MIC4607A datasheet(24/41 Pages) MICROCHIP

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MIC4607A

DS-20006957A-page 24

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voltage rating must be greater than the high-voltage

input supply (VIN). Larger values of resistance can be

used if necessary.

Adding a series resistor in the switch node limits the

peak high-side driver current during turn-off, which

affects the switching speed of the high-side driver. The

resistor in series with the HO pin may be reduced to

help compensate for the extra HS pin resistance.

FIGURE 6-5:

Negative HS Pin Voltage.

6.3

Power Dissipation Considerations

Power dissipation in the driver can be separated into

three areas:

• Internal diode dissipation in the bootstrap circuit

• Internal driver dissipation

• Quiescent current dissipation used to supply the

internal logic and control functions.

6.4

Bootstrap Circuit Power

Dissipation

Power dissipation of the internal bootstrap diode pri-

marily comes from the average charging current of the

bootstrap capacitor (CB) multiplied by the forward volt-

age drop of the diode. Secondary sources of diode

power dissipation are the reverse leakage current and

reverse recovery effects of the diode.

The average current drawn by repeated charging of the

high-side MOSFET is calculated by Equation 6-1.

EQUATION 6-1:

The average power dissipated by the forward voltage

drop of the diode equals:

EQUATION 6-2:

There are three phases in the MIC4607A. The power

dissipation for each of the bootstrap diodes must be

calculated and summed to obtain the total bootstrap

diode power dissipation for the package.

The value of VF should be taken at the peak current

through the diode; however, this current is difficult to

calculate because of differences in source imped-

ances. The peak current can either be measured or the

value of VF at the average current can be used, which

will yield a good approximation of diode power dissipa-

tion.

The reverse leakage current of the internal bootstrap

diode is typically 3 µA at a reverse voltage of 85V at

125°C. Power dissipation due to reverse leakage is typ-

ically much less than 1 mW and can be ignored.

An optional external bootstrap diode may be used

instead of the internal diode (Figure 6-6). An external

diode may be useful if high gate charge MOSFETs are

being driven and the power dissipation of the internal

diode is contributing to excessive die temperatures.

The voltage drop of the external diode must be less

than the internal diode for this option to work. The

reverse voltage across the diode will be equal to the

input voltage minus the VDD supply voltage. The above

equations can be used to calculate power dissipation in

the external diode; however, if the external diode has

significant reverse leakage current, the power dissi-

pated in that diode due to reverse leakage can be cal-

culated with the formula in Equation 6-3:

EQUATION 6-3:

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

ing. In most topologies, the diode is reverse biased

during the switching cycle off-time.

Level

Shift

VDD

BST

VSS

CLAMP

NEG

Phase A

Phases

B&C

ALI

AHI

AHB

AHO

AHS

ALO

IF AVE



QGATE fS



Where:

QGATE = Total gate charge at VHB – VHS.

fS =

Gate drive switching frequency.

PdiodeFWD

IFAVE





Where:

VF =

Diode forward voltage drop.

PdiodeREV

IR VREV

1 D

–





Where:

IR =

Reverse current flow at VREV and TJ.

VREV = Diode reverse voltage.

D =

Duty cycle (tON × fS).

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