Ultra-Low-Noise, High PSRR, Low-Dropout,

150mA Linear Regulators in µDFN

6

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Detailed Description

The MAX8840/MAX8841/MAX8842 are ultra-low-noise,

low-dropout, low-quiescent current linear regulators

designed for space-restricted applications. The parts

are available with preset output voltages ranging from

1.5V to 4.5V. These devices can supply loads up to

150mA. As shown in the Functional Diagram, the

MAX8840/MAX8841 consist of an innovative bandgap

core and noise bypass circuit, error amplifier, p-chan-

nel pass transistor, and internal feedback voltage-

divider. The MAX8842 allows for adjustable output with

an external feedback network.

The 1.225V bandgap reference is connected to the

error amplifier’s inverting input. The error amplifier com-

pares 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 low. This allows more current to pass to

the output and increases the output voltage. If the feed-

back voltage is too high, the pass transistor gate is

pulled high, allowing less current to pass to the output.

The output voltage is fed back through an internal resis-

tor voltage-divider connected to the OUT pin.

An external bypass capacitor connected to BP

(MAX8840) reduces noise at the output. Additional

blocks include a current limiter, thermal sensor, and

shutdown logic.

Internal p-Channel Pass Transistor

The MAX8840/MAX8841/MAX8842 feature a 1

Ω (typ)

p-channel MOSFET pass transistor. This provides seve-

ral advantages over similar designs using a pnp pass

transistor, including longer battery life. The p-channel

MOSFET requires no base drive, which considerably

reduces quiescent current. PNP-based regulators waste

considerable current in dropout when the pass transis-

tor saturates. They also use high base-drive current

under heavy loads. The MAX8840/MAX8841/MAX8842

do not suffer from these problems and consume only

40µA of quiescent current in light load and 220µA in

dropout (see the Typical Operating Characteristics).

Output Voltage Selection

The MAX8840/MAX8841 are supplied with factory-set

output voltages from 1.5V to 4.5V, in 100mV increments

(see the Ordering Information). The MAX8842 features

a user-adjustable output through an external feedback

network (see the Typical Operating Circuits).

To set the output of the MAX8842, use the following

equation:

where R2 is chosen to be less than 240k

1.225V. Use 1% or better resistors.

Shutdown

The MAX8840/MAX8841/MAX8842 feature a low-power

shutdown mode that reduces quiescent current less

than 1µA. Driving SHDN low disables the voltage refer-

ence, error amplifier, gate-drive circuitry, and pass

transistor (see the Functional Diagram), and the device

output enters a high-impedance state. Connect SHDN

to IN for normal operation.

Current Limit

The MAX8840/MAX8841/MAX8842 include a current

limiter, which monitors and controls the pass transis-

tor’s gate voltage, limiting the output current to 200mA.

RR X

V

V

OUT

REF

12

1

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⎝⎜

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⎠⎟

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Pin Description

PIN

MAX8840

MAX8841

MAX8842

NAME

FUNCTION

1

1

1

IN

Unregulated Input Supply

2

2

2

GND

Ground

333

SHDN

Shutdown. Pull low to disable the regulator.

4—

—

BP

Noise Bypass for Low-Noise Operation. Connect a 10nF capacitor

from BP to OUT. It is shorted to OUT in shutdown mode.

—

—

4

FB

Adjustable Output Feedback Point

5

4, 5

5

N.C.

No Connection. Not internally connected.

666

OUT

Regulated Output Voltage. Bypass with a capacitor to GND. See the

Capacitor Selection and Regulator Stability section.