2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3953SB/SLB
In stepper-motor applications applying a constant current to the
load, slow-decay mode PWM is typically used to limit the switch-
ing lossess in the device and iron losses in the motor.
(F) DC Motor Applications.
In closed-loop systems, the speed of a dc motor can be con-
trolled by PWM of the PHASE or ENABLE inputs, or by varying
the reference input voltage (REF). In digital systems (micropro-
cessor controlled), PWM of the PHASE or ENABLE input is used
typically thus avoiding the need to generate a variable analog
voltage reference. In this case, a dc voltage on the REF input is
used typically to limit the maximum load current.
In dc servo applications, which require accurate positioning at
low or zero speed, PWM of the PHASE input is selected typi-
cally. This simplifies the servo control loop because the transfer
function between the duty cycle on the PHASE input and the
average voltage applied to the motor is more linear than in the
case of ENABLE PWM comtrol (which produces a discontinu-
ous current at low current levels).
With bidirectional dc servo motors, the PHASE terminal can be
used for mechanical direction control. Similar to when branking
the motor dynamically, abrupt changes in the direction of a ro-
tating motor produces a current generated by the back-EMF.
The current generated will depend on the mode of operation. If
the internal current control circuitry is not being used, then the
maximum load current generated can be approximated by
I
LOAD
=(V
BEMF
+V
BB
)/R
LOAD
where V
BEMF
is proportional to the motor’s
speed. If the internal slow current-decay control circuitry is used,
then the maximum load current generated can be approximated
by I
LOAD
=V
BEMF
/R
LOAD
. For both cases care must be taken to en-
sure that the maximum ratings of the device are not exceeded.
If the internal fast current-decay control circuitry is used, then
the load current will regulate to a value given by:
I
LOAD
V
REF
R
S
CAUTION:
In fast current-decay mode, when the direction of
the motor is changed abruptly, the kinetic energy stored in the
motor and load inertia will be converted into current that charges
the V
BB
supply bulk capacitance (power supply output and
decoupling capacitance). Care must be taken to ensure that the
capacitance is sufficient to absorb the energy without exceed-
ing the voltage rating of any devices connected to the motor
supply.
See also “Brake Operation” above.
A3953SB/SLB
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