LP2982 Datasheet, PDF (13/17 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, the external capacitors used
with the LP2982 must be carefully selected to assure regula-
tor loop stability.
INPUT CAPACITOR: An input capacitor whose value is ≥
1 µF is required with the LP2982 (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 LP2982 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 val-
ues (refer to Figure 1, Figure 2).
IMPORTANT: The output capacitor must maintain its ESR in
the stable region over the full operating temperature to as-
sure 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 LP2982 and returned to a clean analog
ground.
LOW-CURRENT OPERATION: In applications where the
load current is < 1 mA, special consideration must be given
to the output capacitor.
Circuitry inside the LP2982 is specially designed to reduce
operating (quiescent) current at light loads down to about
65 µA.
The mode of operation which yields this very low quiescent
current also means that the output capacitor ESR is critical.
For optimum stability and minimum output noise, it is recom-
mended that a 10Ω resistor be placed in series with the out-
put capacitor in any applications where I_L < 1 mA.
CAPACITOR CHARACTERISTICS
TANTALUM: Tantalum capacitors are the best choice for
use with the LP2982. Most good quality tantalum can be
used with the LP2982, but check the manufacturer’s data
sheet to be sure the ESR is in range.
It is important to remember that ESR increases sharply at
lower temperatures (< 10˚C) and a capacitor that is near the
upper limit for stability at room temperature can cause insta-
bility when it gets cold.
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 LP2982. This is because the ESR of
a ceramic can be low enough to go below the minimum
stable value for the LP2982. A good 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-
trolytic are not typically used with the LP2982. They must
meet the same ESR requirements over the operating tem-
perature range, which is more difficult because of their large
increase in ESR at cold temperature.
An aluminum electrolytic can exhibit an ESR increase of as
much as 50X when going from 20˚C to −40˚C. Also, some
aluminum electrolytic are not operational below −25˚C be-
cause the electrolyte can freeze.
DS012679-43
FIGURE 1. 5V/2.2 µF ESR Curves
DS012679-44
FIGURE 2. 3V/4.7 µF ESR Curves
BYPASS CAPACITOR
The 0.01 µF capacitor connected to the bypass pin to reduce
noise must have very low leakage.
The current flowing out of the bypass pin comes from the
bandgap reference, which is used to set the output voltage.
This capacitor leakage current causes the output voltage to
decline by an amount proportional to the current. Typical val-
ues are −0.015%/nA @ −40˚C, −0.021%/nA @ 25˚C, and
−0.035%/nA @ +125˚C.
This data is valid up to a maximum leakage current of about
500 nA, beyond which the bandgap is so severely loaded
that it can not function.
Care must be taken to ensure that the capacitor selected will
not have excessive leakage current over the operating tem-
perature range of the application.
A high quality ceramic capacitor which uses either NPO or
COG type dielectric material will typically have very low leak-
age. Small surface mount polypropylene or polycarbonate
film capacitors also have extremely low leakage, but are
slightly larger than ceramics.
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