Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
ON/OFF
INPUT
C3
+5V ALWAYS ON
4.7μF
10Ω
0.1μF
V+ SHDN SECFB VL
0.1μF
SYNC
BST3
4.7μF
0.1μF
BST5
DH5
Q3
Q4
Q1
Q2
DH3
0.1μF
L1
0.1μF
0.1μF
L2
R1
+5V OUTPUT
C1
R2
+3.3V OUTPUT
LX5
DL5
LX3
MAX1631A
MAX1634A
C2
*
*
DL3
PGND
CSH3
CSL3
FB3
CSH5
CSL5
FB5
RESET OUTPUT
RESET
TIME/ON5
RUN/ON3
5V ON/OFF
SKIP
3.3V ON/OFF
STEER
GND
REF
SEQ
+2.5V ALWAYS ON
1μF
*1A SCHOTTKY DIODE REQUIRED
FOR THE MAX1631A (SEE OUTPUT
OVERVOLTAGE PROTECTION SECTION).
Figure 1. Standard 3.3V/5V Application Circuit (MAX1631A/MAX1634A)
cy of these circuits without first recalculating compo-
nent values (particularly inductance value at maximum
battery voltage). Adding a Schottky rectifier across
each synchronous rectifier improves the efficiency of
these circuits by approximately 1%, but this rectifier is
otherwise not needed because the MOSFETs required
for these circuits typically incorporate a high-speed sili-
con diode from drain to source. Use a Schottky rectifier
rated at a DC current equal to at least 1/3 of the load
current.
_______Standard Application Circuit
The basic MAX1631A/MAX1634A dual-output 3.3V/5V
buck converter (Figure 1) is easily adapted to meet a
wide range of applications with inputs up to 28V by
substituting components from Table 1. These circuits
represent a good set of tradeoffs between cost, size,
and efficiency, while staying within the worst-case
specification limits for stress-related parameters, such
as capacitor ripple current. Do not change the frequen-
8
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