[hpsdr] LTC 2208 A/D Thermal Characteristics

Wed Jan 14 08:53:32 PST 2009

Here is another temp measurement. My mercury measured 54 deg C on the  
top and about 46 deg C on the bottom heat sink.

Ron W9KFB
On Jan 14, 2009, at 11:27 AM, Eduardo Jacob wrote:

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>
> Hi
>
> I don't know if this can add some data to the discussion. I measured  
> temperatures in my mercury with a (old but unused)  Tektronix  
> temperature probe with an old Tektronix 2465DMS scope and with an  
> ambient temperature of 18.5 C it gives 44.7 C (after an hour  
> running) . The probe is in contact with the center of the IC. The  
> ambient temperature measured with the probe is quite close to  
> another mercury (btw) thermometer I have (which is also uncalibrated)
>
> Eduardo/EA2BAJ
>
>
> At 12:20 14/01/2009, Rob Frohne wrote:
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>>
>>
>> Hi Scott,
>>
>> That extremely small thermal resistance cannot be between the  
>> junction and the place where Charles and others are measuring the  
>> temperature.
>> It must be between the junction and the part of the case under the  
>> chip where it is supposed to be soldered to a good heat sink.  I am  
>> not saying there is a problem, only that your analysis is not  
>> conclusive, given the measurements that we can fairly easily  
>> perform.  Here are the words of wisdom from the LTC2208 data sheet  
>> regarding heat considerations:
>>
>> Most of the heat generated by the LTC2208 is transferred
>> from the die through the bottom-side exposed pad. For
>> good electrical and thermal performance, the exposed
>> pad must be soldered to a large grounded pad on the PC
>> board. It is critical that the exposed pad and all ground
>> pins are connected to a ground plane of sufficient area
>> with as many vias as possible.
>>
>> The interesting thing is that their example board didn't contain  
>> any vias to the ground plane that I could see anyway, so it seems  
>> that they didn't follow their own advice, or maybe I'm blind.  :-)
>>
>> In any case, increasing the heat sinking will not cause the device  
>> to operate worse, only perhaps a little better, so I hesitate to  
>> discourage folk like Francis from doing as they have with more heat  
>> sinking at least at this point.
>>
>> 73,
>>
>> Rob, KL7NA
>>
>> Scott Cowling wrote:
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>>>
>>> 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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>>
>> --
>> Rob Frohne, Ph.D., P.E.
>> E.F. Cross School of Engineering
>> Walla Walla University
>> 100 SW 4th Street
>> College Place, WA 99324
>> (509) 527-2075                  http://people.walllawalla.edu/~rob.frohne
>>
>>
>>
>>
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