MCP8021 datasheet(43/66 Pages) MICROCHIP

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DS20006265D-page 43

MCP8021/2

5.2.2

BOOTSTRAP VOLTAGE

SUPPRESSION

The pins which handle the highest voltage during

motor operation are the bootstrap pins (VBx). The

bootstrap pin voltage is typically VBOOT (12V) higher

than the associated phase voltage. When the high-

side MOSFET is conducting, the phase pin voltage is

typically at VDD and the bootstrap pin voltage is

typically at VDD + 12V. When the phase MOSFETs

switch, current induced voltage transients occur on the

phase pins. These currents are caused by the

MOSFET body diode reverse recovery and MOSFET

turn-on/turn-off times. Those induced voltages cause

the bootstrap pin voltages to also increase. Depending

on the magnitude of the phase pin voltage, the

bootstrap pin voltage may exceed the safe operating

voltage of the device. The current induced transients

may be reduced by slowing down the turn-on and turn-

off times of the MOSFETs. The external MOSFETs

may be slowed down by adding a 10 to 100 ohm resis-

tor in series with the gate drive. A 1 nF to 10 nF

ceramic capacitor may be added that connects each

MOSFET gate and source terminal. The added capac-

itance slows down the switching times of the MOSFET

while allowing the gate resistance to remain small

enough to keep the gate clamped off. The added

capacitance also results in a lower slew rate of the

phase node and limits the shoot-through current

caused by the body diode reverse recovery.

The high-side MOSFETs may also be slowed down by

inserting a 10

 to 25 resistor between each boot-

strap pin and the associated bootstrap diode capacitor

junction. Another 25

 to 50 resistor is then added

between the gate drive and the MOSFET gate. This

results in a high-side turn-on resistance of 25

 plus

the series gate resistor. The high-side turn-off

resistance only consists of the series gate resistance

and allows for a faster shut-off time. Care must be

taken to make sure the voltage drop across the boot-

strap pin resistor does not cause an external MOSFET

undervoltage Fault.

When a system motor power supply voltage clamp is

not used, 33V or 36V transzorbs may be connected

from each bootstrap pin (VBx) to the ground. This will

ensure that the bootstrap voltage does not exceed the

absolute maximum voltage allowed on the pins. The

resistors connected between the bootstrap pins and

the bootstrap diode/capacitor junctions, mentioned in

the previous paragraph, may also be used in order to

limit the transzorb current and reduce the transzorb

package size.

5.2.3

FLOATING GATE SUPPRESSION

The gate drive pins may float when the supply voltage

is lost or an overvoltage situation shuts down the

driver. When an overvoltage condition exists, the

driver high-side and low-side outputs are tri-state.

Each external MOSFET that is connected to the gate

driver should have a gate-to-source resistor to bleed

off any charge that may accumulate due to the tri-

state. This will help prevent inadvertent turn-on of the

MOSFET.

Figure 5-3 shows the location of the overvoltage

transzorbs (or equivalent circuits), gate resistors,

bootstrap resistors and gate-to-source resistors.

5.2.4

MOSFET BODY DIODE REVERSE

RECOVERY SNUBBER

When motor current is flowing through the external

MOSFET body diodes and the complimentary

MOSFET of the phase pair turns on, the body diode

reverse recovery creates a momentary short circuit

until the reverse recovery time is complete. When the

body diode reverse recovery is complete, the current

path is opened, causing the phase node voltage to

slew rapidly towards ground or VDD levels. The rapid

slew rate may cause an inversion of the gate-to-

source voltage on the MOSFET that is turning on and

result in that MOSFET turning off.

The fast slew rate may also cause ringing on the

phase node and also the sense resistor if the turn-off

is too fast.

The first remedy for the low-side turn-off is to slow down

the MOSFET gate-to-source turn-off. This remedy the

RDSON of the low-side MOSFET to gradually increase

as the gate voltage drops and the low-side MOSFET

slowly turns off. The slow turn-off allows the phase volt-

age, generated by the motor current flowing through the

low-side MOSFET RDSON, to slowly rise towards the

positive motor supply level.

The same scenario is also valid for turning on the low-

side MOSFET when the high-side MOSFET has just

been turned off and current was flowing from the high-

side into the motor.

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