[hpsdr] LTC 2208 A/D Thermal Characteristics

[Scott Cowling]

Hi Mercurians,

The thermal resistance of the LTC 64-pin UP 
package (the one that the LTC2208CUP comes in) is 
specified at 0.24°C/W from the junction to the case.

The maximum allowed operating junction temperature is 125°C.

The A/D dissipates roughly 1.25W at 3.3V and 
1.45W at 3.6V. We are running at 3.3V, but lets 
assume a worst-case power dissipation of 1.5W, 
which is higher that it will ever be.

1.5W * 0.24°C/W = 0.36°C temperature rise from the case to the junction.

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.

Since the maximum junction temperature is 125°C, 
the case can get as hot as 124°C WITHOUT 
EXCEEDING THE MAXIMUM ALLOWED JUNCTION TEMPERATURE.

Since the case temperatures mentioned here are in 
the 50°C to 70°C range, the chip die is operating 
at well below the maximum allowable temperature.

While I will never argue against "cooler is 
better", I will argue against making a problem where there is none.

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? 
Maybe. Designs are about tradeoffs. The designers 
traded off board area, noise pickup and thermal 
resistance and came up with a design that works.

It is cool *enough*
It is quiet *enough*
It fits on the board
It works

That doesn't mean that it is necessarily the 
optimum choice. By all means, modify it, change 
it, make it better; that is what HPSDR is all 
about. There are many solutions, some of which 
are certainly better that the one that we chose. 
But please don't call it "broken" just because it feels hot to your finger. :-)

Here are some reference points:
At 50°C, it takes 2 minutes to give you a third degree burn.
At 60°C, it takes 5 seconds to inflict a third-degree burn.
At 70°C, it takes only 1 second to give you a third degree burn.
source: http://www.cqc.state.ny.us/newsletter/estime.htm

It's hot! Be careful!

73,
Scotty WA2DFI





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