Low-Noise, High PSRR,

Low-Dropout, 120mA Linear Regulator

6

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Detailed Description

The MAX8940 is a low-noise, low-dropout, low-quies-

cent current linear regulator designed for space-

restricted applications. The part is available with preset

output voltages 2.8V or 3.0V. This device can supply

loads up to 120mA. As shown in the

Functional

Diagram, the MAX8940 consist of an innovative

bandgap core and noise bypass circuit, error amplifier,

p-channel pass transistor, and internal feedback volt-

age-divider.

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 reduces

noise at the output. Additional blocks include a current

limiter, thermal sensor, and shutdown logic.

Internal P-Channel Pass Transistor

The MAX8940 features a 1Ω (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 cur-

rent. PNP-based regulators waste considerable current

in dropout when the pass transistor saturates. They

also use high base-drive current under heavy loads.

The MAX8940 does not suffer from these problems and

consume only 40µA of quiescent current in light load

and 220µA in dropout (see the

Typical Operating

Characteristics).

Shutdown

The MAX8940 features a low-power shutdown mode that

reduces quiescent current less than 1µA. Driving SHDN

low disables the voltage reference, error amplifier, gate-

drive circuitry, and pass transistor (see the

Functional

Diagram), and the device output enters a high-imped-

ance state. Connect SHDN to IN for normal operation.

Current Limit

The MAX8940 includes a current limiter, which monitors

and controls the pass transistor’s gate voltage, limiting

the output current to 200mA. For design purposes, con-

sider the current limit to be 130mA (min) to 300mA (max).

The output can be shorted to ground for an indefinite

amount of time without damaging the part.

Thermal-Overload Protection

Thermal-overload protection limits total power dissipation

in the MAX8940. When the junction temperature exceeds

logic, turning off the pass transistor and allowing the IC

to cool down. The thermal sensor turns the pass transis-

tor on again after the IC’s junction temperature drops by

10°C, resulting in a pulsed output during continuous

thermal-overload conditions.

Thermal-overload protection is designed to protect the

MAX8940 in the event of a fault condition. For continual

operation, do not exceed the absolute maximum junc-

Operating Region and Power Dissipation

The MAX8940 maximum power dissipation depends on

the thermal resistance of the case and circuit board,

Pin Description

PIN

NAME

FUNCTION

1

IN

Unregulated Input Supply

2

GND

Ground

3

SHDN

Shutdown. Pull low to disable the regulator.

4BP

Noise Bypass for Low-Noise Operation. Connect a 10nF capacitor from BP to OUT. It is short to OUT

in shutdown mode.

5

OUT

Regulated Output Voltage. Bypass with a capacitor to GND. See the Capacitor Selection and Regulator

Stability section for more details.