Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, the external capacitors used
with the LP2981 must be carefully selected to assure regu-
lator loop stability.
Input Capacitor: An input capacitor whose value is ≥ 1 µF is
required with the LP2981 (amount of capacitance can be
increased without limit).
This capacitor must be located a distance of not more than
0.5" from the input pin of the LP2981 and returned to a clean
analog ground. Any good quality ceramic or tantalum can be
used for this capacitor.
Output Capacitor: The output capacitor must meet both the
requirement for minimum amount of capacitance and E.S.R.
(equivalent series resistance) value. Curves are provided
which show the allowable ESR range as a function of load
current for various output voltages and capacitor values
(refer to Figures 1, 2, 3, 4).
01250638
FIGURE 1. 5V/3.3 µF ESR Curves
Important: The output capacitor must maintain its ESR in
the stable region over the full operating temperature range to
assure stability. Also, capacitor tolerance and variation with
temperature must be considered to assure the minimum
amount of capacitance is provided at all times.
This capacitor should be located not more than 0.5" from the
output pin of the LP2981 and returned to a clean analog
ground.
CAPACITOR CHARACTERISTICS
Tantalum: Tantalum capacitors are the best choice for use
with the LP2981. Most good quality tantalums can be used
with the LP2981, but check the manufacturer’s data sheet to
be sure the ESR is in range.
It is important to remember that ESR increases at lower
temperatures and a capacitor that is near the upper limit for
stability at room temperature can cause instability when it
gets cold.
01250639
FIGURE 2. 5V/10 µF ESR Curves
In applications which must operate at very low temperatures,
it may be necessary to parallel the output tantalum capacitor
with a ceramic capacitor to prevent the ESR from going up
too high (see next section for important information on ce-
ramic capacitors).
Ceramic: Ceramic capacitors are not recommended for use
at the output of the LP2981. This is because the ESR of a
ceramic can be low enough to go below the minimum stable
value for the LP2981. A 2.2 µF ceramic was measured and
found to have an ESR of about 15 mΩ, which is low enough
to cause oscillations.
If a ceramic capacitor is used on the output, a 1Ω resistor
should be placed in series with the capacitor.
Aluminum: Because of large physical size, aluminum elec-
trolytics are not typically used with the LP2981. They must
meet the same ESR requirements over the operating tem-
perature range, more difficult because of their steep increase
at cold temperature.
01250640
An aluminum electrolytic can exhibit an ESR increase of as
much as 50X when going from 20˚C to −40˚C. Also, some
aluminum electrolytics are not operational below −25˚C be-
cause the electrolyte can freeze.
FIGURE 3. 3V/3.3 µF ESR Curves
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