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LM1085IT-3.3 データシート(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1085IT-3.3
部品情報  3A Low Dropout Positive Regulators
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1085IT-3.3 データシート(HTML) 9 Page - National Semiconductor (TI)

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


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