BD9D321EFJ Datasheet, PDF (18/28 Pages) Rohm – 4.5V to 18V Input, 3.0A Integrated MOSFET 1ch Synchronous Buck DC/DC Converter

BD9D321EFJ
Datasheet
PCB Layout Design
In the step-down DC/DC converter, a large pulse current flows into two loops. The first loop is the one into which the current
flows when the high side FET is turned ON. The flow starts from the input capacitor C_IN, runs through the FET, inductor L
and output capacitor C_OUT and back to ground of C_IN via ground of C_OUT. The second loop is the one into which the current
flows when the low side FET is turned on. The flow starts from the low side FET, runs through the inductor L and output
capacitor C_OUT and back to ground of the low side FET via ground of C_OUT. Route these two loops as thick and as short as
possible to allow noise to be reduced for improved efficiency. It is recommended to connect the input and output capacitors
directly to the ground plane. The PCB layout has a great influence on the DC/DC converter in terms of all of the heat
generation, noise and efficiency characteristics.
V_IN
C_IN
MOS FET
L
V_OUT
C_OUT
Figure 32. Current Loop of Buck Converter
Accordingly, design the PCB layout considering the following points.
 Connect an input capacitor as close as possible to the IC VIN terminal on the same plane as the IC.
 If there is any unused area on the PCB, provide a copper foil plane for the ground node to assist heat dissipation from
the IC and the surrounding components.
 Switching nodes such as SW are susceptible to noise due to AC coupling with other nodes. Route the coil pattern as
thick and as short as possible.
 Provide lines connected to FB and SS far from the SW nodes.
 Place the output capacitor away from the input capacitor in order to avoid the effect of harmonic noise from the input.
EN GND_S GND VIN VIN_S
EN GND_S GND VIN VIN_S
R1 R2
C1
CVREG CSS
CIN CBOOT
L
VOUT_S
VOUT
COUT
GND
GND_S
TOP Layer
Figure 33. Example of PCB layout
Bottom Layer
VOUT_S
VOUT
GND
GND_S
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TSZ22111•15•001
18/25
TSZ02201-0J3J0AC00490-1-2
07.Aug.2013 Rev.001