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3932
THREE-PHASE POWER MOSFET CONTROLLER
Braking. The 3932 will dynamically brake by forcing all
sink side MOSFETs on, and all source side MOSFETs off.
This will effectively short out the BEMF and brake the
motor. During braking the load current can be approximated
by:
Current Regulation. Load current is regulated by 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:
ITRIP = VREF /RSENSE
IBRAKEPEAK = VBEMF/RLOAD
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
As the current does not flow through the sense resistor
during a dynamic brake, care should be taken to ensure that
the power MOSFETs maximum ratings are not exceeded.
determined by the configuration of the MODE and SR input
pins. The fixed off time is determined by an external resistor
(RT) and capacitor (CT) connected in parallel from the RC
terminal to AGND. The fixed off time is approximated by:
The RESET pin overrides the BRAKE input. RESET will
always drive all gate outputs Low.
tOFF = RT * CT
TOFF should be in the range 10ms to 50ms. Larger values for
TOFF could result in 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 pin to set an absolute
maximum current limit.
PWM Blank. The capacitor (CT) also serves as the means
to set the BLANK time duration. At the end of a 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 reverse recovery time (trr) of the intrinsic body
diodes of the power MOSFETs. To prevent false tripping of
the sense comparator, the blank function will disable the
comparator for a time defined by:
TBLANK = (1.9 * CT )/(1mA-2/ RT)
The user must ensure that the CT is large enough to cover the
current spike duration.
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
