2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3952SB/SLB/SW
Application Notes
sOutline
Designed for bidirectional pulse-width modulated current con-
trol of inductive loads, the A3952S- is capable of continuous
output currents to
±2A
and operating voltages to 50V. Internal
fixed off-time PWM current-control circuitry can be used to regu-
late the maximum load current to a desired value. The peak
load current limit is set by the user’s selection of an input refer-
ence voltage and external sensing resistor. The fixed OFF-time
pulse duration is set by a user-selected external RC timing net-
work. Internal circuit protection includes thermal shutdown with
hysteresis, transient suppression diodes, and crossover-current
protection. Special power-up sequencing is not required.
With the ENABLE input held low, the PHASE input controls load
current polarity by selecting the appropriate source and sink
driver pair. The MODE input determines whether the PWM cur-
rent-control circuitry operates in a slow current-decay mode (only
the selected sink driver switching) or in a fast current-decay
mode (selected source and sink switching). A user-selectable
blanking window prevents false triggering of the PWM current
control circuitry. With the ENABLE input held high, all output
drivers are disabled. A sleep mode is provided to reduce power
consumption when inactive.
When a logic low is applied to the BRAKE input, the braking
function is enabled. This overrides ENABLE and PHASE to turn
OFF both source drivers and turn ON both sink drivers. The
brake function can be safely used to dynamically brake brush
dc motors.
Fig. 1 Load-Current Paths
V
BB
DRIVE CURRENT
RECIRCULATION (SLOW-DECAY MODE)
RECIRCULATION (FAST-DECAY MODE)
R
S
The user selects an external resistor (R
T
) and capacitor (C
T
) to
determine the time period (t
off
=R
T
C
T
) during which the drivers
remain disabled (see “RC Fixed OFF Time” below). At the end
of the R
T
C
T
interval, the drivers are re-enabled allowing the load
current to increase again. The PWM cycle repeats, maintaining
the load current at the desired value (see figure 2).
Fig. 2 Fast and Slow Current-Decay Waveforms
ENABLE
MODE
I
TRIP
RC
LOAD
CURRENT
RC
sFUNCTIONAL
DESCRIPTION
(A) INTERNAL PWM CURRENT CONTROL DURING FOR-
WARD AND REVERSE OPERATION
The A3952S- contains a fixed OFF-time pulse-width modulated
(PWM) current-control circuit that can be used to limit the load
current to a desired value. The value of the current limiting (I
TRIP
)
is set by the selection of an external current sensing resistor
(R
S
) and reference input voltage (V
REF
). The internal circuitry
compares the voltage across the external sense resistor to one
tenth the voltage on the REF input terminal, resulting in a func-
tion approximated by
I
TRIP
V
REF
10
R
S
(B)INTERNAL PWM CURRENT CONTROL DURING BRAKE
MODE OPERATION
The brake circuit turns OFF both source drivers and turns ON
both sink drivers. For dc motor applications, this has the effect
of shorting the motor’s back-EMF voltage, resulting in current
flow that brakes the motor dynamically. However, if the back-
EMF voltage is large, and there is no PWM current limiting, then
the load current can increase to a value that approaches a locked
rotor condition. To limit the current, when the I
TRIP
level is reached,
the PWM circuit disables the conducting sink driver. The energy
stored in the motor’s inductance is then discharged into the load
supply causing the motor current to decay.
As in the case of forward/reverse operation, the drivers are re-
enabled after a time given by t
off
=R
T
C
T
(see”RC Fixed OFF Time”
below). Depending on the back-EMF voltage (proportional to
the motor’s decreasing speed), the load current again may in-
crease to I
TRIP
. If so, the PWM cycle will repeat, limiting the load
current to the desired value.
(1) Brake Operation-MODE Input High
During braking, when the MODE input is high, the current limit
can be approximated by
In forward or reverse mode the current-control circuitry limits
the load current. When the load current reaches I
TRIP
, the com-
parator resets a latch to turn OFF the selected sink driver (in the
slow-decay mode) or selected sink and source driver pair (in
the fast-decay mode). In slow-decay mode, the selected sink
driver is disabled; the load inductance causes the current to
recirculate through the source driver and flyback diode (see fig-
ure 1). In fast-decay mode, the selected sink and source driver
pair are disabled; the load inductance causes the current to flow
from ground to the load supply via the ground clamp and flyback
diodes.
72
A3952SB/SLB/SW
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