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MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
__________Applications Information
Efficiency Considerations
Refer to the MAX796–MAX799 data sheet for informa-
tion on calculating losses and improving efficiency.
PC Board Layout Considerations
Good PC board layout and routing are required in high-
current, high-frequency switching power supplies to
achieve good regulation, high efficiency, and stability.
The PC board layout artist must be provided with
explicit instructions concerning the placement of
power-switching components and high-current routing.
It is strongly recommended that the evaluation kit PC
board layouts be followed as closely as possible.
Contact Maxim’s Applications Department concerning
the availability of PC board examples for higher-current
circuits.
In most applications, the circuit is on a multilayer
board, and full use of the four or more copper layers is
recommended. Use the top layer for high-current
power and ground connections. Leave the extra cop-
per on the board as a pseudo-ground plane. Use the
bottom layer for quiet connections (REF, FB, AGND),
and the inner layers for an uninterrupted ground plane.
A ground plane and pseudo-ground plane are essential
for reducing ground bounce and switching noise.
Place the high-power components (C1, R1, N1, D1, N2,
L1, and C2 in Figure 1) as close together as possible.
Minimize ground-trace lengths in high-current paths.
The surface-mount power components should be
butted up to one another with their ground terminals
almost touching. Connect their ground terminals using
a wide, filled zone of top-layer copper (the pseudo-
ground plane), rather than through the internal ground
plane. At the output terminal, use vias to connect the
top-layer pseudo-ground plane to the normal inner-
layer ground plane at the output filter capacitor ground
terminals. This minimizes interference from IR drops
and ground noise, and ensures that the IC’s AGND is
sensing at the supply’s output terminals.
Minimize high-current path trace lengths. Use very
short and wide traces. From C1 to N1: 0.4 in. (10mm)
max length; D1 anode to N2: 0.2 in. (5mm) max length;
LX node (N1 source, N2 drain, D1 cathode, inductor
L1): 0.6 in. (15mm) max length.
___________________Chip Information
