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Date: 3/14/06

SPX2945 400mA Low Dropout Voltage Regulator with Shutdown

©Copyright 2006 Sipex Corporation

SPX2945 - 5.0V BLOCK DIAGRAM

FEEDBACK

INPUT

OUTPUT

UNREGULATED DC

SHUTDOWN

GROUND

ERROR

______

5V TAP

SENSE

5V at 400mA

MAX

TO CMOS OR

TTL

REFERENCE

ERROR DETECTION

COMPARATOR

ERROR

AMPLIFIER

1.23V

FROM

CMOS OR

TTL

+

+

+

+

+

+

60 mV

3

7

8

1

2

6

5

4

.

60kΩ

330kΩ

180kΩ

.

.

1µF

_

_

External Capacitors

The stability of the SPX2945 requires a 2.2

µF or

greater capacitor between output and ground.

Oscillation could occur without this capacitor.

Most types of tantalum or aluminum electrolytic

works fine here. For operations of below -25

°C

solid tantalum is recommended since the many

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 capaci-

tor. The capacitor value can be increased with-

out limit.

At lower values of output current, less output

capacitance is required for stability. For the

currents below 10mA the value of the capacitor

can be reduced to 0.5

µF and 0.15µF for 1mA.

More output capacitance needed for the 8-pin

version at voltages below 5V since it runs the

error amplifier at lower gain. At worst case

4.7

µF or greater must be used for the condition

of 250mA load at 1.23V output.

The SPX2945, unlike other low dropout regu-

lators will remain stable and in regulation with

no load in addition to the internal voltage di-

vider. This feature is especially important in

applications like CMOS RAM keep-alive.

If there is more than 10 inches of wire between

the input and the AC filter capacitor, or if a

battery is used as the input, then a 0.1

µF tanta-

lum or aluminum electrolytic capacitor should

be placed from the input to the ground.

Instability can occur if there is stray capaci-

tance to the SPX2945 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 3.3

µF.

APPLICATION INFORMATION