APPLICATION INFORMATION
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 2.2µF alumi-
num electrolytic capacitor works in most appli-
cation circuits, but the same stability often can
be obtained with a 1µF 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
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
C
BYP
. However, if a rapid turn-on is necessary,
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
resistors (R
1
and R
2
) set the output voltage. The
output voltage is calculated using the formula:
V
OUT
= 1.235V x
[
1 + R
1
/R
2
]
R
2
must be > 10 kΩ and for best results, R
2
should be between 22 kΩ and 47kΩ.
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
V
IN
1
5
V
OUT
V
IN
1
5
V
OUT
+
2
+
GND
3
EN
ENABLE may be tied directly to V
IN
SPX3819
+
GND
2
SPX3819
SOT-23
3
4
R1
+
4
BYP
(Opt.)
EN
Hi-ON
Lo-OFF
R2
TOP View
TOP View
Figure 1. Standard Application Circuit
Figure 2. Typical Adjustable Output Voltage Configuration
July 2008 Rev F
SPX3819 500mA, Low Noise LDO Voltage Regulator
© 2008 Exar Corporation
7
EXAR [ EXAR CORPORATION ]
