Dual DMOS Full Bridge Motor Driver
With Serial Port Control and Dual Regulators
A3998
16
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
PCB Layout
Switcher
The board layout has a significant impact on the performance of
the device. It is important to isolate high current ground returns,
in order to minimize ground bounce that could produce reference
errors in the device. The method used to isolate power ground
from noise sensitive circuitry is a star ground. This approach
makes sure the high current components such as the input capaci-
tor, output capacitor, and diode have very low impedance paths
to each other. Figure 5 illustrates the technique. The ground from
each of the components should be very close to each other and
be connected on the same surface as the components. Internal
ground planes should not be used for the star ground connec-
tion, because vias add impedance to the current path. In order to
further reduce noise effects on the PCB, noise sensitive traces
should not be connected to internal ground planes.
The feedback network from the switcher output should have an
independent ground trace that goes directly to the exposed pad
underneath the device. The exposed pad should be connected to
internal ground planes and to any exposed copper used for heat
dissipation. If the grounds from the device also are connected
directly to the exposed pad, the ground reference from the feed-
back network will be less susceptible to noise injection or ground
bounce.
To reduce radiated emissions from the high frequency switching
nodes it is important to have an internal ground plane directly
under the LX node. The plane should not be broken directly
under the switching path because the lowest impedance path for
radiated emissions is back to the star ground using the ground
plane directly under the signal trace. If another trace does break
the return path, the energy will have to find another path, which
is through radiated emissions or through stray eddy currents.
Motor Driver
In order to use PWM current control, a low-value resistor is
placed between the LSSx pin and ground for current sensing
purposes. To minimize ground-trace IR drops in sensing the
output current level, the current sensing resistor should have an
independent ground return to the star ground point. This trace
should be as short as possible. For low-value sense resistors, the
IR drops in the PCB can be significant, and should be taken into
account. When selecting a value for the sense resistor be sure not
to exceed the maximum voltage on the SENSEx pin of ±500 mV
at maximum load. During overcurrent events, this rating may be
exceeded for short durations.
Thermal Considerations
The PCB should have a thick ground plane. For optimum
electrical and thermal performance, the A3998 must be soldered
directly onto the board. On the underside of the A3998 package is
an exposed pad, which provides a path for enhanced thermal dis-
sipation. The thermal pad must be soldered directly to an exposed
surface on the PCB in order to achieve optimal thermal conduc-
tion. Thermal vias are used to transfer heat to other layers of the
PCB. The load supply pin, VBB, should be decoupled with an
electrolytic capacitor (typically 100 F) in parallel with a lower
valued ceramic capacitor placed as close as practicable to the
device
Switching Regulator Component Selection
External component recommended values are provided in table 3.
V
OUT1
The regulator requires an external clamping diode, D1,
inductor, L1, and filter capacitor, COUT1 (see figure 1).
The output voltage is determined by an external resistive voltage
divider, according to the following formula:
V
OUT1
= V
FB1
× (1 + R1 / R2) (5)
Application Information
Star Ground
Current path (on-cycle )
Current path (off-cycle )
C
IN
L
Q1
D
C
OUT
R
LOAD
Figure 5. Star Ground Connection
