DGD0506

Document number: DS38740 Rev. 8 - 3

11 of 14

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June 2021

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DGD0506

Application Information

Bootstrap Capacitor Selection

The capacitance of the bootstrap capacitor should be high enough to provide the charge required by the gate of the high side MOSFET with only a

minimal loss of voltage across it. As a general guideline, it is recommended to make sure the charge stored by the bootstrap capacitor is about 50

times more than the required gate charge at operating VCC(usually about 10V to 12V).

The formula to calculate the change in VBS to provide a certain amount of gate charge is shown below;

Q = C * V where Q is the gate charge required by the external MOSFET to raise its gate voltage to 10V. C is the bootstrap capacitance and V is

the voltage drop across the Vbs.

Example: To switch a high side MOSFET that requires 20nC of gate charge to raise its gate voltage to 10V, the capacitor size can be calculated

as below;

QG(MOSFET) = C(BOOTSTRAP) * ∆VBS ;

∆

VBS = voltage drop acorss the bootstrap capacitor while providing the required gate charge.

In this example, let’s say the acceptable ∆VBS is 200mV.

The required bootstrap capacitor for the job is;

C(BOOTSTRAP) = QG(MOSFET)/ ∆VBS = 20nC/200mV = 100nF

Bootstrap Diode Current

The DGD0506 comes with an integrated bootstrap Schottky diode. The forward characteristics of the diode is shown in the figure 28. The

maximum recommended operating current is 400mA pulsed. Under steady state conditions the only current flowing through the internal diode is

the charge current required by the high-side MOSFET’s gate capacitance, however, it is important to cover applications where the inrush current

exceeds this rating. In such applications to limit the current flowing through the internal diode to the recommended value, two techniques are

suggested as shown in figures 29 and 30.

Figure 28. DGD0506 ‘Internal Diode + Internal Resistor’ VF vs. IF