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CS5421 Datasheet(PDF) 13 Page - ON Semiconductor

Part No. CS5421
Description  Dual Out?뭥f?뭁hase Synchronous Buck Controller with Remote Sense
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

CS5421 Datasheet(HTML) 13 Page - ON Semiconductor

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CS5421
http://onsemi.com
13
ΘSA = the sink−to−ambient thermal resistance (in degrees
C per watt).
The value for ΘJC is included in the component
manufacturer’s data sheets. Its value is dependent on the
mold compound and lead frames used in assembly of the
semiconductor device in question.
ΘCS is the thermal impedance from the surface of the case
to the heatsink. This component of the thermal resistance is
dependent on the roughness of the heatsink and component
as well as on the pressure applied between the two. ΘCS can
be reduced by using thermal pads or by applying a thin layer
of thermal grease between the case and the heatsink. Such
materials reduce the air gap normally found between the
heatsink and the case and provide a better path for thermal
energy. Values of ΘCS are found in catalogs published by
manufacturers of heatsinks and thermal compounds.
Finally, ΘSA is the thermal impedance from the heatsink
to the ambient environment. ΘSA is the important parameter
when selecting a heatsink. Low values of ΘSA allow
increased power dissipation without exceeding the
maximum junction temperature of the component. Values of
ΘSA are found in catalogs published by heatsink
manufacturers.
The basic equation for selecting a heatsink is:
P +
TJ * TA
QJC ) QCS ) QSA
where:
PD = power dissipated by part in question (in watts);
TJ = IC or FET junction temperature (in degrees C);
TA = ambient temperature (in degrees C);
ΘJC = the junction−to−case thermal resistance (in degrees
C per watt);
ΘCS = the case−to−sink thermal resistance (in degrees C
per watt);
ΘSA = the sink−to−ambient thermal resistance (in degrees
C per watt).
The choice of heatsink is dependent on the value of ΘSA
required to keep the calculated junction temperature at the
given level of power dissipation below the component
manufacturer’s maximum junction temperature.
EMI MANAGEMENT
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing for
compliance with EMI/EMC regulations, additional
components may be added to reduce noise emissions. These
components are not required for regulator operation and
experimental results may allow them to be eliminated. The
input filter inductor may not be required because bulk filter
and bypass capacitors, as well as other loads located on the
board will tend to reduce regulator di/dt effects on the circuit
board and input power supply. Placement of the power
component to minimize routing distance will also help to
reduce emissions.
LAYOUT GUIDELINES
When laying out the CPU buck regulator on a printed
circuit board, the following checklist should be used to
ensure proper operation of the CS5421.
1. Rapid changes in voltage across parasitic capacitors
and abrupt changes in current in parasitic inductors
are major concerns for a good layout.
2. Keep high currents out of sensitive ground
connections.
3. Avoid ground loops as they pick up noise. Use star or
single point grounding.
4. For high power buck regulators on double−sided
PCB’s a single ground plane (usually the bottom) is
recommended.
5. Even though double sided PCB’s are usually
sufficient for a good layout, four−layer PCB’s are the
optimum approach to reducing susceptibility to
noise. Use the two internal layers as the power and
GND planes, the top layer for power connections and
component vias, and the bottom layers for the noise
sensitive traces.
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 short,
straight, and wide as possible.
8. Use fewer, but larger output capacitors, keep the
capacitors clustered, and use multiple layer traces
with heavy copper to keep the parasitic resistance
low.
9. Place the switching MOSFET as close to the +5.0 V
input capacitors as possible.
10. Place the output capacitors as close to the load as
possible.
11. Place the COMP capacitor as close as possible to the
COMP pin.
12. Connect the filter components of the following pins:
VFB, VOUT, and COMP to the GND pin with a single
trace, and connect this local GND trace to the output
capacitor GND.
13. Place the VCC bypass capacitors as close as possible
to the IC.


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