Rainbow Electronics MAX1625 - Manual (Page 21)

MAX1624/MAX1625
High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
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Bypassing the Internal Reference
Bypass the internal 3.5V reference at the REF pin by
connecting a 0.1µF capacitor to AGND. Use a larger
value, such as 1µF, for high-current applications.
Choosing the GlitchCatcher MOSFETs
P-channel and N-channel switches and a series resistor
are required for the current-boost circuit (Figure 10).
Current through the MOSFETs and current-limiting
resistors must be sufficient to supply the load current,
with enough extra for prompt output regulation without
excessive overshoot. Design for boost-current values
1.5 times the maximum load current, and choose
MOSFETs and current-limiting resistors such that:
__________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.
Follow these steps:
1) Place the high-power components (C1, R1, N1,
D1, N2, L1, and C2 in Figure 1) as close together
as possible, following these priorities:
• 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 out-
put 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.
R R
V V
I
and
R R
V
I
DSON P MAX LIMIT
IN OUT
OUT MAX
DSON N MAX LIMIT
OUT
OUT MAX
, ( )
( )
, ( )
( )
.
.
+ ≈
−
+ ≈
1 5
1 5
R11
INPUT 5V
C3
C2
C1
OUTPUT
1.1V TO 3.5V
N3 NDRV
PDRV
LOAD
MAX1624
P1
Figure 10. GlitchCatcher Circuit