MAX747 Maxim, MAX747 Datasheet - Page 7

• Manufacturer Part Number: MAX747
• Description: High-Efficiency PWM / Step-Down P-Channel DC-DC Controller
• Manufacturer: Maxim

The switching frequency is nominally 100kHz and the
duty cycle varies from 5% to 96%, depending on the
input/output voltage ratio. EXT, which provides the gate
drive for the external P-FET, is switched between V+
and GND at the switching frequency. EXT is controlled
by a unique two-comparator control scheme composed
of a PWM comparator and an idle-mode comparator
Figure 2. Block Diagram
OUT
REF
AV+
LBI
CC
CS
FB
SS
The Oscillator and EXT Control
SOFT-START
CIRCUITRY
LOW-BATTERY
COMPARATOR
_______________________________________________________________________________________
100mV
60k
40k
DUAL-MODE
COMPARATOR
High-Efficiency PWM, Step-Down
CURRENT-SENSE
CURRENT-LIMIT
COMPARATOR
AMPLIFIER
AGND
LBO
N
COMPENSATION
P-Channel DC-DC Controller
SLOPE
RAMP
REFERENCE
+2V
V
(Figure 2). The PWM comparator determines the cycle-
by-cycle peak current with heavy loads, and the
light-load comparator sets the light-load peak current.
As V
until both comparators trip. With heavy loads, the idle-
mode comparator trips quickly, and the PWM control
comparator determines the EXT on-time; with light
loads, the idle-mode comparator sets the EXT on-time.
RAMP
OUT
GND
AMPLIFIER
begins to drop, EXT goes low and remains low
ERROR
50mV
EXT
COMPARATOR
COMPARATOR
IDLE-MODE
PWM
V+
OSCILLATOR
CONTROL
100kHz
EXT
SHDN
7