MIC4607 datasheet(26/42 Pages) MICROCHIP

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MIC4607

DS20005610A-page 26

 2016 Microchip Technology Inc.

FIGURE 6-6:

Optional External Bootstrap

Diode.

6.5

Gate Driver Power Dissipation

Power dissipation in the output driver stage is mainly

caused by charging and discharging the gate to source

and gate to drain capacitance of the external MOSFET.

Figure 6-7 shows a simplified equivalent circuit of the

MIC4607 driving an external high-side MOSFET.

FIGURE 6-7:

MIC4607 Driving an

External High-Side MOSFET.

6.5.1

DISSIPATION DURING THE

EXTERNAL MOSFET TURN-ON

Energy from capacitor CB is used to charge up the input

capacitance of the MOSFET (CGD and CGS). The

energy delivered to the MOSFET is dissipated in the

three resistive components, RON, RG and RG_FET. RON

is the on resistance of the upper driver MOSFET in the

MIC4607. RG is the series resistor (if any) between the

driver and the MOSFET. RG_FET is the gate resistance

of the MOSFET and is typically listed in the power

MOSFET’s specifications. The ESR of capacitor CB

and the resistance of the connecting etch can be

ignored since they are much less than RON and

RG_FET.

The effective capacitances of CGD and CGS are difficult

to calculate because they vary non-linearly with ID,

VGS, and VDS. Fortunately, most power MOSFET spec-

ifications include a typical graph of total gate charge

versus VGS. Figure 6-8 shows a typical gate charge

curve for an arbitrary power MOSFET. This chart

shows that for a gate voltage of 10V, the MOSFET

requires about 23.5 nC of charge. The energy dissi-

pated by the resistive components of the gate drive cir-

cuit during turn-on is calculated as:

EQUATION 6-4:

but

EQUATION 6-5:

so,

EQUATION 6-6:

FIGURE 6-8:

Typical Gate Charge vs.

VGS.

--- C

ISS



Where:

CISS

Total gate capacitance of the MOSFET.



--- Q



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