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.

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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 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

m

Ω 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.

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