[hpsdr] LTC 2208 A/D Thermal Characteristics
Eduardo Jacob
edu at kender.es
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
>>>>
>>>>
>>>>
>>>>
>>>>_______________________________________________
>>>>HPSDR Discussion List
>>>>To post msg: hpsdr at hpsdr.org
>>>>Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
>>>>HPSDR web page: http://hpsdr.org
>>>>Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
>>>
>>>--
>>>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
>>>
>>>
>>>
>>>
>>>_______________________________________________
>>>HPSDR Discussion List
>>>To post msg: hpsdr at hpsdr.org
>>>Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
>>>HPSDR web page: http://hpsdr.org
>>>Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
>>>
>>>No virus found in this incoming message.
>>>Checked by AVG - http://www.avg.com
>>>Version: 8.0.176 / Virus Database:
>>>270.10.6/1888 - Release Date: 12/01/2009 7:04
>
>
>_______________________________________________
>HPSDR Discussion List
>To post msg: hpsdr at hpsdr.org
>Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
>HPSDR web page: http://hpsdr.org
>Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
>
>
>No virus found in this incoming message.
>Checked by AVG - http://www.avg.com
>Version: 8.0.176 / Virus Database: 270.10.7/1893
>- Release Date: 14/01/2009 6:59
1232055097.0
More information about the Hpsdr
mailing list