Datasheet

MAX1818
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 0.25Ω 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 25kΩ to 100kΩ range. Calculate R1
with the following equation:
R1 = R2 [(V
OUT
/ V
SET
) – 1]
where V
SET
= 1.25V, and V
OUT
may range from 1.25V
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
at OUT determine the rate at which V
OUT
decays.
SHDN can be pulled as high as 6V, regardless of the
input and output voltage.
100mV
SHDN
IN
POK
ON
OFF
C
IN
1μF
V
IN
= 2.5V TO 5.5V
OUT
V
OUT
= 1.25V TO 5.0V
C
OUT
3.3μF
THERMAL
SENSOR
SET
MOSFET
DRIVER WITH
I
LIM
SHUTDOWN
LOGIC
V
REF
1.25V
ERROR
AMPLIFIER
GND
R1
R2
R
POK
100k
LOGIC SUPPLY
VOLTAGE (V
OUT
)
93% V
REF
TO
μC
5k
MAX1818
Figure 1. Functional Diagram