31
Thermal Model
Since θ and θ
are
9,10A
If we, however, assume a worst
case PCB layout and no air flow
4A
dependent on PCB layout and
airflow, their exact number may
not be available. Therefore, a
more accurate method of calcu-
lating the junction temperature is
with the following equations:
The HCPL-316J is designed to
dissipate the majority of the heat
through pins 4 for the input IC
and pins 9 and 10 for the output
where the estimated θ and
4A
θ
are 100°C/W. Then the
9,10A
junction temperatures become
IC. (There are two V pins on
EE
T = (90.8 mW)(60°C/W
ji
the output side, pins 9 and 10,
for this purpose.) Heat flow
through other pins or through the
package directly into ambient are
considered negligible and not
modeled here.
+ 100°C/W) + 100°C = 115°C
T = P θ + T
ji
i i4
P4
T
= P θ
+ T
T = (240 mW)(30°C/W
jo
o o9,10
P9,10
jo
+ 100°C/W) + 100°C = 131°C
These equations, however,
require that the pin 4 and pins
9,10 temperatures be measured
with a thermal couple on the pin
at the HCPL-316J package edge.
The output IC junction
temperature exceeds the absolute
maximum specification of 125°C.
In this case, PCB layout and
airflow will need to be designed
so that the junction temperature
of the output IC does not exceed
125°C.
In order to achieve the power
dissipation specified in the
absolute maximum specification,
it is imperative that pins 4, 9, and
10 have ground planes connected
to them. As long as the maximum
power specification is not
From the earlier power
dissipation calculation
example:
exceeded, the only other limita-
tion to the amount of power one
can dissipate is the absolute
maximum junction temperature
specification of 125°C. The
junction temperatures can be
calculated with the following
equations:
P = 90.8 mW, P = 314 mW, T
A
If the calculated junction
temperatures for the thermal
model in Figure 78 is higher than
125°C, the pin temperature for
pins 9 and 10 should be
measured (at the package edge)
under worst case operating
environment for a more accurate
estimate of the junction
i
o
= 100°C, and assuming the
thermal model shown in Figure
77 below.
T = (90.8 mW)(60°C/W
ji
+ 50°C/W) + 100°C = 110°C
T
= (240 mW)(30°C/W
jo
T = P (θ + θ ) + T
ji
i
i4
4A
A
+ 50°C/W) + 100°C = 119°C
temperatures.
T
= P (θ
+ θ
) + T
jo
o
o9,10
9,10A A
both of which are within the
absolute maximum specification
of 125°C.
where P = power into input IC
i
and P = power into output IC.
o
T = junction temperature of input side IC
ji
T = junction temperature of output side IC
jo
T
T
θ
= pin 4 temperature at package edge
P4
T
T
jo
ji
= pin 9 and 10 temperature at package edge
P9,10
θ
= 60°C/W
θ
= 30°C/W
O9,10
i4
= input side IC to pin 4 thermal resistance
I4
θ
θ
θ
= output side IC to pin 9 and 10 thermal resistance
= pin 4 to ambient thermal resistance
I9,10
T
T
P9,10
P4
4A
9,10A
θ
= 50°C/W*
θ
= 50°C/W*
9,10A
4A
= pin 9 and 10 to ambient thermal resistance
T
A
*The θ and θ
values shown here are for PCB layouts shown in Figure 78 with
4A
9,10A
reasonable air flow. This value may increase or decrease by a factor of 2 depending
on PCB layout and/or airflow.
Figure 78. HCPL-316J Thermal Model.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
