FAN53168MTC FAIRCHILD [Fairchild Semiconductor], FAN53168MTC Datasheet - Page 25

• Manufacturer Part Number: FAN53168MTC
• Description: 6-Bit VID Controlled 2-4 Phase DC-DC Controller
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

PRODUCT SPECIFICATION
Since the FAN53168 turns off all of the phases (switches
inductors to ground), there is no ripple voltage present dur-
ing load release. Thus, you do not have to add headroom for
ripple, allowing your load release V
than V
If V
droop, this implies that capacitors can be removed. When
removing capacitors, make sure to check the output ripple
voltage as well to make sure it is still within spec.
LAYOUT AND COMPONENT PLACEMENT
The following guidelines are recommended for optimal per-
formance of a switching regulator in a PC system. Key lay-
out issues are illustrated in Figure 11.
General Recommendations
•
• Whenever high currents must be routed between PCB
• If critical signal lines (including the output voltage sense
• An analog ground plane should be used around and under
• The components around the FAN53168 should be located
• The output capacitors should be connected as closely as
• Avoid crossing any signal lines over the switching power
REV. 1.0.0 6/9/03
layers, vias should be used liberally to create several
parallel current paths so that the resistance and inductance
introduced by these current paths is minimized and the via
current rating is not exceeded.
lines of the FAN53168) must cross through power
circuitry, it is best if a signal ground plane can be
interposed between those signal lines and the traces of the
power circuitry. This serves as a shield to minimize noise
injection into the signals at the expense of making signal
ground a bit noisier.
the FAN53168 for referencing the components associated
with the controller to. This plane should be tied to the
nearest output decoupling capacitor ground and should
not tie to any other power circuitry to prevent power
currents from flowing in it.
close to the controller with short traces. The most
important traces to keep short and away from other traces
are the FB and CSSUM pins. Refer to Figure 11 for more
details on layout for the CSSUM node.
possible to the load (or connector) that receives the power
(e.g., a microprocessor core). If the load is distributed, the
capacitors should also be distributed, and generally in
proportion to where the load tends to be more dynamic.
path loop, described next.
TRAN1
TRAN1
Ω
and V
by that amount and still be meeting spec.
TRANREL
are less than the desired final
TRANREL
to be larger
Power Circuitry
• The switching power path should be routed on the PCB to
• Whenever a power dissipating component (e.g., a power
• The output power path should also be routed to
• For best EMI containment, a solid power ground plane
encompass the shortest possible length in order to
minimize radiated switching noise energy (i.e., EMI) and
conduction losses in the board. Failure to take proper
precautions often results in EMI problems for the entire
PC system as well as noise related operational problems
in the power converter control circuitry. The switching
power path is the loop formed by the current path through
the input capacitors and the power MOSFETs including
all interconnecting PCB traces and planes. The use of
short and wide interconnection traces is especially critical
in this path for two reasons: it minimizes the inductance in
the switching loop, which can cause high-energy ringing,
and it accommodates the high current demand with
minimal voltage loss.
MOSFET) is soldered to a PCB, the liberal use of vias,
both directly on the mounting pad and immediately
surrounding it, is recommended. Two important reasons
for this are: improved current rating through the vias, and
improved thermal performance from vias extended to the
opposite side of the PCB where a plane can more readily
transfer the heat to the air. Make a mirror image of any
pad being used to heatsink the MOSFETs on the opposite
side of the PCB to achieve the best thermal dissipation to
the air around the board. To further improve thermal
performance, the largest possible pad area should be used.
encompass a short distance. The output power path is
formed by the current path through the inductor, the
output capacitors, and the load.
should be used as one of the inner layers extending fully
under all the power components.
Figure 11. Layout Recommendations
FAN53168
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