Application Hints

EXTERNAL CAPACITORS

To assure regulator stability, input and output capacitors are

required as shown in the Typical Application Circuit.

OUTPUT CAPACITOR

An output capacitor is required on the LP3884X devices for

loop stability. The minimum value of capacitance necessary

depends on type of capacitor: if a solid Tantalum capacitor is

used, the part is stable with capacitor values as low as 4.7µF.

If a ceramic capacitor is used, a minimum of 22 µF of

capacitance must be used (capacitance may be increased

without limit). The reason a larger ceramic capacitor is re-

quired is that the output capacitor sets a pole which limits the

loop bandwidth. The Tantalum capacitor has a higher ESR

than the ceramic which provides more phase margin to the

loop, thereby allowing the use of a smaller output capacitor

because adequate phase margin can be maintained out to a

higher crossover frequency. The tantalum capacitor will typi-

cally also provide faster settling time on the output after a

fast changing load transient occurs, but the ceramic capaci-

tor is superior for bypassing high frequency noise.

The output capacitor must be located less than one centi-

meter from the output pin and returned to a clean analog

ground. Care must be taken in choosing the output capacitor

to ensure that sufficient capacitance is provided over the full

operating temperature range. If ceramics are selected, only

X7R or X5R types may be used because Z5U and Y5F types

suffer severe loss of capacitance with temperature and ap-

plied voltage and may only provide 20% of their rated ca-

pacitance in operation.

INPUT CAPACITOR

The input capacitor is also critical to loop stability because it

provides a low source impedance for the regulator. The

minimum required input capacitance is 10 µF ceramic (Tan-

talum not recommended). The value of C

creased without limit. As stated above, X5R or X7R must be

used to ensure sufficient capacitance is provided. The input

capacitor must be located less than one centimeter from the

input pin and returned to a clean analog ground.

BIAS CAPACITOR

The 0.1µF capacitor on the bias line can be any good quality

capacitor (ceramic is recommended).

BIAS VOLTAGE

The bias voltage is an external voltage rail required to get

gate drive for the N-FET pass transistor. Bias voltage must

be in the range of 4.5 - 5.5V to assure proper operation of

the part.

UNDER VOLTAGE LOCKOUT

The bias voltage is monitored by a circuit which prevents the

regulator output from turning on if the bias voltage is below

approximately 4V.

SHUTDOWN OPERATION

Pulling down the shutdown (S/D) pin will turn-off the regula-

tor. Pin S/D must be actively terminated through a pull-up

resistor (10 k

Ω to 100 kΩ) for a proper operation. If this pin

is driven from a source that actively pulls high and low (such

as a CMOS rail to rail comparator), the pull-up resistor is not

required. This pin must be tied to V

POWER DISSIPATION/HEATSINKING

A heatsink may be required depending on the maximum

power dissipation and maximum ambient temperature of the

application. Under all possible conditions, the junction tem-

perature must be within the range specified under operating

conditions. The total power dissipation of the device is given

by:

P

where I

The maximum allowable temperature rise (T

on the maximum ambient temperature (T

cation, and the maximum allowable junction temperature

(T

T

The maximum allowable value for junction to ambient Ther-

mal Resistance,

θ

θ

These parts are available in TO-220 and TO-263 packages.

The thermal resistance depends on amount of copper area

or heat sink, and on air flow. If the maximum allowable value

of

θ

≥ 60 ˚C/W for TO-220 package

and

≥ 60 ˚C/W for TO-263 package no heatsink is needed

since the package can dissipate enough heat to satisfy these

requirements. If the value for allowable

θ

limits, a heat sink is required.

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO220 package can be reduced

by attaching it to a heat sink or a copper plane on a PC

board. If a copper plane is to be used, the values of

θ

be same as shown in next section for TO263 package.

The heatsink to be used in the application should have a

heatsink to ambient thermal resistance,

θ

HA

≤θ

θ

θ

In this equation,

θ

to the surface of the heat sink and

θ

tance from the junction to the surface of the case.

θ

about 3˚C/W for a TO220 package. The value for

θ

pends on method of attachment, insulator, etc.

θ

between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,

2˚C/W can be assumed.

HEATSINKING TO-263 PACKAGE

The TO-263 package uses the copper plane on the PCB as

a heatsink. The tab of this package is soldered to the copper

plane for heat sinking. The graph below shows a curve for

the

θ

using a typical PCB with 1 ounce copper and no solder mask

over the copper area for heat sinking.

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