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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
