[hpsdr] LTC 2208 A/D Thermal Characteristics

[Scott Cowling]

I have reread my post several times, and I do not 
see where I ever said "at the top of the case".

The 0.24°C/W is the number off of the LTC package information.

If you feel that it is an unrealistic number, 
feel free to substitute any other number that you like.

If you think it is the number to the underside, 
feel free to measure it there. Add a few °C/W for 
the chip-to-board connection so you don't have to 
unsolder your chip to measure it.

If you think that 20°C/W (specified thermal 
resistance between junction and ambient) is a 
realistic number just because it appears on the data sheet, use that one.

Every one of these choices will result in a 
conclusion that the part is operating within 
spec. There will not be a problem until you reach 
nearly 100°C (measured at your favorite spot) 
using the worst-case numbers above.

Phil, if you choose to oversimplify grounding and 
thermal tradeoffs and call it "nonsense", feel 
free to do so (I oversimplified it too). My 
experience, on the other hand, tells me otherwise.

None of this changes the fact that there is 
plenty of thermal headroom on Mercury.

My intent was not to discourage experimentation 
(I specifically stated the opposite), but to 
dispel the myth that there is somehow a design or 
manufacturing defect in Mercury boards with ADC 
temperatures at 50°C, 60°C or even 70°C.

You are going to have to come up with 
temperatures a lot higher than these before you have a *real* problem. :-)

73,
Scotty WA2DFI




At 10:27 AM 1/14/2009 -0500, Philip Covington wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>On Wed, Jan 14, 2009 at 12:31 AM, Scott Cowling <scotty at tonks.com> wrote:
>
> > That means the junction temperature, T(j), will be only about 1/3 of a
> > degree higher than whatever temperature you measure on the case.
>
>I agree with Rob that this analysis is flawed.  There is no way that
>the junction temperature is only 0.33 C higher than the temperature
>measure at the top of the LTC2208 package.  If people are measuring 70
>C at the top of the chip then the junction temperature is considerably
>higher.
>
> > Could we make it cooler? Sure we could. Do you want to add noise by hooking
> > up a BIG ground plane to the pad on the underside of the chip?
>
>This is exactly what Linear Tech suggests because this is the only way
>to get the heat out through the thermal pad on the LTC2208.   The
>notion that somehow this would increase noise is nonsense.
>
>Phil N8VB
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