LM1085 datasheet(21/29 Pages) TI1

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LM1085

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SNVS038H – JULY 1999 – REVISED JANUARY 2015

Thermal Considerations (continued)

IIN = IL + IG

(4)

PD = (VIN−VOUT) IL + VINIG

(5)

Figure 28 shows the voltages and currents which are present in the circuit.

Figure 28. Power Dissipation Diagram

Once the devices power is determined, the maximum allowable (

θJA (max)) is calculated as:

θJA (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 Thermal Information table shows the junction to case thermal resistances for each of

these sections, while the maximum junction temperatures (TJ(max)) for each section is listed in the Absolute

Maximum Ratings section of the datasheet. TJ(max) is 125°C for the control section, while TJ(max) is 150°C for the

output section.

θJA (max) should be calculated separately for each section as follows:

θJA (max, CONTROL SECTION) = (125°C - TA(max))/PD

(6)

θJA (max, OUTPUT SECTION) = (150°C - TA(max))/PD

(7)

The required heat sink is determined by calculating its required thermal resistance (

θHA (max)).

θHA (max) = θJA (max) − (θJC + θCH)

(8)

(

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

(

θHA (max)) = θJA (max, CONTROL SECTION) - (θJC (CONTROL SECTION) + θCH)

(9)

(

θHA (max)) = θJA(max, OUTPUT SECTION) - (θJC (OUTPUT SECTION) + θCH)

(10)

If thermal compound is used,

θCH can be estimated at 0.2 C/W. If the case is soldered to the heat sink, then a

θCH can be estimated as 0 C/W.

After,

θHA (max) is calculated for each section, choose the lower of the two θHA (max) values to determine the

appropriate heat sink.

If PC board copper is going to be used as a heat sink, then Figure 29 can be used to determine the appropriate

area (size) of copper foil required.

Figure 29. Heat Sink Thermal Resistance vs Area

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