[eagle] Re: URx

Howard Long eagle at howardlong.com
Tue Feb 20 03:17:15 PST 2007

Hi Dick, John,
> Device       Dissipation     Delivered 
> U16           0.340             0.850
> U17           0.411
> U18           0.384             1.200
> U19           0.108             0.090
> U20           0.040             0.224
>                 ------------        ------------
> Totals       1.283W          2.364W
> For a grand total power dissipation of the power system of 3.647W.  

I am particularly interested in this as the DSP device on the SDX
(TMS320C6726) typically dissipates 857mW at its full 250MHz speed. It is a
144 pin LQFP PowerPad device and has a metal plate directly underneath the
die that is designed to be soldered directly to the PCB ground plane.

In order to conduct heat away, TI recommend the optimum to be 100 0.33mm
vias in a 10x10 matrix.

See http://www.g6lvb.com/dspthermal.jpg for the design I developed for the
prototype I use for my software development.

We will almost certainly need to conduct heat away from the device on the
underside, so that would suggest to me a direct metallic interface between
the board and the module housing, either milled out of the chassis itself or
with an interfacing block. IT should be noted that for electrical reasons
there are also 44 decupling capacitors on the bottom of the PCB that need to
be considered when providing the thermal interface.

We also would need some space rated goo to ensure good thermal conductivity
between the PCB and the heatsink - I am sure someone will know what this is!

One final thing - 3.647W seems very high for the receiver function. Is this
based on real figures or device spec sheets, and if the latter are these
worst case or typical values?

73, Howard G6LVB

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