MAX8863

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4

DETAILED DESCRIPTION

The MAX8863 is a fixed output, low–dropout,

low–quiescent current linear regulator designed specifically

for portable, battery–operated equipment such as cellular

phones, cordless phones, and modems. A 1.20 V reference,

error amplifier, MOSFET driver, P–channel pass transistor,

comparator, and internal feedback voltage divider comprise

the MAX8863 (see Figure 3).

The bandgap reference is connected to the error

amplifier’s inverting input. The error amplifier then

compares the reference with the selected feedback voltage

and amplifies the difference. The MOSFET driver, reading

the error signal, applies the correct drive to the P–channel

pass transistor. If the feedback voltage is lower than the

reference, the pass–transistor is pulled lower to allow more

current through, and to increase the output voltage.

Conversely, if the feedback voltage is higher than the

reference, the pass–transistor is pulled up, which allows less

current through to the output.

Turn On Response

The turn on response is defined as two separate response

The MAX8863 has a fast Wake Up Time (10

µsec typical)

when released from shutdown. See Figure 2 for the Wake

value after being released from shutdown.

SHDN

98%

2%

Figure 2. Wake Up Response Time

The total turn on response is defined as the Settling Time

defined as the condition when the output is within 2.0% of

its fully enabled value (140

µsec typical) when released

from shutdown. The settling time of the output voltage is

dependent on load conditions and output capacitance on

Internal P–Channel Pass Transistor

Featuring a 1.1

Ω P–channel MOSFET pass transistor, the

MAX8863 offers longer battery life than similar designs

using PNP pass transistors, which waste current in dropout

when the pass transistor saturates. PNP–based regulators

also use high base–drive currents under large loads. The

P–channel MOSFET, however, does not require a base drive

current, which reduces quiescent current. The MAX8863

uses only 50

µA of quiescent current.

SHDN

–

+

MOS DRIVER

THERMAL

SENSOR

1.2 V

REF

SHUTDOWN

LOGIC

IN

GND

GND

OUT

P

N

(MAX8863 Only)

Figure 3. Functional Block Diagram