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LP3879SDX-1.2 Datasheet(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part # LP3879SDX-1.2
Description  Micropower 800mA Low Noise Ceramic Stable Voltage Regulator for Low Voltage Applications
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LP3879SDX-1.2 Datasheet(HTML) 9 Page - National Semiconductor (TI)

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Application Information (Continued)
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: The 10 nF capacitor on the
Bypass pin significantly reduces noise on the regulator out-
put and is required for loop stability. However, the capacitor
is connected directly to a high-impedance circuit in the band-
gap 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 LP3879 was designed to work with ceramic
capacitors on the output to take advantage of the benefits
they offer: for capacitance values in the 10 µF range, ceram-
ics 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 10 µF ceramic ca-
pacitor is in the range of 5 m
Ω to 10 mΩ, which meets the
ESR limits required for stability by the LP3879.
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.
Another significant problem with Z5U and Y5V dielectric
devices is that the capacitance drops severely with applied
voltage. A typical Z5U or Y5V capacitor can lose 60% of its
rated capacitance with half of the rated voltage applied to it.
For these reasons, X7R and X5R type ceramic capaci-
tors must be used on the input and output of the
LP3879.
SHUTDOWN INPUT OPERATION
The LP3879 is shut off by pulling the Shutdown input low,
and turned on by pulling it high. If this feature is not to be
used, the Shutdown input should be tied to V
IN to keep the
regulator output on at all times.
To assure proper operation, the signal source used to drive
the Shutdown input must be able to swing above and below
the specified turn-on/turn-off voltage thresholds listed in the
Electrical Characteristics section under V
ON/OFF.
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP3879 has an inherent diode connected between the regu-
lator output and input.
During normal operation (where the input voltage is higher
than the output) this diode is reverse-biased.
However, if the output is pulled above the input, this diode
will turn ON and current will flow into the regulator output.
In such cases, a parasitic SCR can latch which will allow a
high current to flow into V
IN (and out the ground pin), which
can damage the part.
In any application where the output may be pulled above the
input, an external Schottky diode must be connected from
V
IN to VOUT (cathode on VIN, anode on VOUT), to limit the
reverse voltage across the LP3879 to 0.3V (see Absolute
Maximum Ratings).
www.national.com
9


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