2919
DUAL FULL-BRIDGE
MOTOR DRIVER
APPLICATIONS INFORMATION
PWM CURRENT CONTROL
PWM OUTPUT CURRENT WAVE FORM
The A2919SB/SLB 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 (RS), internal
comparator, and monostable multivibrator.
V
P HAS E
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’s input:
+
I
0
–
OUT
ITRIP = VREF/10 RS
I
TR IP
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’s OFF time (and therefore
the magnitude of the current decrease) is determined by
the monostable’s external RC timing components, where
toff = RTCT within the range of 20 kΩ to 100 kΩ and
t
t
off
d
Dwg. WM-003-1A
LOAD CURRENT PATHS
.
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. Be-
cause 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.
V
BB
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, main-
taining the average motor winding current at the desired
level.
BR IDG E ON
S OUR C E OFF
ALL OFF
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).
R
S
Dwg. E P -006-1
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
