7

July 2008 Rev F

SPX3819 500mA, Low Noise LDO Voltage Regulator

© 2008 Exar Corporation

The SPX3819 requires an output capacitor for

device stability. Its value depends upon the

application circuit. In general, linear regulator

stability decreases with higher output currents.

In applications where the SPX3819 is sourcing

less current, a lower output capacitance may be

sufficient. For example, a regulator outputting

only 10mA, requires approximately half the

capacitance as the same regulator sourcing

150mA.

Bench testing is the best method for determining

the proper type and value of the capacitor since

the high frequency characteristics of electro-

lytic capacitors vary widely, depending on type

and manufacturer. A high quality 2.2

µF alumi-

num electrolytic capacitor works in most appli-

cation circuits, but the same stability often can

be obtained with a 1

µF tantalum electrolytic.

With the SPX3819 adjustable version, the mini-

mum value of output capacitance is a function of

the output voltage. The value decreases with

higher output voltages, since closed loop gain is

increased.

Typical Applications Circuits

A 10nF capacitor on the BYP pin will signifi-

cantly reduce output noise, but it may be left

unconnected if the output noise is not a major

concern. The SPX3819 start-up speed is in-

versely proportional to the size of the BYP

capacitor. Applications requiring a slow ramp-

up of the output voltage should use a larger

the BYP capacitor can be omitted.

The SPX3819’s internal reference is available

through the BYP pin.

Figure 1 represents a SPX3819 standard appli-

cation circuit. The EN (enable) pin is pulled

high (>2.0V) to enable the regulator.

To disable the regulator, EN < 0.4V.

The SPX3819 in Figure 2 illustrates a typical

adjustable output voltage configuration. Two

output voltage is calculated using the formula:

should be between 22 k

Ω and 47kΩ.

APPLICATION INFORMATION

+

+

GND

EN

BYP

(Opt.)

ENABLE may be tied directly to V

IN

SPX3819

1

2

3

4

5

V

IN

V

OUT

TOP View

+

+

GND

EN

Hi-ON

Lo-OFF

SPX3819

SOT-23

1

2

3

4

5

V

IN

V

OUT

R1

R2

TOP View

Figure 1. Standard Application Circuit

Figure 2. Typical Adjustable Output Voltage Configuration