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LP2978 Fiches technique(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce LP2978
Description  Micropower SOT, 50 mA Low-Noise Ultra Low-Dropout Regulator
Download  7 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LP2978 Fiches technique(HTML) 5 Page - National Semiconductor (TI)

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Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP2978 requires external
capacitors for regulator stability. These capacitors must be
correctly selected for good performance.
Input Capacitor
An input capacitor whose capacitance is
≥ 1 µF is required
between the LP2978 input and ground (the amount of ca-
pacitance may be increased without limit).
This capacitor must be located a distance of not more than 1
cm 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 fail-
ure
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
≥ 1 µF over the entire operating temperature range.
Output Capacitor:
The LP2978 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 5 m
Ω.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).
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 LP2978 requires a minimum of 2.2 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
operating temperature range. It should be noted that ceramic
capacitors can exhibit large changes in capacitance with
temperature (see next section,
Capacitor Characteristics).
The output capacitor must be located not more than 1 cm
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. It should be noted that
the capacitor is connected directly to a high-impedance cir-
cuit 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
The LP2978 was designed to work with ceramic capacitors
on the output to take advantage of the benefits they offer: for
capacitance values in the 2.2 µF to 4.7 µF range, ceramics
are the least expensive and also have the lowest ESR val-
ues (which makes them best for eliminating high-frequency
noise). The ESR of a typical 2.2 µF ceramic capacitor is in
the range of 10 m
Ω to 20 mΩ, which easily meets the ESR
limits required for stability by the LP2978.
One disadvantage of ceramic capacitors is that their capaci-
tance can vary with temperature. Most 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 2.2 µF capacitor were used
on the output since it will drop down to approximately 1 µF at
high ambient temperatures (which could cause the LP2978
to oscillate). If Z5U or Y5V capacitors are used on the output,
a minimum capacitance value of 4.7 µF must be observed.
A better choice for temperature coefficient in ceramic capaci-
tors is X7R, which holds the capacitance within ±15%. Un-
fortunately, the larger values of capacitance are not offered
by all manufacturers in the X7R dielectric.
Tantalum:
Tantalum capacitors are less desirable than ceramics for use
as output capacitors because they are more expensive 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.
DS100935-7
www.national.com
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