SNVS037G – SEPTEMBER 1999 – REVISED JANUARY 2015
Thermal Considerations (continued)
IIN = IL + IG
PD = (VIN−VOUT) IL + VINIG
Figure 29 shows the voltages and currents which are present in the circuit.
Figure 29. 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 LM1084 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
θJA (max) should be calculated separately for each section as follows:
θJA (max, CONTROL SECTION) = (125°C - TA(max))/PD
θJA (max, OUTPUT SECTION) = (150°C - TA(max))/PD
The required heat sink is determined by calculating its required thermal resistance (θHA (max)).
θHA (max) = θJA (max) − (θJC + θCH)
(θHA (max)) should also be calculated twice as follows:
(θHA (max)) = θJA (max, CONTROL SECTION) - (θJC (CONTROL SECTION) + θCH)
(θHA (max)) = θJA(max, OUTPUT SECTION) - (θJC (OUTPUT SECTION) + θCH)
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 30 can be used to determine the appropriate
area (size) of copper foil required.
Figure 30. Heat Sink Thermal Resistance vs Area
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