A3957SA 데이터 시트보기 (PDF) - Allegro MicroSystems
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A3957SA Datasheet PDF : 16 Pages
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3957
FULL-BRIDGE PWM
MICROSTEPPING
MOTOR DRIVER
cost of decreased average current accuracy or increased driver
and motor losses.
Mixed Current-Decay Mode. If VPFD is between 1.2 V and
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 ITRIP.
When the voltage at the RC terminal decays to a value below
VPFD, the sink drivers are re-enabled, placing the device in slow
current-decay mode for the remainder of the fixed off time
(figure 2). The percentage of fast decay (PFD) is user deter-
mined by VPFD or two external resistors.
PFD = 100 ln (0.6[R1+R2]/R2)
where
VCC
R1
PFD
R2
Dwg. EP-062-1
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, tOFF, is determined by the selection of an
external resistor (RT) and capacitor (CT) connected from the RC
timing terminal to ground. The off-time, over a range of values
of CT = 470 pF to 1500 pF and RT = 12 kΩ to 100 kΩ, is
approximated by:
tOFF ≈ RTCT.
When the load current is increasing, but has not yet reached
the sense-current comparator threshold (ITRIP), the voltage on
the RC terminal is approximately 0.6VCC. When ITRIP is
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 0.22VCC. The PWM latch is then set,
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.
With increasing values of tOFF, switching losses will
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 tOFF can be chosen for optimi-
zation of these parameters. For applications where audible
noise is a concern, typical values of tOFF are chosen to be in the
range of 15 µs to 35 µs.
RC Blanking. In addition to determining the fixed off-time of
the PWM control circuit, the CT component sets the comparator
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.
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 0.22VCC by an internal current source of
approximately 1 mA. The comparator output remains blanked
until the voltage on CT reaches approximately 0.6VCC. The
blanking time, tBLANK, can be calculated as:
tBLANK = RTCT ln (RT/[RT - 3 kΩ]).
When a transition of the PHASE input occurs, CT is
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,
CT is charged by an internal current source of approximately 1
mA. The comparator output remains blanked until the voltage
on CT reaches approximately 0.6VCC.
Similarly, when the device is disabled, via the DAC data
inputs, CT is discharged to near ground. When the device is re-
enabled, CT is charged by an internal current source of approxi-
mately 1 mA. The comparator output remains blanked until the
voltage on CT reaches approximately 0.6VCC. The blanking
time, tBLANK′, can be calculated as:
tBLANK′ ≈ 1900 CT.
The minimum recommended value for CT is 470 pF
± 5 %. This value ensures that the blanking time is sufficient to
avoid false trips of the comparator under normal operating
conditions. For optimal regulation of the load current, this
value for CT is recommended and the value of RT can be sized
to determine tOFF.
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