# A typical calculation of a bootstrap capacitor’s value

## A typical calculation of a bootstrap capacitor’s value

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The bootstrap capacitor on the high side of a gate driver provides the energy required to turn on a high-side power switch. The bootstrap capacitor discharges as the gate driver tries to turn on the high-side power switch.

The value of, the total electric charge required to turn on a power switch, is crucial.  can be expressed as:

which,

is the gate charge,

is the quiescent current volume responsible for supplying gate driver turn ON a power switch,

is the volume of leakage current from the bootstrap-capacitor to the driver IC, subsequently passes to the IC’s ground,

is the maximum duty of the frequency,

is the operation frequency.

Therefore, the value of the bootstrap capacitor can be derived as:

which the

is the voltage of ripple on bootstrap-capacitor. The value of "D" can be set to "1" in order to take the margin into consideration.

In most cases, the magnitudes of

and

range from a few Pico-amperes to a few nano-amperes. As a result, (2) can be approximated as,

Furthermore, the volume of

is generally less than or equal to 0.1V to ensure proper functioning of the high-side signal. Therefore, (3) can be expressed as,