less power. SKIP mode allows higher efficiency than

PWM under light-load conditions.

Light-Load Operation in Normal Mode

At light loads, the MAX1765 operates by turning on the

DC-DC converter’s N-channel field-effect transistor

edge of the oscillator. The N-channel FET will remain

on, ramping up the inductor current past the minimum

inductor current, until the internal error amplifier and

current mode circuitry determine that the needs of the

system have been met or the device hits the ISET cur-

rent limit. The N-channel is then turned off and the P-

channel is turned on until current decays to the

P-channel turn-off current level. The N-channel will

edge of the oscillator occurs.

800mA, Low-Noise, Step-Up DC-DC Converter

with 500mA Linear Regulator

______________________________________________________________________________________

11

2.15V

ON

OUTL

POUT

LX

PGND

REF

SHUTDOWN

LOGIC

THERMAL

SENSOR

MAX1765

ERROR

AMP

STARTUP

OSCILLATOR

EN

Q

P

P

N

MOSFET DRIVER

WITH CURRENT

LIMITING

EN

1MHz

OSCILLATOR

EN

D

OSC

MODE

SKIP/PWM

Q

Q

IFB

PWM

CONTROLLER

RDY

1.250V

REFERENCE

IC PWR

IREF

ICS

FBL

OUT

ONL

INL

ONB

GND

ONA

ISET

ONL

REF

CLK/SEL

FB

TRACK

TRACK

ISET

Figure 1. Functional Diagram

Table 1. Selecting the Operating Mode

CLK/SEL

MODE

FEATURES

0

Normal

Operation

High-efficiency pulse

skipping at light loads,

PWM at medium and

heavy loads

1

Forced PWM

Low noise, fixed

frequency at all loads

External Clock

500kHz to

1.2MHz

Synchronized

PWM

Low noise, fixed

frequency at all loads