Rainbow Electronics MAX1843 - Manual (Page 7)

MAX1843
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
_______________________________________________________________________________________ 7
Constant-Off-Time Mode
Constant-off-time operation occurs when the current
through the PMOS switch is greater than the idle-mode
threshold current (which corresponds to a load current of
half the idle mode threshold). In this mode, the regulation
comparator turns the PMOS switch on at the end of each
off-time, keeping the device in continuous-conduction
mode. The PMOS switch remains on until the output is in
regulation or the current limit is reached. When the
PMOS switch turns off, it remains off for the programmed
off-time (t
OFF ). To control the current under short-circuit
conditions, the PMOS switch remains off for approxi-
mately 4 x t
OFF when V OUT < V OUT(NOM) / 4.
V CC
470pF
2.2 µF
1 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
V
OUT
LX
C IN
CERAMIC
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
MAX1843
g m
C
OUT
Figure 2. Functional Diagram
TOFF
COMP
V CC
FBSEL
SHDN
IN
PGND
GND
REF
SS
LX
FB MAX1843
R TOFF
1 µF
0.01 µF
L
2.2 µF
470pF
10 Ω
INPUT
C IN
33 µ F
C OUT
150 µ F
OUTPUT
V OUT = +2.5V, FBSEL = V CC
V OUT = +1.8V, FBSEL = REF
V OUT = +1.5V, FBSEL = FLOATING
Figure 1. Typical Circuit