8

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

115 Northeast Cutoff, Box 15036

www.allegromicro.com

A3938

Three-Phase Power MOSFET Controller

each commutation. If a stalled motor results from a fault, the

fault can only be cleared by toggling the RESET pin or by a

power-up sequence.

an internal fixed off-time, PWM-control circuit. When the

outputs of the MOSFETs are turned on, current increases in

the motor winding until it reaches a value given by:

I

At the trip point, the sense comparator resets the source

enable latch, turning off the source driver. At this point, load

inductance causes the current to recirculate for the fixed off-

time period. The current path during recirculation is deter-

mined by the configuration of the MODE and SR input pins.

The fixed off-time is determined by an external resistor, R

and capacitor, C

to AGND. The fixed off-time is approximated by:

t

T

t

values for t

proper circuit operation, 10 kΩ < R

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 pin to set an absolute maximum cur-

rent limit.

to set the BLANK time duration. At the end of a PWM

off-cycle, a high-side gate selected by the commutation logic

turns on. At this time, large current transients can occur dur-

ing the reverse recovery time, t

of the power MOSFETs. To prevent false tripping of the

sense comparator, the BLANK function disables the com-

parator for a time period defined by:

t

The user must ensure that C

current spike duration.

forcing all low-side power MOSFETs on, and all high-side

power MOSFETs off. This effectively short-circuits the

BEMF and brakes the motor. During braking, the load cur-

rent can be approximated by:

I

As the current does not flow through the sense resistor dur-

ing a dynamic brake, care should be taken to ensure that the

maximum ratings of the power MOSFETs are not exceeded.

Note: On its rising edge, a RESET setting of 1 overrides the

BRAKE input pin and latches the condition selected by the

BRKSEL pin.

provide a power-down braking option. A Power-Loss Brake

Trigger Event, which is either an undervoltage on VREG

or a RESET = 1 rising edge, is sensed by the A3938, which

then dynamically brakes or coasts (depending on the stored

BRKSEL setting) the motor. The reservoir capacitor on the

BRKCAP pin provides the positive voltage that forces the

low-side gates of the power MOSFETs high, keeping them

on, even after supply voltage is lost. A stored setting of BRK-

SEL = 1 brakes the motor, but a stored setting of BRKSEL = 0

coasts it. The combined effect of these settings is shown in the

table Brake Control.

BRAKE

BRKSEL

Before Power Loss Brake Trigger Event

After Power Loss Brake Trigger Event

0

0

Normal run mode

Coast mode – All gate drive outputs OFF

0

1

Normal run mode

Brake mode – All low-side gate drivers ON

1

0

Brake mode – All low-side gate drivers ON Coast mode – All gate drive outputs OFF

1

1

Brake mode – All low-side gate drivers ON Brake mode – All low-side gate drivers ON

Brake Control