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Application Note
must be added, one for case to heat-sink (θ
heatsink to ambient (θ
). The junction temperature can be
HA
predicted as follows:
+ θ
+ θ
T
= T
+ P
(θ
J
A
D
JC
CH
T
is junction temperature, T
J
P
is the power consumption of the device. Device power
D
consumption is calculated as follows:
Once the devices power is determined, the maximum allow-
able (θ
) is calculated as:
JA(max)
θ
= T
/P
= T
JA(max)
R(max)
D
J(max
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
section is listed in the Absolute Maximum section of the
datasheet. T
is 125˚C for the control section, while
J(max)
T
is 150˚C for the output section.
J(max)
θ
should be calculated separately for each section as
JA(max)
follows:
θ
(max, CONTROL SECTION) = (125˚C - T
JA
θ
(max, OUTPUT SECTION) = (150˚C - T
JA
The required heat sink is determined by calculating its re-
quired thermal resistance (θ
(Continued)
) and one for
CH
θ
) = T
+ P
HA
A
D
JA
is ambient temperature, and
A
FIGURE 6. Power Dissipation Diagram
− T
)/P
A(max)
D
) for each
J(max)
)/P
A(max)
D
)/P
A(max)
D
).
HA(max)
FIGURE 7. Heat sink thermal Resistance vs Area
I
= I
+ I
IN
L
G
P
= (V
−V
) I
D
IN
OUT
L
Figure 6 shows the voltages and currents which are present
in the circuit.
10094716
θ
= θ
− (θ
HA(max)
JA(max)
JC
θ
should also be calculated twice as follows:
HA(max)
θ
= θ
(max, CONTROL SECTION) - (θ
HA(max)
JA
TROL SECTION) + θ
CH
θ
=θ
(max, OUTPUT SECTION) - (θ
HA(max)
JA
SECTION) + θ
)
CH
If thermal compound is used, θ
C/W. If the case is soldered to the heat sink, then a θ
be estimated as 0 C/W.
After, θ
is calculated for each section, choose the
HA(max)
lower of the two θ
HA(max)
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.
10094764
9
+ V
I
IN
G
+ θ
)
CH
JC
)
(OUTPUT
JC
can be estimated at 0.2
CH
values to determine the appropriate
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(CON-
can
CH
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