LP38856 datasheet(11/14 Pages) NSC

11 / 14 page

Application Information (Continued)

D(PASS) =(VIN -VOUT)xIOUT

(1)

The second part is the power that is dissipated in the bias

and control circuitry, and can be determined with the for-

mula:

D(BIAS) =VBIAS xIGND(BIAS)

(2)

where I

GND(BIAS) is the portion of the operating ground cur-

rent of the device that is related to V

BIAS.

The third part is the power that is dissipated in portions of the

output stage circuitry, and can be determined with the for-

mula:

D(IN) =VIN xIGND(IN)

(3)

where I

GND(IN) is the portion of the operating ground current

of the device that is related to V

IN.

The total power dissipation is then:

D =PD(PASS) +PD(BIAS) +PD(IN)

(4)

The maximum allowable junction temperature rise (

∆T

J) de-

pends on the maximum anticipated ambient temperature

A(MAX)) for the application, and the maximum allowable

operating junction temperature (T

J(MAX)):

(5)

The maximum allowable value for junction to ambient Ther-

mal Resistance,

JA, can be calculated using the formula:

(6)

The LP38856 is available in TO-220 and TO-263 packages.

The thermal resistance in the application depends on

amount of copper area or heat-sink, and on air flow. If the

maximum allowable value of

JA calculated above is

≥ 60

˚C/W for TO-220 package and

≥ 60 ˚C/W for TO-263 pack-

age no heat-sink is needed since the package alone can

dissipate enough heat to satisfy these requirements. If the

value needed for allowable

JA falls below these limits, a

heat-sink is required.

Heat-Sinking The TO-220 Package

The TO-220 package has a

JA rating of 60˚C/W, and a

rating of 3˚C/W. These ratings are for the package only, no

additional heat-sinking, and with no airflow.

The thermal resistance of a TO-220 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

JA will

be same as shown in next section for TO-263 package.

The heat-sink to be used in the application should have a

heat-sink to ambient thermal resistance,

HA:

(7)

where

JA is the required total thermal resistance from the

junction to the ambient air,

CH is the thermal resistance from

the case to the surface of the heat sink, and

JC is the

thermal resistance from the junction to the surface of the

case.

For this equation,

JC is about 3˚C/W for a TO-220 package.

The value for

CH depends on method of attachment, insu-

lator, etc.

CH varies between 1.5˚C/W to 2.5˚C/W. Consult

the heat-sink manufacturer datasheet for details and recom-

mendations.

Heat-Sinking The TO-263 Package

The TO-263 package has a

JA rating of 60˚C/W, and a

rating of 3˚C/W. These ratings are for the package only, no

additional heat-sinking, and with no airflow.

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

a heat-sink. The tab of this package is soldered to the copper

plane for heat-sinking. The graph below 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.

As shown in Figure 1, 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 PCB is

32˚C/W.

Figure 2 shows the maximum allowable power dissipation

for TO-263 packages for different ambient temperatures,

assuming

JA is 35˚C/W and the maximum junction tempera-

ture is 125˚C.

20131125

FIGURE 1.

JA vs Copper (1 Ounce) Area for the

TO-263 package

LP38856 датащи(PDF) 11 Page - National Semiconductor (TI)

LP38856 датащи(HTML) 11 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

датащи скачать

ссылки URL