3959
DMOS FULL-BRIDGE
PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION
(continued)
Current Sensing.
To minimize inaccuracies in sensing
the I
TRIP
current level, which may be caused by ground
trace IR drops, the sense resistor should have an
independent ground return to the ground terminal of the
device. For low-value sense resistors the IR drops in the
PCB sense resistor’s traces can be significant and should
be taken into account. The use of sockets should be
avoided as they can introduce variation in R
S
due to their
contact resistance.
The maximum value of
R
S
is given as
R
S
≤
0.5/I
TRIP
where
I
TRIP
≤
3.0 A.
Thermal Protection.
Circuitry turns off all drivers
when the junction temperature reaches 165°C typically. It
is intended only to protect the device from failures due to
excessive junction temperatures and should not imply that
output short circuits are permitted. Thermal shutdown has
a hysteresis of approximately 15°C.
5
SUFFIX 'B', R
θJA
= 26°C/W
SUFFIX 'LP', R
θJA
= 28°C/W
SUFFIX 'LB', R
θJA
= 35°C/W
MULTI-LAYER HIGH-K BOARD
Layout.
A star ground system located close to the driver
is recommended. The printed wiring board should use a
heavy ground plane. For optimum electrical and thermal
performance*, the driver should be soldered directly onto
the board. The ground side of
R
S
should have an indi-
vidual path to the ground terminals of the device. This
path should be as short as is possible physically and
should not have any other components connected to it. It
is recommended that a 0.1
µF
capacitor be placed between
SENSE and ground as close to the device as possible; the
load supply terminal, V
BB
, should be decoupled with an
electrolytic capacitor (> 47
µF
is recommended) placed as
close to the device as is possible. On the 28-lead TSSOP
package, the copper ground plane located under the
exposed thermal pad is typically used as a star ground.
ALLOWABLE PACKAGE POWER DISSIPATION IN WATTS
4
* The thermal resistance, R
θJA
, and absolute maximum
allowable package power dissipation specified on page 1
is measured on a typical two-sided PCB with one square
inch copper ground area on each side. With minimal
copper on a single-sided PCB (worst-case), the ‘B’
package R
θJA
is 40°C/W, ‘LB’ is 77°C/W, and ‘LP’ is
80°C/W. See also, Application Note 29501.5,
Improving
Batwing Power Dissipation.
For specification purposes, the multi-layer high-K board
performance graphed here is per JEDEC Standard
JESD51.
The products described here are manufactured under one or more
U.S. patents or U.S. patents pending.
Allegro MicroSystems, Inc. reserves the right to make, from time to
time, such departures from the detail specifications as may be
required to permit improvements in the performance, reliability, or
manufacturability of its products. Before placing an order, the user is
cautioned to verify that the information being relied upon is current.
3
2
1
SUFFIX 'B', R
θJA
= 38°C/W
SUFFIX 'LP', R
θJA
= 40°C/W
SUFFIX 'LB', R
θJA
= 50°C/W
DOUBLE-SIDED BOARD,
1 SQ. IN. COPPER EA. SIDE
0
25
50
75
100
TEMPERATURE IN
°
C
125
150
Dwg. GP-049-6
Allegro products are not authorized for use as critical components
in life-support devices or systems without express written approval.
The information included herein is believed to be accurate and
reliable. However, Allegro MicroSystems, Inc. assumes no responsi-
bility for its use; nor for any infringement of patents or other rights of
third parties which may result from its use.
www.allegromicro.com
7
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
