Bootstrap or “Boopstrapping” is a method for boosting a voltage to greater than the supply voltage. It is a type of charge pump that uses the same low side and high-side FETs in a half bridge to also generate the boosted voltage used to drive the highside NMOS.

A bootstrap is constructed but connecting a diode with anode connected to the power supply and cathode connected to one terminal of the capacitor, the other terminal is then connected to the center point of the half bridge. In this configuration, when the low side fet is switched on, the capacitor charges with current through the diode, then when the low side fet turns off the voltage of the capacitor shifts up to the voltage of the output capacitor. The diode blocks current from sinking into the power supply and the voltage measured with respect to ground at the diode’s cathode is 2x the supply voltage minus the drop caused by the diode. This voltage may then be used to drive the gate of the high-side NMOS thus switching it on. #Long

This advantage to using a bootstrap is that it is very long cost and uses mostly circuit components that are already in place. One major disadvantage of the bootstrap method is that it requires continues switching of the low-side fet in order to maintain the boosted voltage. This implies that there will be a maximum duty cycle limit to the fets driven, 100% duty cycle is not possible when using a bootstrap, for some applications, this is important, for others such as a buck converter, this is typically not an issue.

Triad engineers and Mixed Signal Integration Architects understand the tradeoffs associated with the bootstrap technique and will work with customers to determine the correct implemenation for the application. In many cases, bootstrapping can be a cost efficent way to drive a high-side NMOS and we want to take full advantage of it.