
M
150mA, Low-Dropout Linear Regulator
with Power-OK Output
6
_______________________________________________________________________________________
Pin Description
FUNCTION
Regulator Output. Fixed 5.0V, 3.3V, 3.0V, 2.7V, or 2.5V output. Sources up to 150mA. Bypass
with 4.7μF (>0.5
typ ESR) tantalum capacitor to GND.
OUT
5
Power-OK Output. Active low, open-drain output indicates an out-of-regulation condition.
Connect a 100k
pull-up resistor to OUT for logic levels. If not used, leave this pin unconnected.
POK
4
Active-Low Shutdown Input. A logic low reduces the supply current to below 1μA. Connect to IN
for normal operation.
SHDN
3
PIN
Ground. This pin also functions as a heatsink. Solder to a large pad or the circuit-board ground
plane to maximize power dissipation.
GND
2
Regulator Input. Supply voltage can range from +2.5V to +6.5V. Bypass with 1μF capacitor to GND
(see
Capacitor Selection
and
Regulator Stability
).
IN
1
NAME
Detailed Description
The MAX8885 is a low-dropout, low-quiescent-current
linear regulator designed primarily for battery-powered
applications using low-cost, high-ESR tantalum capaci-
tors. The device delivers up to 150mA and is available
with preset output voltages of 2.5V, 2.7V, 3.0V, 3.3V, or
5.0V. The MAX8885 consists of a 1.25V reference, error
amplifier, P-channel pass transistor, power-OK compara-
tor, and internal feedback voltage divider (Figure 1).
The 1.25V bandgap reference is connected to the error
amplifier
’
s inverting input. The error amplifier compares
this reference with the feedback voltage and amplifies
the difference. If the feedback voltage is lower than the
reference voltage, the pass-transistor gate is pulled
lower, which allows more current to pass to the output
and increases the output voltage. If the feedback volt-
age is too high, the pass-transistor gate is pulled up,
allowing less current to pass to the output. The output
voltage is fed back through an internal resistor voltage
divider connected to the OUT pin.
Additional blocks include a current limiter, reverse-bat-
tery protection, thermal sensor, and shutdown logic.
Output Voltage
The MAX8885 is supplied with factory-set output volt-
ages of 2.5V, 2.7V, 3.0V, 3.3V, or 5.0V. The part num-
ber
’
s two-digit suffix identifies the nominal output
voltage. For example, the MAX8885EUK33 indicates a
preset output voltage of 3.3V (see
Output Voltage
Selector Guide
).
Internal P-Channel Pass Transistor
The MAX8885 features a 1.1
(typ) P-channel MOSFET
pass transistor. This provides several advantages over
similar designs using PNP pass transistors, including
longer battery life. The P-channel MOSFET requires no
base drive, which reduces quiescent current signifi-
cantly. PNP-based regulators waste considerable cur-
rent in dropout when the pass transistor saturates. They
also use high base-drive currents under large loads.
The MAX8885 does not suffer from these problems and
consumes only 100μA of quiescent current whether in
dropout, light-load, or heavy-load applications (see
Typical Operating Characteristics
).
Power-OK Output (POK)
When the output voltage goes out of regulation
—
as dur-
ing dropout, current limit, or thermal shutdown
—
POK
goes low. POK is an open-drain N-channel MOSFET. To
obtain a logic-level output, connect a pull-up resistor
from POK to OUT. To minimize current consumption,
make this resistor as large as practical. A 100k
resis-
tor works well for most applications. The POK function is
not active during shutdown. A capacitor to GND may be
added to generate a power-on-reset (POR) delay, which
can operate down to V
IN
≤
1V. (See POK Startup
Response in the
Typical Operating Circuit
.)
Current Limit
The MAX8885 includes a current limiter that monitors
and controls the pass transistor
’
s gate voltage, limiting
the output current to 390mA (typ). For design purpos-
es, consider the current limit to be 160mA (min) to
600mA (max). The output can be shorted to ground for
an indefinite period of time without damaging the part.
Thermal-Overload Protection
When the junction temperature exceeds T
J
= +170
°
C,
the thermal sensor signals the shutdown logic, turning
off the pass transistor and allowing the IC to cool. The