2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3953SB/SLB
crease to I
TRIP
. If so, the PWM cycle will repeat, limiting the peak
load current to the desired value.
During braking, when the MODE input is high, the peak current
limit can be approximated by:
I
TRIP BRAKE MH
V
REF
R
SENSE
comparator’s output is blanked and C
T
begins to be charged
from approximately 0.22 V
CC
by an internal current source of
approximately 1 mA. The comparator output remains blanked
until the voltage on C
T
reaches approximately 0.60 V
CC
.
When a transition of the PHASE input occurs, C
T
is discharged
to near ground during the crossover delay time (the crossover
delay time is present to prevent simultaneous conduction of the
source and sink drivers). After the crossover delay, C
T
is charged
by an internal current source of approximately 1 mA. The com-
parator output remains blanked until the voltage on C
T
reaches
approximately 0.60V
CC
.
When the device is disabled, via the ENABLE input, C
T
is dis-
charged to near ground. When the device is reenabled, C
T
is
charged by an internal current source of approximately 1 mA.
The comparator output remains blanked until the voltage on C
T
reaches approximately 0.60 V
CC
.
For 3.3 V operation,
the minimum recommended value for C
T
is 680pF±5%.
For 5.0V operation,
the minimum recommended value for C
T
is 470pF±5%.
These values ensure that the blanking time is sufficient to avoid
false trips of the comparator under normal operating conditions.
For optimal regulation of the load current, the ablove values for
C
T
are recommended and the value of R
T
can be sized to deter-
mine t
OFF
. For more information regarding load current regula-
tion, see below.
CAUTION:
Because the kinetic energy stored in the motor and
load inertia is being converted into current, which charges the
V
BB
supply bulk capacitance (power supply output and
decoupling capacitance), care must be taken to ensure the ca-
pacitance is sufficient to absorb the energy without exceeding
the voltage rating of any devices connected to the motor sup-
ply.
(2) Brake Operation-MODE Input Low.
During braking, with the MODE input low, the internal current-
control circuitry is disabled. Therefore, care should be taken to
ensure that the motor’s current does not exceed the ratings of
the device. The braking current can be measured by using an
oscilloscope with a current probe connected to one of the motor’s
leads, or if the back-EMF voltage of the motor is known, ap-
proximated by:
I
PEAK BRAKE ML
V
BEMF
−1V
R
LOAD
(C) RC Fixed Off-Time.
The internal PWM current-control circuitry uses a one shot to
control the time the driver (s) remain (s) off. The one-shot time,
t
OFF
(fixed off-time), is determined by the selection of an exter-
nal resistor (R
T
) and capacitor (C
T
) connected in parallel from
the RC timing terminal to ground. The fixed off-time, over a range
of values of C
T
=470pF to 1500pF and R
T
=12kΩ to 100kΩ, is
approximated by:
(E) LOAD CURRENT REGULATION WITH INTERNAL PWM
CURRENT-CONTROL CIRCUITRY
When the device is operating in slow current-decay mode, there
is a limit to the lowest level that the PWM current-control cir-
cuitry can regulate load current. The limitation is the minimum
duty cycle, which is a function of the user-selected value of t
OFF
and the minimum on-time pulse t
ON (min)
max that occurs each
time the PWM latch is reset. If the motor is not rotating (as in the
case of a stepper motor in hold/detent mode, a brush dc motor
The operation of the circuit is as follows: when the PWM latch is
reset by the current comparator, the voltage on the RC terminal
will begin to decay from approximately 0.60V
CC
. When the volt-
age on the RC terminal reaches approximately 0.22 V
CC
, the
PWM latch is set, thereby enabling the driver (s).
where t
OFF
=R
T
C
T
, R
LOAD
is the series resistance of the load, V
BB
(D) RC Blanking.
In addition to determining the fixed off-time of the PWM control
circuit, the C
T
component sets the comparator blanking time.
This function blanks the output of the comparator when the out-
puts are switched by the internal current-control circuitry (or by
the PHASE, BRAKE, or ENABLE inputs). The comparator out-
put is blanked to prevent false over-current detections due to
reverse recovery currents of the clamp diodes, and/or switching
transients related to distributed capacitance in the load.
During internal PWM operation, at the end of the t
OFF
time, the
is the motor supply voltage and t
ON (min)
max is specified in the
electrical characteristics table. When the motor is rotating, the
back EMF generated will influence the above relationship. For
brush dc motor applications, the current regulation is improved.
For stepper motor applications, when the motor is rotating, the
effect is more complex. A discussion of this subject is included
in the section on stepper motors below.
The following procedure can be used to evaluate the worst-case
slow current-decay internal PWM load current regulation in the
system:
I
AVE
t
off
R
T
C
T
when stalled, or at startup), the worst case value of current regu-
lation can be approximated by:
[(V
BB
−V
SAT (source
+
sink)
)
t
on (min)
max]−[1.05
(V
SAT (sink)
+ V
F
)
t
off
]
1.05
(t
on (min)
max + t
off
)
R
LOAD
64
A3953SB/SLB
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