4

Date: 03/02/06

SPX2920 400mA Low Drop Out Voltage Regulator with Shutdown

©Copyright 2006 Sipex Corporation

UNREGULATED DC

5V @ 400mA

MAX

TO CMOS OR

TTL

REFERENCE

ERROR DETECTION

COMPARATOR

ERROR

AMPLIFIER

1.23V

FROM

TTL

+

+

+

+

+

+

60 mV

3

7

8

1

2

6

5

4

60k

330k

180k

1 uF

_

_

SPX2920-5.0 Block Diagram

External Capacitors

The stability of the SPX2920 requires a

2.2µF or greater capacitor between output

and ground. Oscillation could occur without

this capacitor. Most types of tantalum or

aluminum electrolytic capacitors work fine

here. For operations of below -25°C, solid

tantalum is recommended since many of the

aluminum types have electrolytes that freeze

at about -30°C. The ESR of about 5Ω or less

and resonant frequency above 500kHz are

the most important parameters in the value

of the capacitor. The capacitor value can be

increased without limit.

At lower values of output current, less output

capacitance is required for stability. For the

currents below 10mAthe value of the capaci-

tor can be reduced to 0.5µF and 0.15µF for

1mA. More output capacitance is needed

for the 8-pin version at voltages below 5V

since it runs the error amplifier at lower

gain. At worst case, 4.7µF or greater must

be used for the condition of 250mA load at

1.23V output.

The SPX2920, unlike other low dropout

regulators, will remain stable and in regula-

tion with no load in addition to the internal

voltage divider. This feature is especially

important in applications like CMOS RAM

keep-alive.

If there is more than 10 inches of wire be-

tween the input and the AC filter capacitor,

or if a battery is used as the input, then a

0.1µF tantalum or aluminum electrolytic

capacitor should be placed from the input

to the ground.

Instability can occur if there is stray capaci-

tance to the SPX2920 feedback terminal (pin

7). This could cause more problems when

using a higher value of external resistors to

set the output voltage.

This problem can be fixed by adding a 100pF

capacitor between output and feedback

and increasing the output capacitor to at

least 3.3µF.

APPLICATION HINTS

BLOCK DIAGRAM