[hpsdr] LTC 2208 A/D Thermal Characteristics

Thu Jan 15 13:31:37 PST 2009

After another hour running...

IC: 42.9 C
PCB: 63.8 C

Eduardo/EA2BAJ

At 17:39 14/01/2009, Scott Cowling wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>Thanks for the data point, Eduardo. Can you 
>please also measure it on the pad on the rear of the board?
>
>73,
>Scotty WA2DFI
>
>At 17:27 2009-01-14 +0100, Eduardo Jacob wrote:
>>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:
>>>***** High Performance Software Defined Radio Discussion List *****
>>>
>>>
>>>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:
>>>>***** High Performance Software Defined Radio Discussion List *****
>>>>
>>>>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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