A3931 Allegro MicroSystems, Inc., A3931 Datasheet - Page 10

• Manufacturer Part Number: A3931
• Description: Automotive 3-Phase BLDC Controller and MOSFET Driver
• Manufacturer: Allegro MicroSystems, Inc.

A3930
A3931
greater than 20 kΩ. The upper limit for the resistor must be low
enough to ensure that the input voltage reaches the input high
threshold, V
COAST An active-low input which turns all FETs off without
disabling the supplies or control logic. This allows the external
FETs and the motor to be protected in case of a short circuit.
MODE Sets the current-decay method. Referring to table 4, when
in slow-decay mode, MODE = 1, only the high-side MOSFET
is switched off during a PWM-off cycle. In the fast-decay mode,
MODE = 0, the device switches both the high-side and low-side
MOSFETs.
Slow decay allows a lower ripple current in the motor at the
PWM frequency, but reduces the dynamic response of the cur-
rent control. It is suitable for motors which run at a more-or-less
constant speed. Fast decay provides improved current-control
dynamic response, but increases the motor current ripple. It is
suitable for motors used in start-stop and positioning applications.
DIR Determines the direction of motor torque output, as shown in
table 3. For an unloaded, low-inertia motor, this will also usually
be the direction of mechanical rotation. With a motor that has a
high inertial load, the DIR input can be used to apply a controlled
breaking torque, when fast decay is used (MODE = 0).
BRAKE An active-low input that provides a braking function.
When BRAKE = 0 (see table 4), all the low-side FETs are turned
on and the high-side FETs are turned off. This effectively short-
circuits the back EMF in the windings, and brakes the motor.
The braking torque applied depends on the speed. RESET = 0 or
COAST = 0 overrides BRAKE and coasts the motor. Note that
when BRAKE is used to dynamically brake the motor, the wind-
ings are shorted with no control over the winding current.
ESF The state of the enable stop on fault (ESF) pin determines
the action taken when a short is detected. See the Diagnostics
section for details.
TEST Test is for Allegro production use and must be connected
to AGND.
Current Regulation
Load current can be regulated by an internal fixed frequency
PWM control circuit or by external input on the PWM pin.
Current Sense Amplifier: CSP, CSN, and CSOUT A dif-
ferential current sense amplifier with a gain, A
provided to allow the use of low-value sense resistors or current
shunts as the current sensing elements. Because the output of
this sense amplifier is available at CSOUT, it can be used for
either internal or external current sensing. With the sense resistor,
Subject to Change Without Notice
Preliminary Data Sheet
April 6, 2006
INR
.
and
V
, of 19 typical, is
Automotive 3-Phase BLDC Controller
RSENSE, connected between CSP and CSN, the output of the
sense amplifier will be approximately:
where V
current), and A
amplifier, 19 typical.
Internal Current Control: REF A fixed reference voltage
can be applied to provide a maximum current limit. A variable
reference voltage will provide a variable torque control. The
output voltage of the current sense differential amplifier, V
is compared to the reference voltage available on the REF
pin. When the outputs of the MOSFETs are turned on, current
increases in the motor winding until it reaches a trip point value,
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 until the start of the next PWM
period.
The current path during recirculation is determined by the
configuration of the MODE pin. Torque control can therefore be
implemented by varying the voltage on the REF pin, provided
that the PWM input remains high. If direct control of the torque
or current by PWM input is desired, a voltage can be applied to
the REF pin to set an absolute maximum current limit. The REF
input is internally limited to 4 V, which allows the use of a simple
pull-up resistor to V5, RREF, to set the maximum reference
voltage, avoiding the need for an externally generated reference
voltage. RREF should have a value between 20 kΩ and 200 kΩ.
Internal PWM Frequency The internal oscillator frequency,
f
connected in parallel from the RC pin to AGND. The frequency
is approximately:
where f
PWM Input Can be used to control the motor torque by an
external control circuit signal on the PWM pin. Referring to
table 4, when PWM = 0, the selected drivers are turned off and
the load inductance causes the current to recirculate. The current
path during recirculation is determined by the configuration of
the MODE pin. Setting PWM = 1 will turn on selected drivers as
determined by the Hx input logic (see table 3). Holding PWM=1
allows speed and torque control solely by the internal current-
limit circuit, using the voltage on the REF pin.
TRIP
OSC
, is determined by an external resistor, RT, and capacitor, CT,
, given by:
OSC
OOS
in the range 20 to 50 kHz.
is the output offset voltage (the voltage at zero load
V
I
CSOUT
TRIP
V
f
OSC
is the differential voltage gain of the sense
= (V
≈ 1 / (R
≈ (I
REF
LOAD
and MOSFET Driver
T
115 Northeast Cutoff, Box 15036
– V
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
C
T
× A
OOS
+ t
V
BLANK
) / (R
× R
SENSE
SENSE
+ t
) + V
DEAD
× A
OOS
V
) .
) .
,
CSOUT
10
,