Detailed Description
The MAX8887/MAX8888 are low-dropout, low-quies-
cent-current linear regulators designed primarily for
battery-powered applications. The devices supply
loads up to 300mA and are available in several fixed
output voltages in the 1.5V to 3.3V range. The
MAX8887 is optimized for low-noise operation, while
the MAX8888 includes an open-drain POK output flag.
As illustrated in Figure 1, the MAX8888 consists of a
1.25V reference, error amplifier, p-channel pass tran-
sistor, and internal feedback voltage divider.
Internal p-Channel Pass Transistor
The MAX8887/MAX8888 feature a 0.5Ω 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 regulators also waste considerable current in
dropout when the pass transistor saturates and uses
high base drive currents under large loads. The
MAX8887/MAX8888 do not suffer from these problems
and consume only 55µA of quiescent current under
heavy loads as well as in dropout.
Output Voltage Selection
The MAX8887/MAX8888 are supplied with various facto-
ry-set output voltages ranging from 1.5V to 3.3V. The
part number’s two-digit suffix identifies the nominal out-
put voltage. For example, the MAX8887EZK33 has a pre-
set output voltage of 3.3V (see the
Ordering Information
).
Shutdown
Drive SHDN low to enter shutdown. During shutdown,
the output is disconnected from the input and supply
current drops to 0.1µA. When in shutdown, POK and
OUT are driven low. SHDN can be pulled as high as
6V, regardless of the input and output voltages.
Power-OK Output
The power-OK output (POK) 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 logic level output, connect a pullup resistor
from POK to OUT. A 100kΩ resistor works well for most
applications. POK can be used as a power-on-reset
(POR) signal to a microcontroller (µC) or to drive other
logic. Adding a capacitor from POK to ground creates
POK delay. When the MAX8887 is shut down, POK is
held low independent of the output voltage. If unused,
leave POK grounded or unconnected.
Current Limit
The MAX8887/MAX8888 monitor and control the pass
transistor’s gate voltage, limiting the output current to
0.8A (typ). This current limit is reduced to 500mA (typ)
when the output voltage is below 93% of the nominal
value to provide foldback current limiting.
Thermal Overload Protection
Thermal overload protection limits total power dissipa-
tion in the MAX8887/MAX8888. When the junction tem-
perature exceeds T
J
=+170°C, a thermal sensor turns
off the pass transistor, allowing the device to cool. The
thermal sensor turns the pass transistor on again after
the junction temperature cools by 20°C, resulting in a
pulsed output during continuous thermal overload con-
ditions. Thermal overload protection protects the
MAX8887/MAX8888 in the event of fault conditions. For
continuous operation, do not exceed the absolute maxi-
mum junction-temperature rating of T
J
=+150°C.
Operating Region and Power Dissipation
The MAX8887/MAX8888’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
727mW or:
P
MAX
= (T
J(MAX)
- T
A
) / (θ
JC
+ θ
CA
)
where T
J(MAX)
-T
A
is the temperature difference
between the MAX8887/MAX8888 die junction and the
surrounding air; θ
JC
is the thermal resistance from the
junction to the case; and θ
CA
is the thermal resistance
from the case through PC board, copper traces, and
other materials to the surrounding air.
Refer to Figure 2 for the MAX8887/MAX888 valid oper-
ating region.
Noise Reduction
For the MAX8887 only, an external 0.01µF bypass
capacitor at BP creates a lowpass filter for noise reduc-
tion. The MAX8887 exhibits 42µV
RMS
of output voltage
noise with C
BP
= 0.01µF and C
OUT
= 2.2µF (see the
Typical Operating Characteristics
)
.
Applications Information
Capacitor Selection and Regulator
Stability
Connect a 2.2µF ceramic capacitor between IN and
ground and a 2.2µF ceramic capacitor between OUT
and ground. The input capacitor (C
IN
) lowers the
source impedance of the input supply. Reduce noise
and improve load-transient response, stability, and
power-supply rejection by using a larger ceramic out-
put capacitor such as 4.7µF.
The output capacitor’s (C
OUT
) equivalent series resis-
tance (ESR) affects stability and output noise. Use out-
put capacitors with an ESR of 0.1Ω or less to ensure
MAX8887/MAX8888
Low-Dropout, 300mA Linear Regulators in SOT23
6 _______________________________________________________________________________________
