a larger time constant, which leads to larger ripple. Likewise, a larger time constant can result in less ripple due to longer charge or discharge times. The capacitor value can be estimated using the following parameters:

1. PWM switching frequency
2. PWM duty cycle
3. Current required to power the isolated amplifier
4. Allowable ripple

We can rearrange Equation 2 as shown in Equation 3 to solve for capacitance.

Assuming a 20 kHz switching frequency with a 50% duty cycle, using the maximum current draw from the AMC1311-Q1 data sheet, and mandating a 100 mV maximum ripple requirement, the following minimum capacitance value is received:

From there, the bootstrap can be simulated to estimate start-up time, and an appropriate capacitor and resistor can be selected based on the start-up time requirements. The resistor needs to be selected so the resistor does not prevent the high-side of the amplifier from drawing sufficient current.