Low Voltage DC Motor Driver
A3918
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
PAD
A3918
C1
C2 C3
VBB
FL
GND
VCCIO
IN2
IN1
OUTA
OUTB
C4
R4
R6
C5 C6
R5
R2
CP4
CP1
CP3
VCP
IN2
GND
FL
GND
OUTA
SENSE
OUTB
CP2
GND
SLEEP
SLEEP
R1
R3
IN1
VBB
PCB
Thermal Vias
Trace (2 oz.)
Signal (1 oz.)
Ground (1 oz.)
Thermal (2 oz.)
Solder
A3918
Layout The printed circuit board should use a heavy ground-
plane. For optimum electrical and thermal performance, the
A3918 must be soldered directly onto the board. On the under-
side of the A3918 package is an exposed pad, which provides a
path for enhanced thermal dissipation. The thermal pad should be
soldered directly to an exposed surface on the PCB. Thermal vias
are used to transfer heat to other layers of the PCB.
Grounding In order to minimize the effects of ground bounce
and offset issues, it is important to have a low impedance single-
point ground, known as a star ground, located very close to the
device. By making the connection between the exposed thermal
pad and the ground plane directly under the A3918, that area
becomes an ideal location for a star ground point. A low imped-
ance ground will prevent ground bounce during high current
operation and ensure that the supply voltage remains stable at
the input terminal. The recommended PCB layout shown in the
diagram below, illustrates how to create a star ground under the
device, to serve both as low impedance ground point and
thermal path.
The two input capacitors should be placed in parallel, and as
close to the device supply pin as possible. The ceramic capaci-
tor should be closer to the pin than the bulk capacitor. This is
necessary because the ceramic capacitor will be responsible for
delivering the high frequency current components.
Sense Pin The sense resistor, R
SENSE
, should have a very low
impedance path to ground, because it must carry a large current
while supporting very accurate voltage measurements by the cur-
rent sense comparator. Long ground traces will cause additional
voltage drops, adversely affecting the ability of the comparator to
accurately measure the current in the winding. As shown in the
layout below, the SENSE pin has very short traces to the R
SENSE
resistor and very thick, low impedance traces directly to the star
ground underneath the device. If possible, there should be no
other components on the sense circuit.
Note: When selecting a value for the sense resistor, be sure not to
exceed the maximum voltage on the SENSE pin of ±500 mV.
R3
R2
R1
R5
FL
IN2
VCCIO
IN1
C1 C2
C3
U1
GND
VBB
OUTB
OUTA
R4
R6
C4
C5
C6
SLEEP
