1- to 3-Cell, High-Current, Low-Noise,

Step-Up DC-DC Converters with Linear Regulator

10

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_______________Detailed Description

The MAX1705/MAX1706 are designed to supply both

power and low-noise circuitry in portable RF and data-

acquisition instruments. They combine a linear regula-

tor, step-up switching regulator, N-channel power

MOSFET, P-channel synchronous rectifier, precision

reference, and low-battery comparator in a single 16-

pin QSOP package (Figure 1). The switching DC-DC

converter boosts a 1- or 2-cell input to an adjustable

output between 2.5V and 5.5V. The internal low-dropout

regulator provides linear post-regulation for noise-

sensitive circuitry, as well as outputs from 1.25V to

300mV below the switching-regulator output. The

MAX1705/MAX1706 start from a low, 1.1V input and

remain operational down to 0.7V.

These devices are optimized for use in cellular phones

and other applications requiring low noise during full-

power operation, as well as low quiescent current for

maximum battery life during standby and shutdown.

They feature constant-frequency (300kHz), low-noise

pulse-width-modulation (PWM) operation with 300mA or

730mA output capability from one or two cells, respec-

tively, with 3.3V output. A low-quiescent-current stand-

by pulse-frequency-modulation (PFM) mode offers an

output up to 60mA and 140µA, respectively, and

reduces quiescent power consumption to 500µW. In

shutdown mode, the quiescent current is further

reduced to just 1µA. Figure 2 shows the standard appli-

cation circuit for the MAX1705 configured in high-

power PWM mode.

Additional features include synchronous rectification for

high efficiency and improved battery life, and an

uncommitted comparator for low-battery detection. A

CLK/SEL input allows frequency synchronization to

reduce interference. Dual shutdown controls allow shut-

down using a momentary pushbutton switch and micro-

processor control.

LBP

FBLDO

OUT

2.15V

ONA

ON

ONB

REF

GND

CLK/SEL

FB

LDO

POUT

LX

PGND

LBO

LBN

REF

SHUTDOWN

LOGIC

THERMAL

SENSOR

MAX1705

MAX1706

ERROR

AMP

START-UP

OSCILLATOR

EN

Q

P

P

N

MOSFET DRIVER

WITH CURRENT

LIMITING

EN

300kHz

OSCILLATOR

EN

D

OSC

MODE

PFM/PWM

Q

Q

IFB

PFM/PWM

CONTROLLER

N

RDY

1.250V

REFERENCE

TRACK

IC PWR

IREF

ICS

OUT

Figure 1. Functional Diagram