MAX798 Maxim, MAX798 Datasheet - Page 6

• Manufacturer Part Number: MAX798
• Description: High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
• Manufacturer: Maxim

High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
The MAX798 is a BiCMOS, switch-mode power-supply
controller designed primarily for buck-topology regula-
tors in battery-powered applications where high accu-
racy, high efficiency, and low quiescent supply current
are critical. The MAX798 also works well in other
topologies such as boost, inverting, and CUK due to
the flexibility of its floating high-speed gate driver.
Light-load efficiency is enhanced by automatic idle-
mode operation—a variable-frequency pulse-skipping
Figure 1. Standard Application Circuit
6
_______________Detailed Description
_______________________________________________________________________________________
ON
V
IN
Q1, Q2 =
C1, C2 =
, 4.5V TO 30V
0.33 F
0.01 F
C4 =
R1 =
R2 =
L1 =
OFF
f =
2.2 F
SILICONIX Si4410DY or IRF7413
10 F/30V SANYO OS-CON (30SA10)
470 F/4V SPRAGUE 594D SERIES (594D477X0004R2T)
SUMIDA CDRH127 120
6.49k
11.5k
150kHz
SHDN
REF
SS
V+
SYNC
1%
1%
R3
10
MAX798
SKIP
GND
4.7 F
VL
PGND
CSH
BST
CSL
DH
DL
LX
FB
1M
R4
C3
47pF
0.1 F
Q1
Q2
mode that reduces losses due to MOSFET gate charge.
The step-down power-switching circuit consists of two
N-channel MOSFETs, a rectifier, and an LC output filter.
The output voltage is the average of the AC voltage at
the switching node, which is adjusted and regulated by
changing the duty cycle of the MOSFET switches. The
gate-drive signal to the N-channel high-side MOSFET
must exceed the battery voltage and is provided by a
flying capacitor boost circuit that uses a 100nF capaci-
tor connected between BST and LX.
12 H
C1
L1
C2
15m
470pF
C5
C4
V
OUT
, +2.5V @ 4.2A
R1
R2