73
A3952SB/SLB/SW2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3952SB/SLB/SW
I
TRIP
10 • R
S
V
REF
I
TRIP
R
S
1.5V
tOFF
RT
•
CT
CAUTION: Because the kinetic energy stored in the motor and
load inertia is being converted into current, which charges the
VBB 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 exceed-
ing the voltage rating of any devices connected to the motor
supply.
(2)Brake Operation-MODE Input Low
During braking,with the MODE input low, the peak current limit
defaults internally to a value approximated by
In this mode, the value of RS determines the ITRIP value indepen-
dent of VREF. This is useful in applicaions with differing run and
brake currents and no practical method of varying VREF.
Choosing a small value for R S essentially disables the current
limiting during braking. Therefore, care should be taken to en-
sure that the motor ’s current does not exceed the absolute
maximum ratings of the device. The braking current can be
measured by using an oscilloscope with a current probe con-
nected to one of the motor’s leads.
(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, toff (fixed OFF time), is determined by the selection of an
external resistor (RT) and capacitor (CT) connected in parallel
from the RC terminal to ground. The fixed OFF time, over a
range of values of CT=820pF to 1500pF and RT=12kΩ to 100kΩ,
is approximated by
When the PWM latch is reset by the current comparator, the
voltage on the RC terminal will begin to decay from approxi-
mately 3 volts. When the voltage on the RC terminal reaches
approximately 1.1 volt, the PWM latch is set, thereby re-enabling
the driver (s).
(D)RC Blanking
In addition to determining the fixed OFF-time of the PWM con-
trol circuit, the CT 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 toff time, the
comparator’s output is blanked and CT begins to be charged
from approximately 1.1V by an internal current source of ap-
proximately 1mA. The comparator output remains blanked until
the voltage on CT reaches approximately 3.0 volts.
Similarly, 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 con-
duction of the source and sink drivers). After the crossover de-
lay, CT is charged by an internal current source of approximately
1mA. The comparator output remains blanked until the voltage
on CT reaches approximately 3.0 volts.
Similarly, when the device is disabled via the ENABLE input, CT
is discharged to near ground. When the device is re-enabled,
CT is charged by the internal current source. The comparator
output remains blanked until the voltage on CT reaches approxi-
mately 3.0V.
For applications that use the internal fast-decay mode PWM
operation, the minimum recommended value is CT=1200pF±5%.
For all other applications, the minimum recommended value is
CT=820pF±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 above values for CT are recommended and the value of RT
can be sized to determine toff. For more information regarding
load current regulation, see below.
(E)LOAD CURRENT REGULATION WITH THE INTERNAL
PWM CURRENT-CONTROL CIRCUITRY
When the device is operating in slow-decay mode, there is a
limit to the lowest level that the PWM current-control circuitry
can regulate load current. The limitation is the minimum duty
cycle, which is a function of the user-selected value of toff and
the maxuimum value of the minimum ON-time pulse, ton (min), 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,
or a brush dc motor when stalled or at startup, the worst-case
value of current regulation can be approximated by
where toff=RT•CT, RLOAD is the series resistance of the load, VBB is
the load/motor supply voltage, and ton (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 under “Applications”.
The following procedure can be used to evaluate the worst-case
slow-decay internal PWM load current regulation in the system:
Set VREF to 0 volts. With the load connected and the PWM current
control operating in slow-decay mode, use an oscilloscope to
measure the time the output is low (sink ON) for the output that is
chopping. This is the typical minimum ON time (ton (min) t yp ) for the
1.05
•
(t
on (min)
max + t
off
)
•
R
LOAD
[(V
BB
−V
SAT (source
+
sink)
)
•
t
on (min)
max]−[1.05
•
(V
SAT (sink)
+ V
D
)
•
t
off
]
I
(AV)
≅