LP3878 NSC [National Semiconductor], LP3878 Datasheet - Page 9

• Manufacturer Part Number: LP3878
• Description: Micropower 800mA Low Noise Ceramic Stable Voltage Regulator for Low Voltage Applications Designed for Use with Very Low ESR Output Capacitors
• Manufacturer: NSC [National Semiconductor]

Application Hints
LLP PACKAGE DEVICES
The LP3878 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.
EXTERNAL CAPACITORS
Like any low-dropout regulator, the LP3878 requires external
capacitors for regulator stability. These capacitors must be
correctly selected for good performance.
Input Capacitor
An input capacitor whose capacitance is at least 4.7 µF is
required between the LP3878 input and ground (the amount
of capacitance may be increased without limit).
Capacitor tolerance and temperature variation must be con-
sidered when selecting a capacitor (see Capacitor Charac-
teristics section) to assure the minimum requirement of
input capacitance 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 LP3878 requires a ceramic output capacitor whose size
is at least 10 µF. Capacitance tolerance and temperature
characteristics must be considered when selecting an output
capacitor.
The LP3878 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 ultra low ESR output capacitor.
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). Because an
internal zero is built into the error amplifier, the LP3878 is
stable with output capacitor ESR values down to zero ohms.
Stable Region For Output Capacitor ESR
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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 LP3878 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, 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 10 µF ceramic capacitor is in the range of 5
mΩ to 10 mΩ, which meets the ESR limits required for
stability by the LP3878.
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
LP3878.
SHUTDOWN INPUT OPERATION
The LP3878 is shut off by driving 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
regulator output on at all times.
IN
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