LM1085 datasheet(9/13 Pages) NSC

9 / 13 page

Application Note (Continued)

must be added, one for case to heat-sink (

CH) and one for

heatsink to ambient (

HA). The junction temperature can be

predicted as follows:

J =TA +PD (

JC +

CH +

HA)= TA +PD

J is junction temperature, TA is ambient temperature, and

D is the power consumption of the device. Device power

consumption is calculated as follows:

IN =IL +IG

D =(VIN−VOUT)IL +VINIG

Figure 6 shows the voltages and currents which are present

in the circuit.

Once the devices power is determined, the maximum allow-

able (

JA(max)) is calculated as:

JA(max) =TR(max)/PD =TJ(max −TA(max))/PD

The LM1085 has different temperature specifications for two

different sections of the IC: the control section and the output

section. The Electrical Characteristics table shows the junc-

tion to case thermal resistances for each of these sections,

while the maximum junction temperatures (T

J(max)) for each

section is listed in the Absolute Maximum section of the

datasheet. T

J(max) is 125˚C for the control section, while

J(max) is 150˚C for the output section.

JA(max) should be calculated separately for each section as

follows:

JA (max, CONTROL SECTION) = (125˚C - TA(max))/PD

JA(max, OUTPUT SECTION) = (150˚C - TA(max))/PD

The required heat sink is determined by calculating its re-

quired thermal resistance (

HA(max)).

HA(max) =

JA(max) −(

JC +

CH)

HA(max) should also be calculated twice as follows:

HA(max) =

JA (max, CONTROL SECTION) - (

JC (CON-

TROL SECTION) +

CH)

HA(max)=

JA(max,

OUTPUT SECTION) - (

JC(OUTPUT

SECTION) +

CH)

If thermal compound is used,

CH can be estimated at 0.2

C/W. If the case is soldered to the heat sink, then a

CH can

be estimated as 0 C/W.

After,

HA(max) is calculated for each section, choose the

lower of the two

HA(max) values to determine the appropriate

heat sink.

If PC board copper is going to be used as a heat sink, then

Figure 7 can be used to determine the appropriate area

(size) of copper foil required.

10094716

FIGURE 6. Power Dissipation Diagram

10094764

FIGURE 7. Heat sink thermal Resistance vs Area

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