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Data subject to change. Copyright © 2005-2012 Avago Technologies. All rights reserved.
AV02-3789EN - October 12, 2012
Thermal Model
De nitions:
R
11
: Junction to Ambient Thermal Resistance of LED
due to heating of LED
R
12
: Junction to Ambient Thermal Resistance of LED
due to heating of Detector (Output IC)
R
21
: Junction to Ambient Thermal Resistance of Detector
(Output IC) due to heating of LED.
R
22
: Junction to Ambient Thermal Resistance of Detector
(Output IC) due to heating of Detector (Output IC).
P
1
: Power dissipation of LED (W).
P
2
: Power dissipation of Detector / Output IC (W).
T
1
: Junction temperature of LED (˚C).
T
2
: Junction temperature of Detector (˚C).
T
A
: Ambient temperature.
Ambient Temperature: Junction to Ambient Thermal Re-
sistances were measured approximately 1.25 cm above
optocoupler at ~23 ˚C in still air
Thermal Resistance °C/W
R
11
118.5
R
12
11.6
R
21
14
R
22
55.5
This thermal model assumes the device is soldered onto a
high conductivity board as per JEDEC 51-7. The tempera-
ture at the LED and Detector junctions of the optocoupler
can be calculated using the equations below.
T
1
= (R
11
* P
1
+ R
12
* P
2
) + T
A
-- (1)
T
2
= (R
21
* P
1
+ R
22
* P
2
) + T
A
-- (2)
Using the given thermal resistances and thermal model
formula in this datasheet, we can calculate the junction
temperature for both LED and the output detector. Both
junction temperatures should be within the absolute
maxi¬mum rating of 125°C.
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