in Thin SOT23-5

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7

up voltage is a function of load current (see Typical

Operating Characteristics). This circuit is powered from

the BATT pin for the MAX1722/MAX1724, guaranteeing

startup at input voltages as low as 0.91V. The MAX1723

lacks a BATT pin; therefore, this circuit is powered

through the OUT pin. Adding a Schottky diode in paral-

lel with the P-channel synchronous rectifier allows for

startup voltages as low as 1.2V for the MAX1723

(Figure 3). The external Schottky diode is not needed

for input voltages greater than 1.8V. Once started, the

output maintains the load as the battery voltage

decreases below the startup voltage.

Shutdown (MAX1723/MAX1724)

The MAX1723/MAX1724 enter shutdown when the

SHDN pin is driven low. During shutdown, the body

diode of the P-channel MOSFET allows current to flow

Shutdown can be pulled as high as 6V, regardless of

the voltage at BATT or OUT. For normal operation, con-

nect SHDN to the input.

P

N

CONTROL

LOGIC

STARTUP

CIRCUITRY

DAMPING

SWITCH

DRIVER

GND

SHDN

BATT

LX

OUT

CURRENT

LIMIT

MAX1724

ERROR

COMPARATOR

ZERO-

CROSSING

DETECTOR

1.235V REFERENCE

Figure 2. MAX1724 Simplified Functional Diagram

Figure 3. MAX1723 Single-Cell Operation

D1

10

10

µF

R2

2.37M

Ω

R1

1.24M

Ω

10

µF

LX

OUT

SHDN

GND

FB

MAX1723