Dual Full Bridge Motor Driver
A3909
5
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Description
Device Operation
The 3909 is designed to operate two DC motors or a single
stepper motor. The outputs are PMOS source drivers combined
with low R
DS(on)
DMOS sink drivers.
Protection circuitry includes internal thermal shutdown, protec-
tion against shorted loads, and against outputs shorted to GND
or supply. Undervoltage lockout prevents damage by keeping the
outputs off until the driver has enough voltage to operate nor-
mally.
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.
Thermal Shutdown
If the die temperature increases to T
JTSD
, then all outputs are dis-
abled until the internal temperature falls below a hysteresis level,
T
TSDHYS
, of 20°C. Internal UVLO is detected on VBB to prevent
output drivers from turning on when below the UVLO threshold.
IN1
IN2
OUT1
OUT2
2 3 4
t
STANDBY
5 6 7
Current (1Æ2)
1) Low power standby (Sleep) mode (I
BB
= 0)
2) On, reverse – 100% d
uty cycle
3) On, forward – 100% duty cycle
4) Slow decay PWM
5) Fast decay PWM
6) Coast
7) Low power standby (Sleep) mode
after t
STANBY
(I
BB
= 0)
DC Motor Timing Diagram
