MIC4600 datasheet(13/26 Pages) MICROCHIP

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DS20005584B-page 13

MIC4600

5.5

Power Dissipation Considerations

Power dissipation in the driver can be separated into

two areas:

• Quiescent current dissipation

• Internal driver dissipation

5.6

Quiescent Current Power

Dissipation

Power is dissipated in the MIC4600 even if nothing is

being driven. The quiescent current is drawn by the

bias for the internal circuitry and the level shifting

circuitry. The quiescent current is proportional to

operating frequency. The typical characteristic graphs

show how quiescent current varies with switching

frequency.

The power dissipated due to quiescent current is

calculated by:

EQUATION 5-5:

5.7

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 5-1 shows a simplified equivalent circuit of the

MIC4600 driving an external high-side MOSFET.

FIGURE 5-1:

MIC4600 Driving an

External MOSFET.

5.7.1

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

MIC4600. RG is the series resistor (if any) between the

driver IC and the MOSFET. RG_FET is the gate

resistance of the MOSFET. RG_FET is usually listed in

the power MOSFET’s specifications. The ESR of

capacitor CB and the resistance of the connecting etch

can be ignored because 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

specifications include a typical graph of total gate

charge vs. VGS. Figure 5-2 is a typical gate charge

curve for a power MOSFET. This chart shows that for a

gate voltage of 4.5V, the MOSFET gate is charged up

to 25 nC of total gate charge. The energy dissipated by

the resistive components of the gate drive circuit during

turn-on is calculated as noted in Equation 5-6.

EQUATION 5-6:

FIGURE 5-2:

Typical Gate Charge vs.

VGS.

PDISS_IQ

VDD IDD



BST

EXTERNAL FET

VDD

G_FET

OFF

MIC4600

HIGH-SIDE DRIVER

--- CISS



VGS



but

QC V



--- QG



VGS



Where:



CISS = Total gate capacitance of the MOSFET.

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