115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

3933

THREE-PHASE POWER

MOSFET CONTROLLER

Applications Information

bootstrap capacitor. When the bootstrap capacitor has been

properly charged, the high side is turned back ON. The circuit

will allow three faults of this type within one commutation

cycle before signaling a fault and coast the motor (all gate

outputs go low).

2)

Bootstrap Monitor. The bootstrap capacitor is charged

whenever a sink-side MOSFET is ON, Sx output goes low, and

the load current recirculates. This happens constantly during

normal operation. A 60

µs timer is started at the beginning of

this cycle and the capacitor is charged with typically 100 mA.

The bootstrap capacitor voltage is clamped at approximately

voltage in 60

µs, a fault is signaled and the motor will coast.

3)

low-side gate driver and the bootstrap charge current. It is

critical to ensure that the voltages are at a proper level before

enabling any of the outputs. The undervoltage circuit is active

during power up and will force a motor coast condition until

4)

Hall Invalid. Illegal codes for the HALL inputs (000 or

111) will force a fault and coast the motor.

Faults are cleared at the beginning of each commutation. If a

stalled motor results from a fault, the fault can only be cleared

by toggling the RESET terminal or by a power-up sequence.

Current Control. Internal fixed off-time PWM circuitry is

implemented to limit load current to a desired value. The

external sense resistor combined with the applied analog

voltage to REF terminal will set the peak current level

approximately

After the peak level is reached, the sense comparator trips and

the load current will decay for a fixed off time.

range of 10

µs to 50 µs. Longer values for t

audible noise problems.

Torque control can be implemented by varying the REF input

voltage as long as the PWM input stays high. If direct control

of the torque/current is desired by PWM input, a voltage can be

applied to the REF input to set an absolute maximum current

limit.

Bootstrap Capacitor Selection. The high-side bootstrap

circuit operates on a charge-transfer principle. The gate charge

taken into consideration. The bootstrap capacitor must be large

enough to turn on the MOSFET without losing significant gate

voltage. If the bootstrap capacitor is too large, it would take too

long to charge up during the off portion of the PWM cycle. The

capacitor value must be selected with both of these constraints

in mind.

1)

Minimum bootstrap capacitor value to transfer charge. The

charge on the bootstrap capacitor should be 20x greater than the

µC, select

the bootstrap capacitor monitor circuit.

The sink-side MOSFET will be held OFF for this minimum

time such that the bootstrap capacitor can be recharged

independently of the PWM input frequency.

The above equation is valid for PWM cycles after the bootstrap

capacitor has been charged once. For the first cycle after a

motor phase commutates from Hi-Z to GHx ON, or during the

first charging cycle at power-up, the circuit will ignore PWM

signals until it has been charged.

The time required to charge up at power up and at commutation

change is approximately:

Protection Circuitry. The A3933 will protect the external

MOSFETs by shutting down the gate drive if any of the

following conditions are detected:

1)

Gate Source Monitor (high side only). The voltage on

the GHx terminals must stay 7 V higher than the source. If this

voltage droops below the threshold, the high side turns OFF,

and the low-side gate will turn ON in an attempt to recharge the