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Detailed Description

The MAX1722/MAX1723/MAX1724 compact, high-effi-

ciency, step-up DC-DC converters are guaranteed to

start up with voltages as low as 0.91V and operate with

an input voltage down to 0.8V. Consuming only 1.5µA of

quiescent current, these devices include a built-in syn-

chronous rectifier that reduces cost by eliminating the

need for an external diode and improves overall efficien-

cy by minimizing losses in the circuit (see Synchronous

Rectification section). The MAX1722/MAX1724 feature a

clamp circuit that reduces EMI due to inductor ringing.

The MAX1723/MAX1724 feature an active-low shutdown

that reduces quiescent supply current to 0.1µA. The

MAX1722/MAX1723 have an adjustable output voltage,

while the MAX1724 is available with four fixed-output

voltage options (see Selector Guide). Figure 1 is the

MAX1723 simplified functional diagram and Figure 2 is

the MAX1724 simplified functional diagram.

PFM Control Scheme

A forced discontinuous, current-limited, pulse-frequency-

modulation (PFM) control scheme is a key feature of the

MAX1722/MAX1723/MAX1724. This scheme provides

ultra-low quiescent current and high efficiency over a

wide output current range. There is no oscillator; the

inductor current is limited by the 0.5A N-channel cur-

rent limit or by the 5µs switch maximum on-time.

Following each on cycle, the inductor current must

ramp to zero before another cycle may start. When the

error comparator senses that the output has fallen

below the regulation threshold, another cycle begins.

Synchronous Rectification

The internal synchronous rectifier eliminates the need

for an external Schottky diode, thus reducing cost and

board space. While the inductor discharges, the P-

channel MOSFET turns on and shunts the MOSFET

body diode. As a result, the rectifier voltage drop is sig-

nificantly reduced, improving efficiency without the

addition of external components.

Low-Voltage Startup Circuit

The MAX1722/MAX1723/MAX1724 contain a low-volt-

age startup circuit to control DC-DC operation until the

output voltage exceeds 1.5V (typ). The minimum start-

P

N

CONTROL

LOGIC

STARTUP

CIRCUITRY

DRIVER

GND

FB

OUT

LX

CURRENT

LIMIT

1.235V REFERENCE

ERROR

COMPARATOR

ZERO-

CROSSING

DETECTOR

MAX1723

SHDN

Figure 1. MAX1723 Simplified Functional Diagram