MIC4225 datasheet(10/20 Pages) MIC

10 / 20 page

Micrel, Inc.

MIC4223/MIC4224/MIC4225

June 2009

M9999-061109-A

(408) 944-0800

Application Information

Power Dissipation Considerations

Power dissipation in the driver can be separated into two

areas:

Output driver stage dissipation

Quiescent current dissipation used to supply the

internal logic and control functions.

Output Driver Stage Power Dissipation

Power dissipation in the driver’s output stage is mainly

caused by charging and discharging the gate to source

and gate to drain capacitance of the external MOSFET.

Figure 3 shows a simplified circuit of the MIC4223 driving

an external MOSFET.

Figure 3. Functional MOSFET/Driver Diagram

Dissipation Caused by Switching the External

MOSFET

Energy from capacitor CVDD is used to charge up the input

capacitance of the MOSFET (CGD and CGS). The energy

delivered to the MOSFET is dissipated in the upper driver

MOSFET

and

Bipolar

impedances.

The

effective

capacitance of CGD and CGS is difficult to calculate since

they vary non-linearly with ID, VGS, and VDS. Fortunately,

most power MOSFET specifications include a typical

graph of total gate charge vs. VGS. Figure 4 shows a

typical MOSFET gate charge curve. The graph illustrates

that for a gate voltage of 10V, the MOSFET requires about

23.5nC of charge.

Figure 4. MOSFET Gate Charge vs. VGS

The energy dissipated during turn-on is calculated as:

iss

1/2

but

capacitanc

gate

total

MOSFET'

the

where

An equivalent amount of energy is dissipated in the

driver’s sink circuit when the MOSFET turns off. The total

energy and power dissipated by the drive components is:

DRIVER

and

Where:

EDRIVER is the energy dissipated per switching cycle

PDRIVER is the power dissipated by switching the

MOSFET on and off

QG is the total Gate charge at VGS

VGS is the MOSFETs Gate to Source voltage

fS is the switching frequency of the Gate drive

circuit

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