ADP3208 ON Semiconductor, ADP3208 Datasheet - Page 36

• Manufacturer Part Number: ADP3208
• Description: 7-bit, Programmable, Dual-phase, Mobile, Cpu, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

ADP3208
2.
3.
LAYOUT AND COMPONENT PLACEMENT
The following guidelines are recommended for optimal
performance of a switching regulator in a PC system.
If both overshoots are larger than desired, try the following
adjustments in the order shown.
a.
b.
c.
If these adjustments do not change the response, it is
because the system is limited by the output decoupling.
Check the output response and the switching nodes each
time a change is made to ensure that the output decoupling
is stable.
For load release (see Figure 47), if V
the value specified by IMVP-6+, a greater percentage of
output capacitance is needed. Either increase the
capacitance directly or decrease the inductor values. (If
inductors are changed, however, it will be necessary to
redesign the circuit using the information from the
spreadsheet and to repeat all tuning guide procedures).
Increase the resistance of the ramp resistor
(R
For V
frequency.
For V
Figure 47. Transient Setting Waveform, Load Release
Figure 46. Transient Setting Waveform, Load Step
RAMP
V
TRAN1
TRAN2
V
TRAN1
) by 25%.
TRANREL
, increase R
, increase C
V
TRAN2
A
B
by 25% and decrease C
or increase the switching
V
DROOP
V
DROOP
TRANREL
is larger than
Rev. 1 | Page 36 of 38 | www.onsemi.com
A
by 25%.
General Recommendations
1.
2.
3.
4.
5.
6.
7.
Power Circuitry
1.
For best results, use a PCB of four or more layers. This
should provide the needed versatility for control circuitry
interconnections with optimal placement; power planes for
ground, input, and output; and wide interconnection traces
in the rest of the power delivery current paths. Keep in
mind that each square unit of 1 oz copper trace has a
resistance of ~0.53 mΩ at room temperature.
When 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 ADP3208) 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 increasing signal
ground noise.
An analog ground plane should be used around and under
the ADP3208 for referencing the components associated
with the controller. This plane should be tied to the nearest
ground of the output decoupling capacitor, but should not
be tied to any other power circuitry to prevent power
currents from flowing into the plane.
The components around the ADP3208 should be located
close to the controller with short traces. The most important
traces to keep short and away from other traces are those
to the FB and CSSUM pins. Refer to Figure 40 for more
details on the layout for the CSSUM node.
The output capacitors should be connected as close as
possible to the load (or connector) that receives the power
(for example, a microprocessor core). If the load is distributed,
the capacitors should also be distributed and generally placed
in greater proportion where the load is more dynamic.
Avoid crossing signal lines over the switching power path
loop, as described in the Power Circuitry section.
The switching power path on the PCB should be routed to
encompass the shortest possible length to minimize
radiated switching noise energy (that is, 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,
wide interconnection traces is especially critical in this