
PRODUCT SPECIFICATION
FAN5019B
REV. 1.0.0 Jul/15/05
27
Figure 10. Transient Setting Waveform
Since the FAN5019B turns off all of the phases (switches
inductors to ground), there is no ripple voltage present
during load release. Thus, you do not have to add headroom
for ripple, allowing your load release V
TRANREL
to be larger
than V
TRAN1
by that amount and still be meeting spec. If
V
TRAN1
and V
TRANREL
are less than the desired final droop,
this implies that capacitors can be removed. When removing
capacitors, check the output ripple voltage as well to ensure
it is still within spec.
Layout and Component Placement
The following guidelines are recommended for optimal
performance of a switching regulator in a PC system.
Key layout issues are illustrated in Figure 11.
Figure 11. Layout Recommendations
General Recommendations
For good results, at least a four-layer PCB is
recommended. This should allow the needed versatility
for control circuitry interconnections with optimal
placement, power planes for ground, input, and output
power, and wide interconnection traces in the rest of the
power delivery current paths. Keep in mind that each
square unit of 1 ounce copper trace has a resistance of
~0.53 m
Ω
at room temperature.
Whenever high currents must be routed between PCB
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.
If critical signal lines (including the output voltage sense
lines of the FAN5019B) 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 to serves as a shield to minimize noise
injection into the signals..
Use an analog ground plane both around and under the
FAN5019B for ground connections to the components
associated with the controller. Tie this plane to the nearest
output decoupling capacitor ground and not to any other
power circuitry to prevent power currents from flowing in
it.
Connect the components around the FAN5019B 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.
Connect the output capacitors as close as 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.
Power Circuitry
Route the switching power path on the PCB to 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. Avoid crossing any signal lines over
the switching power path loop, described below.