POK Output
The power OK (POK) output pulls low when OUT is less
than 93% of the nominal regulation voltage. Once OUT
exceeds 93% of the nominal voltage, POK goes high
impedance. POK is an open-drain N-channel output. To
obtain a voltage output, connect a pullup resistor from
POK to OUT. A 100kΩ resistor works well for most appli-
cations. POK can be used as a power-okay (POK) signal
to a microcontroller (µC), or drive an external LED to indi-
cate power failure. When the MAX1818 is shut down,
POK is held low independent of the output voltage. If
unused, leave POK grounded or unconnected.
Current Limit
The MAX1818 monitors and controls the pass transis-
tor’s gate voltage, limiting the output current to 0.8A
(typ). This current limit doubles when the output voltage
is within 4% of the nominal value to improve perfor-
mance with large load transients.
Thermal Overload Protection
Thermal overload protection limits total power dissipa-
tion in the MAX1818. When the junction temperature
exceeds T
J
= +170°C, a thermal sensor turns off the
pass transistor, allowing the IC to cool. The thermal
sensor turns the pass transistor on again after the junc-
tion temperature cools by 20°C, resulting in a pulsed
output during continuous thermal overload conditions.
Thermal overload protection protects the MAX1818 in
the event of fault conditions. For continuous operation,
do not exceed the absolute maximum junction-temper-
ature rating of T
J
= +150°C.
Operating Region and Power Dissipation
The MAX1818’s maximum power dissipation depends
on the thermal resistance of the IC package and circuit
board, the temperature difference between the die
junction and ambient air, and the rate of air flow. The
power dissipated in the device is P = I
OUT
× (V
IN
-
V
OUT
). The maximum allowed power dissipation is
800mW at T
A
= +70°C or:
P
MAX
= (T
J(MAX)
- T
A
) / ( θ
JC
+ θ
CA
)
where T
J
- T
A
is the temperature difference between
the MAX1818 die junction and the surrounding air, θ
JC
is the thermal resistance of the junction to the case,
and θ
CA
is the thermal resistance from the case
through the PC board, copper traces, and other materi-
als to the surrounding air. For best heatsinking, the
copper area should be equally shared between the IN,
OUT, and GND pins.
The MAX1818 delivers up to 0.5A RMS and operates
with input voltages up to +5.5V, but not simultaneously.
High output currents can only be sustained when input-
output differential voltages are low, as shown in Figure 3.
MAX1818
500mA Low-Dropout
Linear Regulator in SOT23
_______________________________________________________________________________________ 7
vs. INPUT VOLTAGE
