Datasheet

Application Hints
LLP PACKAGE DEVICES
The LP2989 is offered in the 8 lead LLP surface mount
package to allow for increased power dissipation compared
to the SO-8 and Mini SO-8. For details on thermal perfor-
mance as well as mounting and soldering specifications,
refer to Application Note AN-1187.
For output voltages
<
2V, see LP2989LV datasheet.
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP2989 requires external
capacitors for regulator stability. These capacitors must be
correctly selected for good performance.
Input Capacitor
An input capacitor whose size is at least 2.2 µF is required
between the LP2989 input and ground (the amount of ca-
pacitance may be increased without limit).
Characterization testing performed on the LP2989 has
shown that if the amount of actual input capacitance drops
below about 1.5 µF, an unstable operating condition may
result. Therefore, the next larger standard size (2.2 µF) is
specified as the minimum required input capacitance. Ca-
pacitor tolerance and temperature variation must be consid-
ered when selecting a capacitor (see Capacitor Character-
istics section) to assure the minimum requirement of 1.5 µF
is met over all operating conditions.
The input capacitor must be located at a distance of not
more than 0.5" from the input pin and returned to a clean
analog ground. Any good quality ceramic or tantalum may be
used for this capacitor, assuming the minimum capacitance
requirement is met.
Output Capacitor
The LP2989 requires a ceramic output capacitor whose size
is at least 4.7µF. The actual amount of capacitance on the
output must never drop below about 3.5µF or unstable op-
eration may result. For this reason, capacitance tolerance
and temperature characteristics must be considered when
selecting an output capacitor.
The LP2989 is designed specifically to work with ceramic
output capacitors, utilizing circuitry which allows the regula-
tor to be stable across the entire range of output current with
an output capacitor whose ESR is as low as 4 mΩ.Itmay
also be possible to use Tantalum or film capacitors at the
output, but these are not as attractive for reasons of size and
cost (see next section Capacitor Characteristics).
The output capacitor must meet the requirement for mini-
mum amount of capacitance and also have an ESR (equiva-
lent series resistance) value which is within the stable range.
Curves are provided which show the stable ESR range as a
function of load current (see ESR graph below).
10133938
Stable Region For output Capacitor ESR
Important: The output capacitor must maintain its ESR
within the stable region over the full operating temperature
range of the application to assure stability.
It is important to remember that capacitor tolerance and
variation with temperature must be taken into consideration
when selecting an output capacitor so that the minimum
required amount of output capacitance is provided over the
full operating temperature range. (See Capacitor Character-
istics section).
The output capacitor must be located not more than 0.5"
from the output pin and returned to a clean analog ground.
Noise Bypass Capacitor
Connecting a 10 nF capacitor to the Bypass pin significantly
reduces noise on the regulator output. However, the capaci-
tor is connected directly to a high-impedance circuit in the
bandgap reference.
Because this circuit has only a few microamperes flowing in
it, any significant loading on this node will cause a change in
the regulated output voltage. For this reason, DC leakage
current through the noise bypass capacitor must never ex-
ceed 100 nA, and should be kept as low as possible for best
output voltage accuracy.
The types of capacitors best suited for the noise bypass
capacitor are ceramic and film. High-quality ceramic capaci-
tors with either NPO or COG dielectric typically have very
low leakage. 10 nF polypropolene and polycarbonate film
capacitors are available in small surface-mount packages
and typically have extremely low leakage current.
CAPACITOR CHARACTERISTICS
Ceramic
The LP2989 was designed to work with ceramic capacitors
on the output to take advantage of the benefits they offer: for
capacitance values in the 4.7 µF range, ceramics are the
least expensive and also have the lowest ESR values (which
makes them best for eliminating high-frequency noise). The
ESR of a typical 4.7 µF ceramic capacitor is in the range of
10 mΩ to 15 mΩ, which easily meets the ESR limits required
for stability by the LP2989.
One disadvantage of ceramic capacitors is that their capaci-
tance can vary with temperature. Many large value ceramic
capacitors (≥ 2.2 µF) are manufactured with the Z5U or Y5V
LP2989
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