Rainbow-electronics MAX849 Manual de usuario descargar pdf (Pagina 9)

MAX848/MAX849

1-Cell to 3-Cell, High-Power,

Low-Noise, Step-Up DC-DC Converters

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_______________Detailed Description

The MAX848/MAX849 combine a switching regulator,

N-channel power MOSFET, P-channel synchronous

rectifier, precision reference voltage, power-good indi-

cator, and battery voltage monitor, all in a single mono-

lithic device. The MAX848/MAX849 are powered

directly from the output. The output voltage is factory

preset to 3.3V or adjustable from 2.7V to 5V with exter-

nal resistors (Dual Mode™ operation). These devices

start from a low 1V input voltage and remain opera-

tional down to 0.7V. The MAX848/MAX849 operate with

either one to three NiCd/NiMH cells or one Li-Ion cell.

At power-up, an internal low-voltage oscillator drives

the N-channel power switch, and the output voltage

slowly builds up. The oscillator has a 25% nominal duty

cycle to prevent current build-up in the inductor. An

output voltage in excess of the nominal 2.25V lockout

voltage activates the error comparator and internal tim-

ing circuitry. The device resumes operation in either

pulse-frequency-modulation (PFM) low-power mode or

pulse-width-modulation (PWM) low-noise mode, select-

ed by the logic control, CLK/SEL. Figure 2 shows the

standard application circuit for the MAX849 configured

in the high-power PWM mode.

On/Off Control

The MAX848/MAX849 are turned on or off by logic

input pins ON1 and ON2 (Table 1). When ON1 = 1 or

ON2 = 0, the part is on. When ON1 = 0 and ON2 = 1,

the part is off. Both inputs have logic trip points near

0.5 x V

OUT

with 0.15 x V

OUT

hysteresis.

Operating Modes

The MAX848/MAX849 operate in either PFM, PWM, or

PWM synchronized to an externally applied clock sig-

nal. Table 2 lists each operating mode.

REF

1.25V

RDY

START-UP

OSCILLATOR

OUT

ON1

ON2

REF

GND

CLK/SEL

POKIN

AINSEL

AIN1

AIN2

ADC

FEEDBACK

300kHz

OSCILLATOR

2.25V

FEEDBACK

AND

POWER-GOOD

SELECT

PFM/PWM

PCH

0.25Ω

NCH

0.13Ω

POUT

PGND

POK

DATA

MAX848/MAX849

OSC

MODE

PFM/PWM

CONTROLLER

Figure 1. Functional Diagram

ON1

OONN22

MAX848/MAX849

0 0 On

0 1 Off

1 0 On

1 1 On

Table 1. On/Off Logic Control

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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