[hpsdr] LTC 2208 A/D Thermal Characteristics

Wed Jan 14 08:39:50 PST 2009

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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