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 Vin if not used.

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 these packages are soldered to the

copper plane for heat sinking. The graph below shows a

curve for the

θ

sizes, using a typical PCB with 1 ounce copper and no solder

mask over the copper area for heat sinking.

20063225

FIGURE 1.

θ

package

www.national.com

9