LP38841 datasheet(7/18 Pages) TI1

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LP38841

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SNVS289B – DECEMBER 2004 – REVISED SEPTEMBER 2006

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 regulator. 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

VBIAS 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 temperature must be within the range specified under

operating conditions. The total power dissipation of the device is given by:

PD = (VIN−VOUT)IOUT+ (VIN)IGND

where IGND is the operating ground current of the device.

The maximum allowable temperature rise (TRmax) depends on the maximum ambient temperature (TAmax) of the

application, and the maximum allowable junction temperature (TJmax):

TRmax = TJmax− TAmax

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

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

θJA calculated above is ≥ 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

θJA falls below these 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

θJA will 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,

θHA≤ θJA − θCH − θJC.

In this equation,

θCH is the thermal resistance from the case to the surface of the heat sink and θJC is the thermal

resistance from the junction to the surface of the case.

θJC is about 3°C/W for a TO220 package. The value for

θCH depends on method of attachment, insulator, etc. θCH varies 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

θJA of TO-263 package for different

copper area sizes, using a typical PCB with 1 ounce copper and no solder mask over the copper area for heat

sinking.

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