2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3953SB/SLB
Application Notes
sOutline
Designed for bidirectional pulse-width modulated (PWM) cur-
rent control of inductive loads, the A3953S- is capable of con-
tinuous output currents to
±1.3A
and operating voltages to 50V.
Internal fixed off-time PWM current-control circuitry can be used
to regulate the mximum load current to a desired value. The
peak load current limit is set by the user’s selection of an input
reference voltage and external sensing resistor. The fixed off-
time pulse duration is set by a userselected external RC timing
network. Internal circuit protection includes thermal shutdown
with hysteresis, transient-suppression diodes, and crossover cur-
rent 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 source 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 a logic low is applied to the Brake input, the braking func-
tion is enabled. This overrides ENABLE and PHASE to turn OFF
both source drivers and turn ON both sink drivers. The brake
function can be used to dynamically brake brush dc motors.
decay mode, the selected sink and source driver pair are dis-
abled; the load inductance causes the current to flow from ground
to the load supply via the ground clamp and flyback diodes.
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 RC interval, the drivers are enabled allowing the load cur-
rent to increase again. The PWM cycle repeats, maintaing the
peak load current at the desired value (see figure 2).
Fig. 2 Fast and Slow Current-Decay Waveforms
ENABLE
sFUNCTIONAL
DESCRIPTION
(A) Internal PWM Current Control During Forward and Re-
verse Operation.
The A3953S-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 peak value of the current limiting
(I
TRIP
) is set by the selection of an external current sensing re-
sistor (R
S
) and reference input voltage (V
REF
). The internal cir-
cuitry compares the voltage across the external sense resistor
to the voltage on the reference input terminal (REF) resulting in
a transconductance function approximated by:
I
TRIP
V
REF
−I
SO
R
SENSE
LOAD
CURRENT
I
TRIP
RC
RC
MODE
(B)INTERNAL PWM CURRENT CONTROL DURING BRAKE-
MODE OPERATION
(1) Brake Operation-MODE Input High.
The brake circuit turns OFF both source drivers and turns ON
both sink drivers. For dc motor applications, this has the effect
of shoring the motor’s back-EMF voltage resulting in current
flow that dynamically brakes the motor. If the back-EMF volt-
age is large, and there is no PWM current limiting, the load cur-
rent can increase to a value that approaches that of a locked
rotor condition. To limit the current, when the I
TRIP
level is reaced,
the PWM circuit disables the conducting sink drivers. The en-
ergy stored in the motor’s inductance is discharged into the load
supply causing the motor current to decay.
As in the case of forward/reverse operation, the drivers are en-
abled 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-
A3953SB/SLB
where I
SO
is the offset due to base drive current.
In forward or reverse mode the current-control circuitry limits
the load current as follows: when the load current reaches I
TRIP
,
the comparator resets a latch that turns off the selected source
driver or selected sink and source driver pair depending on
whether the device is operating in slow or fast current-decay
mode, respectively.
In slow current-decay mode, the selected source driver is dis-
abled; the load inductance causes the current to recirculate
through the sink driver and ground clamp diode. In fast current-
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