Rainbow Electronics MAX8707 User Manual
Page 35
This is essential for high-power MOSFETs that
require low-impedance gate drivers to avoid shoot-
through currents.
7) When trade-offs in trace lengths must be made, it’s
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it’s better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and the low-
side MOSFET or between the inductor and the out-
put filter capacitor.
Layout Procedure
1) Place the power components first, with ground termi-
nals adjacent (low-side MOSFET source, C
IN
, C
OUT
,
and D1 anode). If possible, make all these connec-
tions on the top layer with wide, copper-filled areas.
2) Mount the driver IC adjacent to the low-side
MOSFETs. The DL gate traces must be short and
wide (50mils to 100mils wide if the MOSFET is 1in
from the driver IC).
3) Group the gate-drive components (BST diodes and
capacitors, V
DD
bypass capacitor) together near the
driver IC.
4) Make the DC-DC controller ground connections as
shown in the Standard Application Circuits. This dia-
gram can be viewed as having three separate
ground planes: input/output ground, where all the
high-power components go; the power ground
plane, where the PGND pin, V
DD
bypass capacitor,
and driver IC ground connection go; and the con-
troller’s analog ground plane, where sensitive ana-
log components, the master’s GND pin, and the V
CC
bypass capacitor go. The controller’s analog ground
plane (GND) must meet the power ground plane
(PGND) only at a single point directly beneath the
IC. The power ground plane should connect to the
high-power output ground with a short, thick metal
trace from PGND to the source of the low-side
MOSFETs (the middle of the star ground).
5) Connect the output power planes (V
CORE
and sys-
tem ground planes) directly to the output-filter-
capacitor positive and negative terminals with
multiple vias. Place the entire DC-DC converter cir-
cuit as close to the CPU as is practical.
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
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