[time-nuts] Trimble GPS board

Angus not.again at btinternet.com
Fri Aug 28 16:46:17 EDT 2015


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.

Angus.


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