Application Note (Continued)
must be added, one for case to
TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA
TJ is junction temperature, TA is ambient temperature, and PD is the power consumption of the device. Device power consumption is calculated as follows:
IIN = IL + IG
PD = (VIN−VOUT) IL + VINIG
Figure 6 shows the voltages and currents which are present in the circuit.
LM1085
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FIGURE 6. Power Dissipation Diagram
Once the devices power is determined, the maximum allow- able (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 Electrical Characteristics table shows the junc- tion 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 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 θJA(max, OUTPUT SECTION) = (150˚C - TA(max))/PD
The required heat sink is determined by calculating its re- quired 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 (CON- TROL 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 7 can be used to determine the appropriate area (size) of copper foil required.
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FIGURE 7. Heat sink thermal Resistance vs Area
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