FG256 datasheet(32/108 Pages) ACTEL

32 / 108 page

SX-A Family FPGAs

2- 12

v5.3

Theta-JA

Junction-to-ambient thermal resistance (θJA) is determined under standard conditions specified by JESD-51 series but

has little relevance in actual performance of the product in real application. It should be employed with caution but is

useful for comparing the thermal performance of one package to another.

A sample calculation to estimate the absolute maximum power dissipation allowed (worst case) for a 329-pin PBGA

package at still air is as follows. i.e.:

EQ 2-11

The device's power consumption must be lower than the calculated maximum power dissipation by the package.

The power consumption of a device can be calculated using the Actel power calculator. If the power consumption is

higher than the device's maximum allowable power dissipation, then a heat sink can be attached on top of the case or

the airflow inside the system must be increased.

Theta-JC

Junction-to-case thermal resistance (θJC) 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 and only

applies to situations where all or nearly all of the heat is dissipated through the surface in consideration. If the power

consumption is higher than the calculated maximum power dissipation of the package, then a heat sink is required.

Calculation for Heat Sink

For example, in a design implemented in a FG484 package, the power consumption value using the power calculator is

3.00 W. The user-dependent data TJ and TA are given as follows:

From the datasheet:

EQ 2-12

The 2.22 W power is less than then required 3.00 W; therefore, the design requires a heat sink or the airflow where the

device is mounted should be increased. The design's junction-to-air thermal resistance requirement can be estimated

by:

EQ 2-13

= 17.1°C/W is taken from Table 2-12 on page 2-11

= 125°C is the maximum limit of ambient (from the datasheet)

Max. Allowed Power

Max Junction Temp

Max. Ambient Temp

–

------------------------------------------------------------------------------------------------------------

150

°C 125°C

–

17.1

°C/W

----------------------------------------

1.46 W

TJ = 110°C

TA = 70°C

JA = 18.0°C/W

JC = 3.2 °C/W

Max Junction Temp

Max. Ambient Temp

–

------------------------------------------------------------------------------------------------------------

110

°C70°C

–

18.0

°C/W

------------------------------------

2.22 W

Max Junction Temp

Max. Ambient Temp

–

------------------------------------------------------------------------------------------------------------

110

°C70°C

–

3.00 W

------------------------------------

13.33

°C/W

FG256 Datasheet(PDF) 32 Page - Actel Corporation

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