2 Bootstrap Power Supply Design

The bootstrap operates from an input voltage and a PWM signal. The input voltage is supplied from the same supply powering the device-under-test (DUT), and the input voltage can be stepped down using an LDO. Since the bootstrap requires a PWM signal to operate, the signal can only be used with DUTs which produce or operate with a PWM signal. The DUT is not necessarily isolated from the low-side of the amplifier, as shown in Figure 1-1, which is why a DC-DC power converter alone cannot be used. The high-side of the amplifier does not share a ground connection with the DUT. The PWM signal is tied to the isolated amplifier's high-side ground. The bootstrap make sure the high-side power supply always floats above the PWM signal, so the high-side power supply has a steady signal, even though the high-side ground is a PWM signal.

The input voltage to the bootstrap circuit determines the output steady state value, so the input voltage must be close to the desired high-side supply voltage to avoid violating the amplifier’s high-side supply specifications. An LDO is required if the DUT VCC bus is outside of the isolated amplifier's recommended operating conditions. LDOs generally require few additional external components, and LDOs produce cleaner signals than switching regulators, which is why LDOs are recommended in this application. The input voltage to the bootstrap circuit is greater than the DUT ground, so when the PWM signal is low, there is a positive voltage drop across the diode, and it conducts, charging the capacitor, as shown in Figure 2-1.

When the PWM signal is high, there is no voltage drop or a negative voltage drop across the capacitor, and the signal stops conducting, so the capacitor discharges into the high-side supply, as shown in Figure 2-2. The bootstrap circuit can achieve steady state when the amount of voltage stored by the capacitor when the PWM signal is low is equal to the amount of voltage discharged by the capacitor when the PWM signal is high. This means that the start-up time and steady state ripple are dependent on the RC time constant and can be impacted by the frequency and duty cycle of the PWM signal.