Rainbow Electronics MAX1842 - Manual (Page 10)

MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
10 ______________________________________________________________________________________
Synchronous Rectification
In a step-down regulator without synchronous rectifica-
tion, an external Schottky diode provides a path for cur-
rent to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS syn-
chronous switch reduces conduction losses and
improves efficiency.
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In
V CC
470pF
2.2 µF
1 µF
10 µF
10 Ω
FBSEL
0.01 µF
FEEDBACK
SELECTION
CURRENT
SENSE
PWM LOGIC
AND
DRIVERS
SS
IN
FB
V IN
3.0V TO 5.5V
LX
PGND TOFF
R
TOFF
GND
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
REF
REF
SUMMING
COMPARATOR
REF
REF
COMP
SKIP
SHDN
TIMER
V IN
CURRENT
SENSE
G m
C OUT
V OUT
MAX1742
MAX1842
Figure 2. Functional Diagram
TOFF
COMP
V
CC
FBSEL
SHDN
IN
PGND
GND
REF
SS
LX
FB MAX1742
R
TOFF
1 µF
0.01 µF
L
2.2 µF
470pF
10 Ω
INPUT
C
IN
= 10 µF (MAX1742)
C
IN
= 33 µF (MAX1842) OUTPUT
C
OUT = 47 µF (MAX1742)
C OUT = 150 µF (MAX1842)
V OUT = 2.5V, FBSEL = V CC
V OUT = 1.8V, FBSEL = REF
V OUT = 1.5V, FBSEL = FLOATING
Figure 1. Typical Circuit