MCP8021 datasheet(42/66 Pages) MICROCHIP

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MCP8021/2

DS20006265D-page 42

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5.1.2

BOOTSTRAP CAPACITOR

The high-side driver bootstrap capacitor needs to

power the high-side driver and gate for 1/3 of the

motor electrical period for a 3-phase BLDC motor

operating in Six-Step mode.

Let:

Solve for the smallest capacitance that can supply:

• 130 nC of charge to the MOSFET gate

• 1 Megohm gate source resistor current

• Driver bias current and switching losses

Sum all of the energy requirements:

•C = (QMOSFET + QRESISTOR + QDRIVER)/VDROP

• C = (130 nC + 0.594 nC + 0.99 nC)/3V

• C = 43.86 nF

Choose a bootstrap capacitor value that is larger than

43.86 nF.

5.2

Device Protection

5.2.1

MOSFET VOLTAGE SUPPRESSION

When a motor shaft is rotating and power is removed,

the magnetism of the motor components will cause the

motor to act like a generator. The current that was

flowing into the motor will now flow out of the motor. As

the motor magnetic field decays, the generator output

will also decay. The voltage across the generator

terminals will be proportional to the generator current

and circuit impedance of the generator circuit. If the

power supply is part of the return path for the current

and the power supply is disconnected, then the volt-

age at the generator terminals will increase until the

current flows. This voltage increase must be handled

externally to the driver. A voltage suppression device

may be used to clamp the motor terminal voltage to a

level that will not exceed the maximum system operat-

ing voltage during the high-voltage transients. A

voltage suppressor circuit may be connected from

power ground to the motor power supply rail to create

a path for the motor current when the supply is discon-

nected (Figure 5-2). The PCB traces must be capable

of carrying the motor current with minimum voltage

and temperature rise.

FIGURE 5-2:

Transient Voltage Clamp.

An additional method is to inactivate the high-side

drivers and to activate the low-side drivers. This allows

current to flow through the low-side external

MOSFETs and prevents the voltage from increasing at

the power supply terminals.

MOSFET Driver Current = 300 mA

PWM Period = 50 µs (20 kHz)

Minimum Duty Cycle = 1% (500 ns)

Maximum Duty Cycle = 99% (49.5 µs)

VIN =12V

Minimum Gate Drive Voltage = 8V (VGS)

Total Gate Charge = 130 nC

(80A MOSFET)

Allowable VGS Drop (VDROP)=3V

Switch RDSON =100 m

Driver Internal Bias Current = 20 µA (IBIAS)

QMOSFET =130 nC

QRESISTOR =[(VGS/R)  TON]

QDRIVER =(IBIAS  TON)

TON = 49.5 µs (99% DC) for worst case

QRESISTOR =QRESISTOR

QDRIVER =20 µA  49.5 µs = 0.99 nC

MOTOR

TVS

CLAMP

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