Full-Bridge PWM Microstepping Motor Driver
A4975
8
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Mixed Current-Decay Mode. If V
PFD
is between 1.1 V
and 3.1 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 the Stepper Motor
Applications section).
As in fast current-decay mode, mixed-decay starts with the
sink and source drivers disabled and the opposite pair turned
on after the load current reaches I
TRIP
. When the voltage at the
RC terminal decays to a value below V
PFD
, the sink drivers are
re-enabled, placing the device in slow current-decay mode for
the remainder of the fi xed off-time (fi gure 2). The percentage
of fast decay (PFD) is user determined by V
PFD
or two external
resistors.
PFD = 100 ln (0.6[R
1
+R
2
]/R
2
)
where:
Dwg. EP-062-1
PFD
V
CC
R
2
R
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, t
OFF
, is determined by the selection of
an external resistor (R
T
) and capacitor (C
T
) connected from the
RC timing terminal to ground. The off-time, over a range of
values of C
T
= 470 pF to 1500 pF and R
T
= 12 kΩ to 100 kΩ, is
approximated by:
t
OFF
≈ R
T
C
T
.
When the load current is increasing, but has not yet reached the
sense-current comparator threshold (I
TRIP
), the voltage on the
RC terminal is approximately 0.6V
CC
. When I
TRIP
is reached,
the PWM latch is reset by the current-sense comparator and
the voltage on the RC terminal will decay until it reaches
approximately 0.22V
CC
. 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 t
OFF,
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 t
OFF
can be chosen for optimization
of these parameters. For applications where audible noise is a
concern, typical values of t
OFF
are chosen to be in the range of
15 to 35 μs.
RC Blanking. In addition to determining the fi xed off-time of
the PWM control circuit, the C
T
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 t
OFF
time, the
comparator’s output is blanked and C
T
begins to be charged
from approximately 0.22V
CC
by an internal current source of
approximately 1 mA. The comparator output remains blanked
until the voltage on C
T
reaches approximately 0.6V
CC
. The
blanking time, t
BLANK
, can be calculated as:
t
BLANK
= R
T
C
T
ln (R
T
/[R
T
– 3 kΩ]).
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 comparator output remains blanked until the voltage on C
T
reaches approximately 0.6V
CC
.
Similarly, when the device is disabled, via the DAC data inputs,
C
T
is discharged to near ground. When the device is re-enabled,
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 0.6V
CC
. The blanking time, t
BLANK
,
can be calculated as:
t
BLANK
= R
T
C
T
ln ([R
T
- 1.1 kΩ]/R
T
- 3 kΩ).
The minimum recommended value for C
T
is 470 pF ± 5 %.
This value ensures that the blanking time is suffi cient to avoid
