U1AFS250-FG256 Datasheet, PDF (8/27 Pages) Actel Corporation – 3 - DC and Power Characteristics

DC and Power Characteristics
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 U1AFS600-FG256
package under forced convection of 1.0 m/s and 75°C ambient temperature is as follows:
Maximum Power Allowed
=
-^T---^J-⁽--^M----^A---^X---⁾--^–-----^T---^A---⁽--^M----^A---^X--⁾
θ_JA
where
θ_JA = 25.20°C/W (taken from Table 3-6 on page 3-7).
T_A = 75.00°C
EQ 3-4
Maximum Power Allowed = ¹----¹---⁰----^.-⁰----⁰---^°--^C------^–----⁷----⁵---^.--⁰---⁰---^°---^C-- = 1.39 W
25.2°C/W
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 U1AFS600-FG256 package with 2.5 m/s airflow, the
power consumption value using the power calculator is 3.00 W. The user-dependent T_a and T_j are
given as follows:
T_J = 110.00°C
T_A = 70.00°C
From the datasheet:
θ_JA = 23.50°C/W
θ_JC = 6.80°C/W
P
=
T_----J---–_----T_----A-
θ_JA
=
1_----1_---0_----°_--C_-----–_-----7_---0_----°_--C_--
23.50°C/W
=
1.70 W
EQ 3-5
3-8
Preliminary v0.4