Valley Current Mode Control Buck Converter
A4403
13
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
To avoid ground offset issues in the output voltage, it is highly
recommended that the ground-referenced feedback resistor R6
should be connected directly to the GND connection of the
A4403. In other words, the R6 ground return should avoid the use
of the internal ground plane.
All ground-referenced support components (C5 and the DIS
switch) should also be located as close to the GND connection as
possible. A “local quiet” ground plane around these components
can be implemented; however, this ground plane should have a
high impedance connection to the star ground connection of the
power stages, as referenced below.
The sense resistor connections should be connected in a Kelvin
circuit (see figure 3) to the corresponding pins on the A4403
(ISEN and SGND). Note that it is imperative that the PCB traces
between the sense resistor pads and the sense connections are as
short as possible to minimize the effects of leakage inductance.
In noisy systems, it is highly recommended that an R-C filter be
used to filter the signal produced across the ISEN pin. See the
Sense Resistor section and the Typical Application schematic.
If an internal ground plane is used, it is recommended that it
does not overlap the switching node, LX, to avoid the possibility
of noise pick up. To minimize the possibility of noise injection
issues, it is recommended to isolate the ground plane around the
high impedance nodes, such as FB and SS.
Q
L
R
LOAD
VIN
V
OUT
D
C
OUT
C
IN
LX
Star Connection
R
Input
Voltage
Figure 5. FET on-cycle current conduction paths
Q
L
VIN
LX
Star Connection
D
R
Input
Voltage
R
LOAD
V
OUT
C
OUT
C
IN
Figure 6. FET off-cycle current conduction paths
Cin
Cout
D
Star Connection
A4403
Internal Ground Plane
Thermal Vias
Power Circuitry
A4403 Support
Com
onents
Local ‘quiet’
Ground Plane
GND
R
R6
Switch
C5
SGND
Figure 4. Ground plane configurations
