LP38511_09 datasheet(11/14 Pages) NSC

11 / 14 page

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FIGURE 2. ERROR Flag Operation, biased from V

POWER DISSIPATION/HEATSINKING

A heatsink may be required depending on the maximum pow-

dissipation

D(MAX)),

maximum

ambient

temperature (T

A(MAX)) of the application, and the thermal re-

sistance (

JA) of the package. Under all possible conditions,

the junction temperature (T

J) must be within the range spec-

ified in the Operating Ratings. The total power dissipation of

the device is given by:

D = ( (VIN−VOUT) x IOUT) + (VIN x IGND)

(1)

where I

GND is the operating ground current of the device

(specified under Electrical Characteristics).

The maximum allowable junction temperature rise (

ΔT

J) de-

pends on the maximum expected ambient temperature (T

(MAX)) of the application, and the maximum allowable junction

temperature (T

J(MAX)):

ΔT

J = TJ(MAX)− TA(MAX)

(2)

The maximum allowable value for junction to ambient Ther-

mal Resistance,

JA, can be calculated using the formula:

JA = ΔTJ / PD(MAX)

(3)

HEATSINKING TO-263 PACKAGE

The TO-263 and the TO-263 THIN packages use the copper

plane on the PCB as a heatsink. The tab, or DAP, of these

packages are soldered to the copper plane for heat sinking.

Figure 3 shows a curve for the

JA 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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FIGURE 3.

JA vs Copper (1 Ounce) Area for TO-263

package

As shown in the figure, increasing the copper area beyond 1

square inch produces very little improvement. The minimum

value for

JA for the TO-263 package mounted to a two-layer

PCB is 32°C/W.

Figure 4 shows the maximum allowable power dissipation for

TO-263 packages for different ambient temperatures, assum-

ing

JA is 35°C/W and the maximum junction temperature is

125°C.

20182936

FIGURE 4. Maximum Power Dissipation vs Ambient

Temperature for TO-263 Package

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