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-

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-

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.

727mW or:

between the MAX8887/MAX8888 die junction and the

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-

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

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.

tance (ESR) affects stability and output noise. Use out-

put capacitors with an ESR of 0.1Ω or less to ensure

Low-Dropout, 300mA Linear Regulators in SOT23

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