A3918 Datasheet, PDF (7/8 Pages) Allegro MicroSystems – Low Voltage DC Motor Driver

A3918
Low Voltage DC Motor Driver
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.
A3918
PCB
Thermal Vias
Solder
Trace (2 oz.)
Signal (1 oz.)
Ground (1 oz.)
Thermal (2 oz.)
C1
C2
C3
SLEEP
IN1
IN2
VCCIO
FL
GND
C1
C2
C3
R3
U1
R1
C4
R2
OUTA
R4
R6
OUTB
R5
C5
VBB
C6
SLEEP
IN1
IN2
VCCIO
FL
GND
CP2 _A3918 OUTA
R4
GND
PAD ^SENSE
R6
SLEEP
OUTB
R3
IN1
R1
VBB
C4 C5 C6
R2
R5
OUTA
OUTB
VBB
Allegro MicroSystems, Inc.
7
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com