High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

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7

LBI

REF

OUT

CC

FB

AV+

CS

SS

LBO

EXT

V+

LOW-BATTERY

COMPARATOR

ERROR

AMPLIFIER

EXT

CONTROL

CURRENT-SENSE

AMPLIFIER

100mV

SHDN

N

100kHz

OSCILLATOR

+2V

REFERENCE

SOFT-START

CIRCUITRY

Σ

SLOPE

COMPENSATION

RAMP

AGND

50mV

GND

CURRENT-LIMIT

COMPARATOR

PWM

COMPARATOR

IDLE-MODE

COMPARATOR

60k

40k

DUAL-MODE

COMPARATOR

Figure 2. Block Diagram

The Oscillator and EXT Control

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). The PWM comparator determines the cycle-

by-cycle peak current with heavy loads, and the

light-load comparator sets the light-load peak current.

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.