Linear Regulators with POK

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5

Detailed Description

The MAX8880/MAX8881 are low-dropout, low-quies-

cent current linear regulators designed primarily for

battery-powered applications (Figure 1). The MAX8880

provides an adjustable output voltage from 1.25V to 5V

using an external resistor-divider. The MAX8881 is

available in factory preset output voltages of 1.8V, 2.5V,

3.3V, and 5V. Both devices have a +1.25V reference,

error amplifier, MOSFET driver, and P-channel pass

transistor (Figure 2).

Low-Dropout Regulator

The 1.25V reference is connected to the error amplifi-

er’s inverting input. The error amplifier compares this

reference with the selected feedback voltage and

amplifies the difference. The MOSFET driver reads the

error signal and applies the appropriate drive to the P-

channel pass transistor. If the feedback voltage is lower

than the reference voltage, the pass-transistor gate is

pulled lower, allowing more current to pass, increasing

the output voltage. If the feedback voltage is higher

than the reference voltage, the pass-transistor gate is

driven higher, allowing less current to pass to the out-

put. The output voltage is fed back through either an

internal resistor voltage divider by externally connect-

ing FB to OUT (MAX8881), or an external resistor net-

work connected to FB (MAX8880). Additional blocks

include an output current limiter, reverse battery pro-

tection, a thermal sensor, shutdown logic, and a POK

comparator to indicate when the output is out of regula-

tion (Figure 2).

Internal P-Channel Pass Transistor

The MAX8880/MAX8881 feature a 2

Ω P-channel MOS-

FET pass transistor. This provides advantages over

similar designs using PNP pass transistors, including

longer battery life. The P-channel MOSFET requires no

base drive, which reduces quiescent current consider-

ably. PNP-based regulators waste considerable current

in dropout when the pass transistor saturates. They

also use high base-drive currents under large loads.

The MAX8880/MAX8881 do not suffer from these prob-

lems and consume only 3.5µA of supply current (see

Typical Operating Characteristics).

Dropout Voltage

A regulator’s minimum input-output differential (or

dropout voltage) determines the lowest usable supply

voltage. In battery-powered systems, this determines

the useful end-of-life battery voltage. Because the

MAX8880/MAX8881 use a P-channel MOSFET pass

Ω) multi-

plied by the load current (see Electrical Characteristics).

IN

R2*

100k

OUT

FB

GND

C2

4.7

μF

C1

1

μF

SHDN

2.5V TO 12V

REGULATION OK

POK

MAX8881

*OPTIONAL

Figure 1. Standard Application Circuit

PIN

MAX888_ (SOT)

MAX888_ (TDFN)

NAME

FUNCTION

1

4

IN

Input Voltage. Bypass with a 1µF capacitor to GND.

2

2

GND

Ground

3

3

OUT

Output Voltage. Bypass with a 4.7µF capacitor (<0.5

Ω ESR) to GND for load

currents up to 200mA. For load currents up to 40mA, 1µF is acceptable.

41

FB

Feedback Set Point, 1.25V (MAX8880 only). Output sense, connect to OUT

externally (MAX8881 only).

5

5

SHDN

IN. If reverse battery protection of the SHDN input is desired, connect a

100k

Ω resistor in series with SHDN.

6

6

POK

POK Output, Open Drain. Low when OUT is out of regulation or in shutdown.

Connect POK to OUT through a high-value resistor for a simple error

—

—

EP

Exposed paddle (TDFN only). Connect to the circuit ground plane.

Pin Description