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DS20005604D-page 23

MIC4606

7.2

HS Pin Clamp

A resistor/diode clamp between the motor phase node

and the xHS pin is necessary to clamp large negative

glitches or pulses on the xHS pin.

Figure 7-5 shows the Phase A section high-side and

low-side MOSFETs connected to one phase of the

motor. There is a brief period of time (dead time)

between switching to prevent both MOSFETs from

being on at the same time. When the high-side

MOSFET is conducting during the on-time state, cur-

rent flows into the motor. After the high-side MOSFET

turns off, but before the low-side MOSFET turns on,

current from the motor flows through the body diode in

parallel with the low-side MOSFET. Depending upon

the turn-on time of the body diode, the motor current

and circuit parasitics, the initial negative voltage on the

switch node can be several volts or more. The forward

voltage drop of the body diode can be several volts,

depending on the body diode characteristics and motor

current.

Even though the xHS pins are rated for negative voltage,

it is good practice to clamp the negative voltage on the

xHS pin with a resistor and diode to prevent excessive

negative voltage from damaging the driver. Depending

upon the application and amount of negative voltage on

the switch node, a 3Ω resistor is recommended. If the

xHS pin voltage exceeds 0.7V, a diode or Schottky diode

between the xHS pin and ground is recommended. The

diode reverse voltage rating must be greater than the

high-voltage input supply (VIN). Larger values of resis-

tance 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 7-5:

Negative HS Pin Voltage.

7.3

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.

7.4

Bootstrap Circuit Power

Dissipation

Power dissipation of the internal bootstrap diode

primarily comes from the average charging current of

the bootstrap capacitor (CB), multiplied by the forward

voltage 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 7-1:

The average power dissipated by the forward voltage

drop of the diode equals:

EQUATION 7-2:

TABLE 7-2:

MIC4606-2 TRUTH TABLE

xPWM

xLO

xHO

Comments

xLO will be delayed an

extra 250 ns if xHS never

falls below 2.2V.

xHO will not go high until

xLO falls below 1.9V.

IF AVE



QGATE fS



Where:

QGATE = Total Gate Charge at VHB

fS = Gate Drive Switching Frequency

PDIODEfwd

IF AVE





Where:

VF = Diode Forward Voltage Drop

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