A4407 Datasheet, PDF (24/29 Pages) Allegro MicroSystems – The A4407 is an automotive power management IC that uses a 2.2 MHz constant on-time (COT) buck pre-regulator to supply a 5 V linear regulator, a 5 V tracking/protected linear regulator,

A4407
2.2 MHz Constant On-Time Buck Regulator
With Two External and Two Internal Linear Regulators
PCB Component Placement and Routing
The board layout has a large impact on the performance of the
device. It is important to isolate high current ground returns to
minimize ground bounce that could produce reference errors in
the device. The method used to isolate power ground from noise
sensitive circuitry is to use a star ground. This approach makes
sure that the high current components such as the input capacitor,
output capacitor, and diode have very low impedance paths to
each other. Figure 16 illustrates the technique.
The ground traces for each of the components should be very
close to each other and should be connected to each other on the
same surface as the components. Internal ground planes should
not be used for the star ground connection, because vias add
impedance to the current path.
In order to further reduce noise effects on the PCB, noise sensi-
tive 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 are also 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. That ground plane should not be broken
directly under the node, because the lowest impedance path back
to the star ground is directly under the signal trace. If another
trace does break the return path, the energy would have to find
another path, which would be through radiated emissions.
The peak-to-peak amplitude of the buck current sense signal
will typically be only tens of millivolts. The current sense pins,
ISEN+ and ISEN–, and internal differential amplifier comprise a
differential signal receiver, and balanced pair of traces should be
routed from the pins of the buck current sense resistor, R_SENSE ,
as shown in figure 17 (upper panel). The ISEN+ pin and the sense
resistor ground should not be separated by simply using local via
connections to the ground plane (figure 17 lower panel). Incorrect
routing of the ISEN+ pin would likely add an offset error to the
buck current sense signal.
VIN
Current path (on-cycle)
LX
L1
C_IN ^Q1
A4407
D_BUCK
R_SENSE
Current path
(off-cycle)
C_OUT1
R_LOAD
Star Ground
Differential
Amplifier
LX
ISEN–
ISEN+
A4407
L1
D_BUCK
(Asynchronous)
R_SENSE
Correct routing of ISEN+ and ISEN– traces
(direct on same plane)
Differential
Amplifier
LX
ISEN–
ISEN+
A4407
L1
D_BUCK
(Asynchronous)
R_SENSE
Figure 16. Illustration of star ground connection
Ground plane
Incorrect routing of ISEN+ and ISEN– traces
(using vias to a ground plane)
Figure 17. Comparison of routing paths for the traces between the
A4407 ISEN+ and ISEN– traces and the sense resistor, R_SENSE
Allegro MicroSystems, Inc.
24
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com