3932

THREE-PHASE POWER

MOSFET CONTROLLER

www.allegromicro.com

9

Applications Information (cont’d)

Current Control. Internal fixed off-time PWM circuitry is

implemented to limit load current to a desired value. When a

high-side and low-side MOSFET are turned on, current will

increase in the motor winding until it reaches a value given by

At the trip point, the sense comparator resets the source-

enable latch, turning off the high-side driver. Load

inductance causes the current to recirculate (decay) for the fixed

off time. The current path during recirculation is determined by

the configuration of the MODE and SR inputs.

parallel from the RC terminal to AGND, are used to set the

of 10 k

Ω to 500 kΩ. The t

50

µs. Larger values for t

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.

set the blank time duration. At the end of the PWM off cycle, a

high-side gate selected by the commutation logic will turn on.

At this time, large current transients can occur during the

external power MOSFETs. To prevent false tripping of the

current-sense comparator, the blank function disables the

comparator for a time

current-spike duration.

Braking. The A3932 will dynamically brake by forcing all

low-side MOSFETs on and all high-side MOSFETs off. This

will effectively short-circuit the BEMF and brake the motor.

During braking, the load current can be approximated by:

Because the load current does not flow through the sense

resistor during a dynamic brake, care must be taken to ensure

that the power MOSFET’s maximum ratings are not exceeded.

RESET = 1 overrides BRAKE and turns all motor bridge

FETs off, coasting the motor.

Low-Voltage Operation. Although VREG can be connected

must be observed including transients. If transients cannot be

adequately controlled, use VREG in the regulator mode (not

voltage level specification may not be met. Note that in this

mode the VREG undervoltage threshold may leave the system

Driving an H Bridge. The A3932 may be used to drive an H

bridge (e.g., a brush dc motor load) by hard wiring one state for

the Hall inputs (e.g., H1 = H2 = 1 (HIGH), H3 = 0 (LOW)).

Leave the appropriate phase driver outputs floating (in this case

CC, GHC, SC, and GLC because, from the Commutation Truth

Table, SC = Z). The DIR input controls the motor rotation

while the PWM, MODE, and SR inputs control the motor

current behavior as described in the Input Logic table.

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.