2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3953SB/SLB
Set V
REF
to 0 volts. With the load connected and the PWM cur-
rent control operating in slow current-decay mode, use and os-
cilloscope to measure the time the output is low (sink ON) for
the output that is chopping. This is the typical minimum ON
time (t
ON (min)
typ) for the device.
The C
T
then should be increased until the measured value of t
ON
(min)
omitted. The PHASE and ENABLE inputs should not be PWM
with this circuit configuration due to the absence of a blanking
function synchronous with their transitions.
Fig. 3 Synchronous Fixed-Frequency Control Circuit
V
CC
is equal to t
ON (min)
max as specified in the electrical charac-
t
2
100 kΩ
20 kΩ
teristics table. When the new value of C
T
has been set, the value
of R
T
should be decreased so the value for t
OFF
=R
T
C
T
(with the
artificially increased value of C
T
) is equal to the nominal design
value. The worst-case load-current regulation then can be mea-
sured in the system under operating conditions.
(F) PWM of the PHASE and ENABLE Inputs.
The PHASE and ENABLE inputs can be pulse-width modulated
to regulate load current. Typical propagation delays from the
PHASE and ENABLE inputs to transitions of the power outputs
are specified in the electrical characteristics table. If the internal
PWM current control is used, the comparator blanking function
is active during phase and enable transitions. This eliminates
false tripping of the over-current comparator caused by switch-
ing transients (see “RC Blanking” above).
(1) Enable PWM.
With the MODE input low, toggling the ENABLE input turns ON
and OFF the selected source and sink drivers. The correspond-
ing pair of flyback and ground-clamp diodes conduct after the
drivers are disabled, resulting in fast current decay. When the
device is enabled the internal current-control curcuitry will be
active and can be used to limit the load current in a slow cur-
rent-decay mode.
For applications that PWM the ENABLE input and desire the
internal current-limiting circuit to function in the fast decay mode,
the ENABLE input signal should be inverted and connected to
the MODE input. This prevents the device from being switched
into sleep mode when the ENABLE input is low.
(2) Phase PWM.
Toggling the PHASE terminal selects which sink/source pair is
enabled, producing a load current that varies with the duty cycle
and remains continuous at all times. This can have added ben-
efits in bidirectional brush dc servo motor applications as 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 control (withch produces a
discontinuous current at low current levels). For more informa-
tion see “DC Motor Applications” below.
(3) Synchronous Fixed-Frequency PWM.
The internal PWM current-control circuitry of multiple A3953S-
devices can be synchronized by using the simple circuit shown
in figure 3. A 555IC can be used to generate the reset pulse/
blanking signal (t
1
) for the device and the period of the PWM
cycle (t
2
). The value of t
1
should be a minimum of 1.5ms. When
used in this configuration, the R
T
and C
T
components should be
I
OS
RC
1
1N4001
2N2222
t
1
RC
N
(G)Miscellaneous Information.
A logic high applied to both the ENABLE and MODE terminals
puts the device into a sleep mode to minimize current consump-
tion when not in use.
An internally generated dead time prevents crossover currents
that can occur when switching phase or braking.
Thermal protection circuitry turns OFF all drivers should the junc-
tion termperature reach 165°C (typical). This is intended only to
protect the device from failures due to excessive junction tem-
peratures and should not imply that output short circuits are
permitted. The hysteresis of the thermal shutdown circuit is ap-
proximately 8°C.
sAPPLICATION
NOTES
(A)Current Sensing.
The actual peak load current (I
PEAK
) will be above the calculated
value of I
TRIP
due to delays in the turn off of the drivers. The
amount of overshoot can be approximated by:
(V
BB-
[(I
TRIP
R
LOAD
) + V
BEMF
])
t
PWM (OFF)
L
LOAD
where V
BB
is the motor supply voltage, V
BEMF
is the back-EMF
voltage of the load, R
LOAD
and L
LOAD
are the resistance and in-
ductance of the load respectively, and t
PWM (OFF)
is specified in
the electrical characteristics table.
The reference terminal has a maximum input bias current of
±5
µ
A. This current should be taken into account when deter-
mining the impedance of the external circuit that sets the refer-
ence voltage value.
To minimize current-sensing inaccuracies caused by ground
trace I
R drops, the current-sensing resistor should have a sepa-
rate return to the ground terminal of the device. For low-value
sense resistors, the I
R drops in the printed wiring board can be
significant and should be taken into account. The use of sock-
ets should be avoided as their contact resistance can cause
variations in the effective value of R
S
.
Generally, larger values of R
S
reduce the aforementioned ef-
fects but can result in excessive heating and power loss in the
A3953SB/SLB
65
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