A2919(2010) Ver la hoja de datos (PDF) - Allegro MicroSystems
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A2919 Datasheet PDF : 8 Pages
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A2919
Dual Full-Bridge PWM Motor Driver
APPLICATIONS INFORMATION
PWM CURRENT CONTROL
The A2919 dual bridges are designed to drive both windings
of a bipolar stepper motor. Output current is sensed and
controlled independently in each bridge by an external
sense resistor (R ), internal comparator, and monostable
S
multivibrator.
When the bridge is turned ON, current increases in the motor
winding and flows through the external sense resistor until
the sense voltage (VS) reaches the level set at the comparator
input:
ITRIP = VREF/10 RS
The comparator then triggers the monostable, which turns off
the source driver of the bridge. The actual load current peak
will be slightly higher than the trip point (especially for low-
inductance loads) because of the internal logic and switching
delays. This delay (td) is typically 2 μs. After turn-off, the
motor current decays, circulating through the ground-clamp
diode and sink transistor. The source driver off-time (and
therefore the magnitude of the current decrease) is determined
by the monostable external RC timing components, where
toff = RTCT within the range of 20 kΩ to 100 kΩ and
100 pF to 1000 pF.
The fixed off-time should be short enough to keep the current
chopping above the audible range (< 46 μs) and long enough
to properly regulate the current. Because only slow-decay
current control is available, short off times (< 10 μs) require
additional efforts to ensure proper current regulation. Factors
that can negatively affect the ability to properly regulate the
current when using short off times include: higher motor-
supply voltage, light load, and longer than necessary blank
time.
When the source driver is re-enabled, the winding current (the
sense voltage) is again allowed to rise to the comparator’s
threshold. This cycle repeats itself, maintaining the average
motor winding current at the desired level.
Loads with high distributed capacitances may cause current
spikes capable of tripping the comparator, resulting in
erroneous current control. An external RCCC time delay should
be used to delay the action of the comparator. Depending on
load type, many applications will not require these external
components (SENSE connected to E).
PWM OUTPUT CURRENT WAVE FORM
V PHAS E
+
I OUT 0
–
ITR IP
td
toff
Dwg. WM-003-1A
LOAD CURRENT PATHS
V
BB
BR IDGE ON
RS
S OUR CE OFF
ALL OFF
Dwg. E P -006-1
Allegro MicroSystems, Inc.
4
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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