Application Information: continued
8) Use fewer, but larger output capacitors, keep the capaci-
Layout Guidelines
tors clustered, and use multiple layer traces with heavy
copper to keep the parasitic resistance low.
When laying out the CPU buck regulator on a printed cir-
cuit board, the following checklist should be used to ensure
proper operation of the CS5132.
9) Place the switching MOSFET as close to the +5V input
capacitors as possible.
1) Rapid changes in voltage across parasitic capacitors and
abrupt changes in current in parasitic inductors are major
concerns for a good layout.
10) Place the output capacitors as close to the load
as possible.
11) Place the VFFB,VOUT filter resistors (510½) in series with
the VFFB and VOUT pins as close as possible to the pins.
2) Keep high currents out of sensitive ground connections.
Avoid connecting the IC Gnd between the source of the
lower FET and the input capacitor Gnd.
12) Place the COFF and COMP capacitors as close as possi-
ble to the COFF and COMP pins.
3) Avoid ground loops as they pick up noise. Use star or
single point grounding.
13) Place the current limit filter capacitors between the
VFFB and VOUT pins, as close as possible to the pins.
4) For high power buck regulators on double-sided PCBs a
single ground plane (usually the bottom) is recommended.
14) Connect the filter components of the following pins:
VFB, VFFB, VOUT, COFF, and COMP to the LGnd pin with a
single trace, and connect this local LGnd trace to the output
capacitor Gnd.
5) Even though double sided PCBs are usually sufficient
for a good layout, four-layer PCBs are the optimum
approach to reducing susceptibility to noise. Use the two
internal layers as the power and Gnd planes, the top layer
for the high current connections and component vias, and
the bottom layer for the noise sensitive traces.
15) The ÒDroopÓ Resistor (embedded PCB trace) has to be
wide enough to carry the full load current.
16) Place the VCC bypass capacitors as close as possible to
the VCC pins and connect them to PGnd.
6) Keep the inductor switching node small by placing the
output inductor, switching and synchronous FETs close
together.
7) The MOSFET gate traces to the IC must be as short,
straight, and wide as possible. Ideally, the IC has to be
placed right next to the MOSFETs.
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