lp2980im5-1.8 National Semiconductor Corporation, lp2980im5-1.8 Datasheet - Page 10

• Manufacturer Part Number: lp2980im5-1.8
• Description: Ic Reg Ldo Micropower Sot23-5
• Manufacturer: National Semiconductor Corporation

www.national.com
OUTPUT CAPACITOR
The LP2980 requires an output capacitor to maintain loop
stability. The capacitor must be selected to meet the require-
ments of capacitance and ESR (equivalent series resistance)
over the full operating temperature range of the application
(see SELECTING THE OUTPUT CAPACITOR).
The minimum amount of capacitance which can be used on
the output is 2.2 µF, but this value may be increased without
limit.
Four curves are provided which show the stable ESR range
of the LP2980-1.8V operated with output capacitances of 2.2,
4.7, 10, and 15 µF:
2.2 µF ESR Curves For 1.8V Output
4.7 µF ESR Curves For 1.8V Output
10 µF ESR Curves For 1.8V Output
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It should be noted that for the lower values of the output ca-
pacitance (< 10 µF), it may be necessary to use a capacitor
and series resistance to provide sufficient ESR. To under-
stand why this is true, the basic characteristics of capacitance
types must be explained:
CAPACITOR CHARACTERISTICS
Ceramic Capacitors have an extremely low ESR (in the
range of 5-15 mΩ), and can only be used on the output of the
LP2980 if an external resistor is placed in series to supply the
needed ESR (a resistance value of about 2Ω +/− 30% is rec-
ommended).
Be careful of the temperature coefficient of ceramics: select
X7R or X5R if possible, because those types typically vary
less than +/− 25% over the range of −40 to +125°C. Z5U types
are worse, and will typically lose half (or more) of their ca-
pacitance over the same temperature range.
A source for large-value ceramics with good performance is
Taiyo-Yuden. Their web address is :
http://www.t-yuden.com/hicap.html
Tantalum capacitors have ESR values that are more difficult
to determine: the manufacturers specify only a maximum lim-
it, which is typically 10X or 20X the typical value. ESR values
can vary considerably from lot to lot and from manufacturer
to manufacturer. For example, some 4.7 µF/10V devices test-
ed showed typical values in the range of 0.5 - 1Ω, but values
as high as 6Ω have been seen.
It should also be noted that the ESR typically increases about
2X - 3X when going from +125°C down to −40°C.
Another factor to consider is that Tantalum manufacturers are
presently designing their products toward the goal of getting
the lowest possible ESR, in an attempt to compete with the
new high-value ceramic capacitors. This means that the typ-
ical values will probably continue to decline in the future.
SELECTING THE OUTPUT CAPACITOR
This section contains guidelines for selecting an output ca-
pacitor which will maintain good regulator stability over the
entire operating temperature range (refer to ESR CURVES).
2.2 µF Output Capacitor
The smallest output capacitor which can be used with the
LP2980-1.8 is 2.2 µF. However, care must be exercised if this
value is used because of the ESR requirement.
At load currents
mately 0.5Ω to 6Ω. This range is probably sufficiently wide
that most 2.2 µF Tantalum capacitors would fall within it.
15 µF ESR Curves For 1.8V Output
≤
25mA, the stable ESR range is approxi-
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