IIN = IL + IG
Application Note (Continued)
must be added, one for case to heat-sink (θCH) and one for
heatsink to ambient (θHA). The junction temperature can be
predicted as follows:
PD = (VIN−VOUT) IL + VIN G
I
Figure 6 shows the voltages and currents which are present
in the circuit.
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:
10094616
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 LM1084 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.
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 appropri-
ate heat sink.
θ
JA (max) should be calculated separately for each section as
follows:
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.
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)).
10094664
FIGURE 7. Heat sink thermal Resistance vs Area
9
www.national.com
NSC [ National Semiconductor ]
