DC and Power Characteristics
3- 8
Prel i minary v1 .7
Theta-JA
Junction-to-ambient thermal resistance (
θ
JA) is determined under standard conditions specified by
JEDEC (JESD-51), but it has little relevance in actual performance of the product. It should be used
with caution but is useful for comparing the thermal performance of one package to another.
A sample calculation showing the maximum power dissipation allowed for the AFS600-FG484
package under forced convection of 1.0 m/s and 75°C ambient temperature is as follows:
EQ 3-4
where
The power consumption of a device can be calculated using the Actel power calculator. The device's
power consumption must be lower than the calculated maximum power dissipation by the
package. If the power consumption is higher than the device's maximum allowable power
dissipation, a heat sink can be attached on top of the case, or the airflow inside the system must be
increased.
Theta-JB
Junction-to-board thermal resistance (
θ
JB) measures the ability of the package to dissipate heat
from the surface of the chip to the PCB. As defined by the JEDEC (JESD-51) standard, the thermal
resistance from junction to board uses an isothermal ring cold plate zone concept. The ring cold
plate is simply a means to generate an isothermal boundary condition at the perimeter. The cold
plate is mounted on a JEDEC standard board with a minimum distance of 5.0 mm away from the
package edge.
Theta-JC
Junction-to-case thermal resistance (
θ
JC) measures the ability of a device to dissipate heat from the
surface of the chip to the top or bottom surface of the package. It is applicable for packages used
with external heat sinks. Constant temperature is applied to the surface in consideration and acts
as a boundary condition. This only applies to situations where all or nearly all of the heat is
dissipated through the surface in consideration.
Calculation for Heat Sink
For example, in a design implemented in an AFS600-FG484 package with 2.5 m/s airflow, the power
consumption value using the power calculator is 3.00 W. The user-dependent Ta and Tj are given as
follows:
From the datasheet:
EQ 3-5
θ
JA
TA
= 75.00°C
TJ
=
110.00°C
TA =
70.00°C
θ
JA
=
17.00°C/W
θ
JC
=
8.28°C/W
Maximum Power Allowed
T
J(MAX)
T
A(MAX)
–
θ
JA
------------------------------------------
=
Maximum Power Allowed
110.00°C
75.00°C
–
19.00°C/W
----------------------------------------------------
1.84 W
==
P
T
J
T
A
–
θ
JA
-----------------
110°C
70°C
–
17.00 W
-----------------------------------
2.35 W
==
=