5V/3.3V or Adjustable, Low-Dropout,

______________________________________________________________________________________

13

When operating from sources other than batteries, sup-

ply-noise rejection and transient response can be

improved by increasing the values of the input and out-

put capacitors and employing passive filtering tech-

niques. Do not use power supplies with ripple voltage

exceeding 200mV at 100kHz.

Overshoot and Transient Considerations

The

Typical Operating Characteristics section shows

power-up, supply, and load-transient response graphs.

On the load-transient graphs, two components of the

output response can be observed: a DC shift from the

output impedance due to the different load currents,

and the transient response. Typical transients for step

changes in the load current from 50mA to 250mA are

200mV. Increasing the output capacitor’s value attenu-

ates transient spikes.

During recovery from shutdown, overshoot is negligible

if the output voltage has been given time to decay ade-

Input-Output (Dropout) Voltage

A regulator’s minimum input-output voltage differential

(or dropout voltage) determines the lowest usable sup-

ply voltage. In battery-powered systems, this deter-

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

the MAX882/MAX883/MAX884 use a P-channel MOS-

FET pass transistor, their dropout voltage is a function

Characteristics). Quickly stepping up the input voltage

from the dropout voltage can result in overshoot.

Short-Term Battery Backup

Using the MAX882

Figure 9 illustrates a scheme for implementing battery

backup for 3.3V circuits using the MAX882. When the

supply voltage drops below some user-specified value

based on resistors R1 and R2, the standby function

activates, turning off the MAX882’s output. Under

these conditions, the backup battery supplies power to

the load. Reverse current protection prevents the bat-

tery from draining back through the regulator to the

input.

This application is limited to short-term battery backup

for 3.3V circuits. The current drawn by the MAX882’s

OUT pin at 3.3V during reverse current protection is

typically 8µA. It should not be used with the MAX883

and MAX884, since the OFF pin is a logic input, and

indeterminate inputs can cause the regulator to turn on

intermittently, draining the battery.

Reverse Battery Protection

Reverse battery protection can be added by including

an inexpensive Schottky diode between the battery

input and the regulator circuit, as shown in Figure 7.

However, the dropout voltage of the regulator will be

increased by the forward voltage drop of the diode. For

example, the forward voltage of a standard 1N5817

Schottky diode is typically 0.29V at 200mA.

BACKUP

BATTERY

MAX882

O.1

µF

2.2

µF

OUTPUT

VOLTAGE

INPUT

VOLTAGE

LBI

SET

STBY

IN

D2

R1

R2

OUT

GND

Figure 9. Short-Term Battery Backup Using the MAX882