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.
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
CBYP. 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.
Benchtestingisthebestmethodfordetermining
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.
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 (R1 and R2) set the output voltage. The
output voltage is calculated using the formula:
With the SPX3819 adjustable version, the mini-
mumvalueofoutputcapacitanceisafunctionof
the output voltage. The value decreases with
higher output voltages, since closed loop gain is
increased.
VOUT = 1.235V x [ 1 + R1/R2]
R2 must be > 10 kΩꢀand for best results, R2
shouldbebetween22kΩ and47kΩ.
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
VIN
1
VOUT
5
VOUT
VIN
1
5
4
+
+
R1
R2
2
2
SPX3819
SOT-23
SPX3819
GND
EN
+
GND
EN
+
3
3
4
BYP
(Opt.)
Hi-ON
Lo-OFF
ENABLE may be tied directly to V
IN
TOP View
TOP View
Figure 2. Typical Adjustable Output Voltage Configuration
Figure 1. Standard Application Circuit
July 2008 Rev F
SPX3819 500mA, Low Noise LDO Voltage Regulator
© 2008 Exar Corporation
7
EXAR [ EXAR CORPORATION ]
