Datasheet

13
Thermal Model
(Discussion applies to HCPL-3180)
The steady state thermal model for the HCPL-3180 is
shown in Figure 27. The thermal resistance values given
in this model can be used to calculate the temperatures
at each node for a given operating condition. As shown
by the model, all heat generated ows through q
CA
which
raises the case temperature TC accordingly. The value of
q
CA
depends on the conditions of the board design and
is, therefore, determined by the designer. The value of
T
JE
= P
E
* (256°C/W + q
CA
) + P
D
* (57°C/W + q
CA
) + T
A
T
JD
= P
E
* (57°C/W + q
CA
) + P
D
* (111°C/W + q
CA
) + T
A
For example, given P
E
= 45 mW,
P
O
= 250 mW, T
A
= +70 °C and q
CA
= +83 °C/W:
T
JE
= P
E
* 339°C/W + P
D
* 140°C/W + T
A
= 45 mW * 339°C/W + 250 mW * 140°C/W + 70°C
= 120°C
T
JD
= P
E
* 140°C/W + P
D
* 194°C/W + T
A
= 45 mW * 140°C/W + 250 mW * 194°C/W + 70°C
= 125°C
T
JE
and T
JD
should be limited to +125 °C based on the board layout and part
placement (q
CA
) specic to the application.
Figure 27. Thermal model.
q
CA
= +83 °C/W was obtained from thermal measure-
ments using a 2.5 x 2.5 inch PC board, with small traces
(no ground plane), a single HCPL- 3180 soldered into the
center of the board and still air. The absolute maximum
power dissipation derating specications assume a q
CA
value of +83 °C/W. From the thermal mode in Figure 27,
the LED and detector IC junction temperatures can be
expressed as:
θ
LD
= 442 °C/W
T
JE
T
JD
θ
LC
= 467 °C/W θ
DC
= 126 °C/W
θ
CA
= 83 °C/W*
T
C
T
A
T
JE
= LED JUNCTION TEMPERATURE
T
JD
= DETECTOR IC JUNCTION TEMPERATURE
T
C
= CASE TEMPERATURE MEASURED AT THE
CENTER OF THE PACKAGE BOTTOM
θ
LC
= LED-TO-CASE THERMAL RESISTANCE
θ
LD
= LED-TO-DETECTOR THERMAL RESISTANCE
θ
DC
= DETECTOR-TO-CASE THERMAL RESISTANCE
θ
CA
= CASE-TO-AMBIENT THERMAL RESISTANCE
*θ
CA
WILL DEPEND ON THE BOARD DESIGN AND
THE PLACEMENT OF THE PART.
T
JE
= P
E
* (q
LC
//q
LD
+ q
DC
) + q
CA
) + P
D
* [
+ q
CA
] + T
A
q
LC
*
q
DC
q
LC
+
q
DC
+ q
LD
T
JD
= P
E
* [
+ q
CA
] + P
D *
(q
LC
//q
LD
+ q
DC
) + q
CA
) + T
A
q
LC
*
q
DC
q
LC
+
q
DC
+ q
LD