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IDT82V3255
WAN PLL
Thermal Management
111
June 19, 2006
7
THERMAL MANAGEMENT
The device operates over the industry temperature range -40°C ~
+85°C. To ensure the functionality and reliability of the device, the maxi-
mum junction temperature T
jmax
should not exceed 125°C. In some
applications, the device will consume more power and a thermal solution
should be provided to ensure the junction temperature T
j
does not
exceed the T
jmax
.
7.1
JUNCTION TEMPERATURE
Junction temperature T
j
is the temperature of package typically at the
geographical center of the chip where the device's electrical circuits are.
It can be calculated as follows:
Equation 1: T
j
= T
A
+ P X
θ
JA
Where:
θ
JA
= Junction-to-Ambient Thermal Resistance of the Package
T
j
= Junction Temperature
T
A
= Ambient Temperature
P = Device Power Consumption
In order to calculate junction temperature, an appropriate
θ
JA
must
be used. The
θ
JA
is shown in
Table 36
:
Power consumption is the core power excluding the power dissipated
in the loads.
Table 35
provides power consumption in special environ-
ments.
7.2
EXAMPLE OF JUNCTION TEMPERATURE
CALCULATION
Assume:
T
A
= 85°C
θ
JA
= 18.8°C/W (TQFP/DK64 Soldered & when airfow rate is 0 m/s)
P = 1.57W
The junction temperature T
j
can be calculated as follows:
T
j
= T
A
+ P X
θ
JA
= 85°C + 1.57W X 18.8°C/W = 114.5°C
The junction temperature of 114.5°C is below the maximum junction
temperature of 125°C so no extra heat enhancement is required.
In some operation environments, the calculated junction temperature
might exceed the maximum junction temperature of 125°C and an exter-
nal thermal solution such as a heatsink is required.
7.3
HEATSINK EVALUATION
A heatsink is expanding the surface area of the device to which it is
attached.
θ
JA
is now a combination of device case and heat-sink thermal
resistance, as the heat flowing from the die junction to ambient goes
through the package and the heatsink.
θ
JA
can be calculated as follows:
Equation 2:
θ
JA
=
θ
JC
+
θ
HA
Where:
θ
JC
= Junction-to-Case (Heatsink) Thermal Resistance
θ
HA
= Heatsink-to-Ambient Thermal Resistance
θ
HA
determines which heatsink can be selected to ensure the junc-
tion temperature does not exceed the maximum junction temperature.
According to Equation 1 and 2, the heatsink-to-ambient thermal resis-
tance
θ
HA
can be calculated as follows:
Equation 3:
θ
HA
= (T
j
- T
A
) / P -
θ
JC
Assume:
T
j
= 125°C (T
jmax
)
T
A
= 85°C
P = 1.57W
θ
JC
= 11.9°C/W (TQFP/DK64)
The heatsink-to-ambient thermal resistance
θ
HA
can be calculated
as follows:
θ
HA
= (125°C - 85°C ) / 1.57W - 11.9°C/W = 13.6°C/W
That is, if a heatsink whose heatsink-to-ambient thermal resistance
θ
HA
is below or equal to 13.6°C/W is used in such operation environ-
ment, the junction temperature will not exceed the maximum junction
temperature.
Table 35: Power Consumption and Maximum Junction Temperature
Package
Power
Consumption (W)
Operating
Voltage
(V)
T
A
(°C)
Maximum
Junction
Temperature (°C)
TQFP/PP64
TQFP/DK64
1.57
1.57
3.6
3.6
85
85
125
125
Table 36: Thermal Data
Package
Pin Count
Thermal Pad
θ
JC
(°C/W)
θ
JB
(°C/W)
θ
JA
(°C/W) Air Flow in m/s
0
1
2
3
4
5
TQFP/PP64
TQFP/DK64
TQFP/DK64
64
64
64
No
12.3
11.9
11.9
35.1
24.9
3.2
43.1
32.6
18.8
40
29.7
15.7
38.1
27.8
14.3
37.3
26.9
13.4
36.5
26
12.8
36.1
25.5
12.5
Yes/Exposed
Yes/Soldered