Dual Full Bridge Motor Driver
Allegro MicroSystems, LLC
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
The 3909 is designed to operate two DC motors or a single
stepper motor. The outputs are PMOS source drivers combined
with low RDS(on) DMOS sink drivers.
Protection circuitry includes internal thermal shutdown, protec-
or supply. Undervoltage lockout prevents damage by keeping the
outputs off until the driver has enough voltage to operate nor-
A low power standby (Sleep) mode is activated when all inputs
are low for longer than 1 ms. Sleep mode disables all of the cir-
cuitry making the IC ideal for battery operated applications.
Overcurrent Protection (OCP)
The A3909 is protected against accidental shorts or motor outputs
to ground and supply, as well as a shorted load condition. For
the source drivers, the current is monitored after the MOSFET is
turned on. If the current exceeds 1.8 A for longer than 2 µs, then
a fault condition is asserted. The sink driver utilizes a drain-to-
source voltage monitor. If the voltage exceeds 2 V for longer than
2 µs, the fault condition is asserted.
When a fault occurs, the IC immediately disables both sides of
the full bridge where the fault occurred. The full bridge input
commands will be ignored for a 2 ms period before being allowed
to retry. Each channel has independent overcurrent protection.
During OCP events, the absolute ratings may be exceeded for a
short period of time before the outputs are disabled.
If the die temperature increases to TJTSD , then all outputs are dis-
abled until the internal temperature falls below a hysteresis level,
TTSDHYS, of 20°C. Internal UVLO is detected on VBB to prevent
output drivers from turning on when below the UVLO threshold.
2 3 4
5 6 7
1) Low power standby (Sleep) mode (IBB = 0)
2) On, reverse – 100% duty cycle
3) On, forward – 100% duty cycle
4) Slow decay PWM
5) Fast decay PWM
7) Low power standby (Sleep) mode
after tSTANBY (IBB = 0)
DC Motor Timing Diagram