3933

THREE-PHASE POWER

MOSFET CONTROLLER

www.allegromicro.com

Applications Information (cont’d)

set the blank time duration. After the off time expires, the

selected gates are turned back ON. At this time, large current

intrinsic body diodes of the external MOSFETs. To prevent the

current-sense comparator from thinking the current spikes are a

real overcurrent event, the comparator is blanked:

current-spike duration.

Load Current Recirculation. If MODE has been set for

slow decay, the high-side (source) driver will turn OFF forcing

the current to recirculate through the pair of sink MOSFETs. If

MODE has been selected for fast decay, both the selected high-

and low-side gates are turned OFF, which will force the current

to recirculate through one sink MOSFET and the high-side

clamp diode. Synchronous rectification (only on the low side)

allows current to flow through the MOSFET, rather than the

clamp diode, during the decay time. This will minimize power

loss during the off period. It is important to take into account

that, when switching, the intrinsic diodes will conduct during

the adjustable deadtime.

Braking. The A3933 will dynamically brake by forcing all

sink-side MOSFETs ON. This will effectively short out the

BEMF. During braking, the load current can be approximated

by:

Power Loss Brake. The BRKCAP and BRKSEL terminals

provide a power-down braking option. By applying a logic

level to input BRKSEL, the system can control if the motor is

dynamically braked or is allowed to coast during an

undervoltage event. The reservoir capacitor on the BRKCAP

terminal provides the power to hold the sink-side gates ON after

supply voltage is lost. A logic high on BRKSEL will brake the

motor, a logic low and it will coast.

Layout. Careful consideration must be given to PCB layout

when designing high-frequency, fast-switching, high-current

circuits.

1)

The analog ground (AGND), the power ground (PGND),

and the high-current return of the external MOSFETs (the

negative side of the sense resistor) should return separately to

the negative side of the motor supply filtering capacitor. This

will minimize the effect of switching noise on the device logic

and analog reference.

2)

Minimize stray inductances by using short, wide copper

runs at the drain and source terminals of all power MOSFETs.

This includes motor lead connections, the input power buss, and

the common source of the low-side power MOSFETs. This will

minimize voltages induced by fast switching of large load

currents.

3)

Kelvin connect the SENSE terminal PC trace to the

positive side of the sense resistor.

Dwg. EP-006-50

R S

BB

V

DRIVE CURRENT

RECIRCULATION

(SLOW-DECAY MODE)

RECIRCULATION

(FAST-DECAY MODE)