A3957SLB Allegro, A3957SLB Datasheet - Page 11

• Manufacturer Part Number: A3957SLB
• Description: Full Bridge PWM Motor Driver 5V 24-Pin SOIC W
• Manufacturer: Allegro
• Type: Full Bridge PWM Motor Driverr

Specifications of A3957SLB

• Package: 24SOIC W
• Operating Supply Voltage: 5 V
• Maximum Operating Current: 4 mA
• Operating Temperature: -20 to 85 °C

cost of decreased average current accuracy or increased driver
and motor losses.
Mixed Current-Decay Mode. If V
2.9 V, the device will be in a mixed current-decay mode.
Mixed-decay mode allows the user to achieve good current
regulation with a minimum amount of ripple current and motor/
driver losses by selecting the minimum percentage of fast decay
required for their application (see also Stepper Motor Applica-
tions).
As in fast current-decay mode, mixed-decay starts with the sink
and source drivers disabled after the load current reaches I
When the voltage at the RC terminal decays to a value below
V
current-decay mode for the remainder of the fixed off time
(figure 2). The percentage of fast decay (PFD) is user deter-
mined by V
where
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 off-time, t
external resistor (R
timing terminal to ground. The off-time, over a range of values
of C
approximated by:
the sense-current comparator threshold (I
the RC terminal is approximately 0.6V
reached, the PWM latch is reset by the current-sense compara-
tor and the voltage on the RC terminal will decay until it
reaches approximately 0.22V
thereby re-enabling the driver(s) and allowing load current to
increase again. The PWM cycle repeats, maintaining the peak
load current at the desired value.
PFD
3957
FULL-BRIDGE PWM
MICROSTEPPING
MOTOR DRIVER
T
When the load current is increasing, but has not yet reached
, the sink drivers are re-enabled, placing the device in slow
= 470 pF to 1500 pF and R
PFD
or two external resistors.
PFD = 100 ln (0.6[R
T
) and capacitor (C
OFF
R 1
R 2
V
CC
t
OFF
, is determined by the selection of an
CC
Dwg. EP-062-1
. The PWM latch is then set,
R
T
PFD
T
= 12 k to 100 k , is
C
PFD
T
1
T
.
+R
) connected from the RC
CC
is between 1.2 V and
. When I
TRIP
2
]/R
), the voltage on
2
)
TRIP
is
TRIP
.
decrease, low-level load-current regulation will improve, EMI
will be reduced, the PWM frequency will decrease, and ripple
current will increase. A value of t
zation of these parameters. For applications where audible
noise is a concern, typical values of t
range of 15 s to 35 s.
RC Blanking. In addition to determining the fixed off-time of
the PWM control circuit, the C
blanking time. This function blanks the output of the current-
sense comparator when the outputs are switched by the internal
current-control circuitry (or by the PHASE input, or when the
device is enabled with the DAC data inputs). The comparator
output 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.
the comparator’s output is blanked and C
from approximately 0.22V
approximately 1 mA. The comparator output remains blanked
until the voltage on C
blanking time, t
discharged to near ground during the crossover delay time (the
crossover delay time is present to prevent simultaneous conduc-
tion of the source and sink drivers). After the crossover delay,
C
mA. The comparator output remains blanked until the voltage
on C
inputs, C
enabled, C
mately 1 mA. The comparator output remains blanked until the
voltage on C
time, t
avoid false trips of the comparator under normal operating
conditions. For optimal regulation of the load current, this
value for C
to determine t
T
5 %. This value ensures that the blanking time is sufficient to
is charged by an internal current source of approximately 1
With increasing values of t
During internal PWM operation, at the end of the t
When a transition of the PHASE input occurs, C
Similarly, when the device is disabled, via the DAC data
The minimum recommended value for C
T
reaches approximately 0.6V
BLANK
T
is discharged to near ground. When the device is re-
T
T
is charged by an internal current source of approxi-
T
, can be calculated as:
is recommended and the value of R
OFF
reaches approximately 0.6V
t
BLANK
BLANK
.
T
, can be calculated as:
= R
t
reaches approximately 0.6V
BLANK
T
CC
C
T
by an internal current source of
ln (R
T
OFF,
component sets the comparator
1900 C
OFF
CC
T
switching losses will
/[R
OFF
.
can be chosen for optimi-
T
T
are chosen to be in the
T
.
- 3 k ]).
CC
begins to be charged
. The blanking
T
is 470 pF
T
can be sized
CC
T
. The
is
OFF
time,