MIC4102 datasheet(16/28 Pages) MICROCHIP

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MIC4102

DS20005575A-page 16

 2016 Microchip Technology Inc.

The on-time is the time the high-side switch is

conducting. In most power supply topologies, the diode

is reverse-biased during the switching cycle off-time.

FIGURE 6-1:

Optional Bootstrap Diode.

6.3

Gate Drive 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 6-2 shows a simplified equivalent circuit of the

MIC4102 driving an external high-side MOSFET.

FIGURE 6-2:

MIC4102 Driving an

External MOSFET.

6.4

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

MIC4102. 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 capacitance of CGD and CGS is 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 6-3 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

dissipated by the resistive components of the gate

drive circuit during turn-on is calculated as:

EQUATION 6-7:

but

EQUATION 6-8:

EQUATION 6-9:

FIGURE 6-3:

Typical Gate Charge vs.

VGS.

EXTERNAL

DIODE

LEVEL

SHIFT

PWM

EXTERNAL

FET

G_FET

OFF

---

ISS



Where:

CISS

Total Gate Capacitance of MOSFET



---



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