LP2989IM-2.5 National Semiconductor, LP2989IM-2.5 Datasheet - Page 10

• Manufacturer Part Number: LP2989IM-2.5
• Description: IC,VOLT REGULATOR,FIXED,+2.5V,BIPOLAR,SOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LP2989IM-2.5

• Rohs Compliant: NO

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Application Hints
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 capacitance
is
(the amount of capacitance may be increased without limit).
This capacitor must be located a distance of not more than
0.5 from the input pin and returned to a clean analog
ground. Any good quality ceramic, tantalum, or film capacitor
may be used at the input.
IMPORTANT: Tantalum capacitors can suffer catastrophic
failure due to surge current when connected to a
low-impedance source of power (like a battery or very large
capacitor). If a Tantalum capacitor is used at the input, it
must be guaranteed by the manufacturer to have a surge
current rating sufficient for the application.
There are no requirements for ESR on the input capacitor,
but tolerance and temperature coefficient must be consid-
ered when selecting the capacitor to ensure the capacitance
will be
OUTPUT CAPACITOR: The LP2989 is designed specifically
to work with ceramic output capacitors, utilizing circuitry
which allows the regulator to be stable across the entire
range of output current with an output capacitor whose ESR
is as low as 5 m . It may also be possible to use Tantalum
or film capacitors at the output, but these are not as attrac-
tive 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).
Important: The output capacitor must maintain its ESR
within the stable region over the full operating temperature
range of the application to assure stability.
The LP2989 requires a minimum of 4.7 µF on the output
(output capacitor size can be increased without limit).
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 re-
quired amount of output capacitance is provided over the full
1 µF is required between the LP2989 input and ground
Stable Region For output Capacitor ESR
1 µF over the entire operating temperature range.
DS101339-38
10
operating temperature range. Some ceramic capacitors can
exhibit large changes in capacitance with temperature, so
X7R or X5R dielectric are strongly recommended.
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 capaci-
tor to the Bypass pin significantly reduces noise on the regu-
lator output. However, the capacitor 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 ca-
pacitor are ceramic and film. High-quality ceramic capacitors
with either NPO or COG dielectric typically have very low
leakage. 10 nF polypropolene and polycarbonate film ca-
pacitors 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, ceram-
ics are the least expensive and also have the lowest ESR
values
high-frequency noise). The ESR of a typical 4.7 µF ceramic
capacitor is in the range of 10 m
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
temperature characteristic, which results in the capacitance
dropping by more than 50% as the temperature goes from
25˚C to 85˚C.
This could cause problems if a 4.7 µF capacitor were used
on the output since it will drop down to approximately 2.4 µF
at high ambient temperatures (which could cause the
LP2989 to oscillate). If Z5U or Y5V capacitors are used on
the output, a minimum capacitance value of 10 µF must be
observed.
A better choice for temperature coefficient in ceramic capaci-
tors is X7R or X5R, which holds the capacitance within about
±
TANTALUM: Tantalum output capacitors are not recom-
mended for use with the LP2989 because:
Tantalum capacitors are less desirable than ceramics for use
as output capacitors because they are typically more expen-
sive when comparing equivalent capacitance and voltage
ratings in the 1 µF to 4.7 µF range.
Another important consideration is that Tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a Tantalum ca-
pacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly) than a ceramic capacitor with the same ESR
value.
It should also be noted that the ESR of a typical Tantalum will
increase about 2:1 as the temperature goes from 25˚C down
to −40˚C, so some guard band must be allowed.
FILM: Polycarbonate and polypropelene film capacitors
have excellent electrical performance: their ESR is the low-
15% over the operating temperature range.
(which
makes
them
to 15 m , which easily
best
for
eliminating