500mA Low-Dropout

Linear Regulator in SOT23

6

_______________________________________________________________________________________

Additional blocks include an output current limiter, ther-

mal sensor, and shutdown logic.

Internal P-Channel Pass Transistor

The MAX1818 features a 0.25

Ω P-channel MOSFET

pass transistor. Unlike similar designs using PNP pass

transistors, P-channel MOSFETs require no base drive,

which reduces quiescent current. PNP-based regula-

tors also waste considerable current in dropout when

the pass transistor saturates, and use high base-drive

currents under large loads. The MAX1818 does not suf-

fer from these problems and consumes only 125µA of

quiescent current under heavy loads as well as in

dropout.

Output Voltage Selection

The MAX1818’s Dual Mode operation allows operation

in either a preset voltage mode or an adjustable mode.

Connect SET to GND to select the preset output volt-

age. The two-digit part number suffix identifies the out-

put voltage (see Selector Guide). For example, the

MAX1818EUT33 has a preset 3.3V output voltage.

The output voltage may also be adjusted by connecting

a voltage-divider from OUT to SET to GND (Figure 2).

Select R2 in the 25k

Ω to 100kΩ range. Calculate R1

with the following equation:

to 5.0V.

Shutdown

Pull SHDN low to enter shutdown. During shutdown, the

output is disconnected from the input and supply cur-

rent drops to 0.1µA. When in shutdown, POK pulls low

and OUT is high impedance. The capacitance and load

SHDN can be pulled as high as 6V, regardless of the

input and output voltage.

100mV

SHDN

IN

POK

ON

OFF

1

µF

OUT

3.3

µF

THERMAL

SENSOR

SET

MOSFET

DRIVER WITH

I LIM

SHUTDOWN

LOGIC

1.25V

ERROR

AMPLIFIER

GND

R1

R2

100k

LOGIC SUPPLY

TO

µC

5k

MAX1818

Figure 1. Functional Diagram