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MAX1765
(Rev.:2000)

800mA, Low-Noise, Step-Up DC-DC Converter with 500mA Linear Regulator

Maxim Integrated 

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

with 500mA Linear Regulator

FBL

OUT

GND

ONA

ONB

ONL

REF

CLK/SEL

FB

TRACK

ISET

IC PWR

2.15V

VOUT

VOUT - 300mV

RDY

ON

1.250V

REFERENCE

Figure 1. Functional Diagram

THERMAL

SENSOR

SHUTDOWN

LOGIC

ONL

REF

ERROR

AMP

MAX1765

MOSFET DRIVER

WITH CURRENT

P

LIMITING

TRACK

STARTUP

EN OSCILLATOR Q

D

Q

P

PWM

CONTROLLER

EN

EN

OSC

Q

N

1MHz

OSCILLATOR

SKIP/PWM

MODE

ICS

IREF

IFB ISET

INL

OUTL

POUT

LX

PGND

Table 1. Selecting the Operating Mode

CLK/SEL

MODE

FEATURES

High-efficiency pulse

0

Normal

Operation

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

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

(FET) when VFB < VREF, synchronized with the rising

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

remain off until VFB is again less than VREF, and a rising

edge of the oscillator occurs.

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