High-Efficiency, Step-Up

DC-DC Converters for 1V Inputs

6

_______________________________________________________________________________________

______________________________________________________________Pin Description

_______________Detailed Description

The MAX1642/MAX1643 each consist of an internal 1

Ω,

N-channel MOSFET power switch, a built-in synchro-

nous rectifier that acts as the catch diode, an oscillator,

a reference, and PFM control circuitry (Figure 1).

These devices are optimized for applications with

power-management features that operate from one

alkaline cell, such as pagers, remote controls, and bat-

tery-powered instruments. They are designed to meet

the specific demands of the operating states character-

istic of such systems:

1)

Primary battery is good and the load is active: In

this state, the system draws tens of milliamperes,

and the MAX1642/MAX1643 typically offer 80% effi-

ciency.

2)

Primary battery is good and the load is sleeping: In

this state, the load is drawing hundreds of microam-

peres, and the DC-DC converter IC draws very low

quiescent current. In many applications, the load is

expected to be in this state most of the time.

Operating Principle

The MAX1642/MAX1643 employ a proprietary pulse-

frequency-modulation (PFM) control scheme that com-

bines the ultra-low quiescent current traditional of

pulse-skipping PFM converters with the high-load effi-

ciency of pulse-width-modulation (PWM) converters.

The on-time and minimum off-times are varied as a

function of the input and output voltages:

where K is typically 25V-µs. This enables the

MAX1642/MAX1643 to maintain high efficiency over a

wide range of loads and input/output voltages. The DC-

DC converter is powered from the OUT pin.

t

=

K

V

t

=

1.2 x K

V

- V

ON

BATT

OFF(MIN)

OUT

BATT

6

7

8

4

—

5

2

1

MAX1643

Ground

GND

6

N-Channel MOSFET Switch Drain and P-Channel Synchronous-Rectifier Drain

LX

7

Power Output. Feedback input for fixed 3.3V operation and IC power input. Connect filter

capacitor close to OUT.

OUT

8

Open-Drain Power-Fail Output. Sinks current when PFI drops below 614mV.

PFO

3

Active-Low Shutdown Input. Connect to BATT for normal operation.

SHDN

4

Feedback Input for adjustable-output operation. Connect FB to an external resistor voltage

divider between OUT and GND. Connect to GND for fixed-output operation.

FB

5

Power-Fail Input. When the voltage on PFI drops below 614mV, PFO sinks current.

PFI

2

IC Battery-Power Input. Sense input for BATTLO comparator (MAX1643 only).

BATT

1

PIN

3

Open-Drain Battery-Low Output. When the voltage at BATT drops below 1V, BATTLO

sinks current.

BATTLO

—

MAX1642

FUNCTION

NAME