[time-nuts] Trimble GPS board

Bob Camp kb8tq at n1k.org
Fri Aug 28 19:36:10 EDT 2015


> On Aug 28, 2015, at 4:46 PM, Angus <not.again at btinternet.com> wrote:
> On Sun, 23 Aug 2015 23:36:27 -0400, you wrote:
>> Hi Angus,
>> Thanks for your reply.  My original suspicion is the +6.5V rail.  That needs
>> to come from somewhere, and you need headroom if it's an unregulated input.
>> Pumping 2A through a regulator like that is no easy feat.  Although rated
>> for 3A, you need to keep the junction temperature below 125C. The part has a
>> 30C/W thermal resistance (Tja) with a 1" square copper pad.  So 6.5V and 3A
>> calculate as follows: (6.5V-5V)*3A=4.5W.   4.5W*30C/W=135C RISE, add that to
>> ambient, 25C, and you are 35C over max junction.  In my world we have a 70C
>> ambient, and that leaves me (125C-70C)/30C/W=1.83W, or a maximum of
>> 1.83W/1.5V=1.22A.  Generally a tab mounted TO-220 (a D2Pak) can have a Tjc
>> (junction to case) or less than 1C/W.  It's all the reest of the mounting
>> that piles on the thermal resistance.
>> The Trimble board has a top layer pad, some far side pad, and probably
>> multiple layer of ground plane over the rest of the board.  The mounting
>> post is mostly likely part of the thermal resistance calculation.  There is
>> also a time constant involved, it probably can't take the 2A forever, just
>> long enough to get the oven up to temp. I suspect they may have gotten the
>> thermal impedance down as low as 10C/W.  I will test it with 12V and let
>> everyone know if I fry the board.
>> Bob
> Hi Bob,
> There's not really any need for a 6V or so rail - a DC-DC converter
> (and probably some filtering) can provide the right voltage just where
> it's needed.
> What looks like a Symmetricom variant is in listing #271483752431.
> It does appear to have a DC-DC converter, etc., on the board.
> Anyway, I dug out my Trimble to have a look at it, and I see that
> there's a 10V tantalum capacitor on the input power, so it's certainly
> not meant to run on 12V.

If they derated that cap 20%, it is not meant to run on anything over 8V. It’s 
rare to see a voltage between 6.3 and 10 on those parts. Obviously 6.3 is to close
to run on a 6V line and get any decent MTBF. 


> Angus.
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