Results 1 to 6 of 6

Thread: Sproggy design questions

  1. #1
    Maximum Bitrate
    Join Date
    Oct 2001
    Location
    Ontario, Canada
    Posts
    845

    Sproggy design questions

    With the sproggy,

    (a) How important is the 2200uF 35V cap on output? There's a cap that goes between the 12V in and ground, if that is attached (or at least if this particular cap is attached) then it will short circuit when it powers on. Actually let me be more specific. Using an old AT power supply that can give 4A on the +12V rail as a power source, if this cap is in place then it will cause the AT PSU to turn off. How much current will that cap draw while it is charging?

    In any case, I removed it and it seems to work fine How necessary is that? IIRC it's for noise supression (cap/ind/cap) right?

    (b) How much leeway do you have on the voltages and capacitances of the caps?

    (c) How much leeway do you have on the voltages and inductances of the inductors?
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  2. #2
    Constant Bitrate
    Join Date
    Jul 2002
    Location
    Seattle WA
    Posts
    166

    Unhappy

    It beats me, but if that's what I think it is, I am a little depressed now. So simple, and now gone.

    Ryan
    New site at
    http://customcarpc.dreamhost.com

    http://home.attbi.com/~rbarr110

    2002 Toyota Tacoma Doublecab

  3. #3
    Maximum Bitrate
    Join Date
    Oct 2001
    Location
    Ontario, Canada
    Posts
    845
    Don't feel so bad, I actually had to replace the transformer and the LM2587T-12 as well, I don't know if they actually were bad but they weren't working either.

    The 3.3 and 5.0V rails were fine though, but I think you did something nasty to the +12V rail with ... whatever you did to it

    Good thing I had 4 of those LM2587T-12 and I just happened to get my delivery from renco in today, as well as your sproggy. I got 4 deliveries today (maxim delivered 1/2 of the samples I asked for).

    It took me about 5 hours in total, and I also removed the wires that you had on there and put on much longer ones, as well as some real thick power input wires Oh you also had the +5VSB wire connected directly to the +5V rail, which might have caused additional problems.

    I tell you, superglue, the de-soldering pump and soldering flux has got to be the 3 most important contributions towards DIY electronics work.

    Oh yeah, both MBR 745s were broken off when I got it, eh? One was completely detached and the other had one broken leg.

    Now I've gotta build a daughtercard to give me a second 5.0V rail (or maybe 3?) and possibly another 12V rail since I have the parts to do that now. I also have to build some sort of shutdown controller onto it, but the jury is out on how to do that. Oh yeah, I have to replace the 78L05 with a LM340T because it needs more than 100mA on the 5VSB rail.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  4. #4
    Constant Bitrate
    Join Date
    Jul 2002
    Location
    Seattle WA
    Posts
    166
    OK

    So now I don't feel so bad. I am pretty sure those parts were not broken before I packaged it, but I don't see how they would have broken in the packaging. Oh well, glad you got it.

    Later
    Ryan
    New site at
    http://customcarpc.dreamhost.com

    http://home.attbi.com/~rbarr110

    2002 Toyota Tacoma Doublecab

  5. #5
    Maximum Bitrate
    Join Date
    Oct 2001
    Location
    Ontario, Canada
    Posts
    845
    Bump bump... I got even more questions now:

    - Relay: why are we using an SPDT instead of an SPST? The SPST's are cheaper and smaller for the same current load.
    - are the 10uH inductors needed for the output of the MAX787/788? According to the spec sheet they are not (but the spec sheet also says to use a 50uH main inductor)
    - are the second 470uF caps (after the 10uH inductor on the voltage output for 5.0 and 3.3) needed? Spec sheet does not seem to think so either.
    - what are the MBR745's used for and why do they need to be heatsinked? Since this is a diode to ground, one would not think that there would be a lot of current flowing through there...
    - do the caps need to be 35V or will 25V do? Why are some 25V and others 35V? From what I understand, the voltage is unimportant provided that it is below the max that you will charge it with?
    - why do people keep saying that you need low-ESR caps, when regular ones will (as people have shown) work?
    - Can the T6 or equiv transformer be ordered from digi-key?

    - If I understand this correctly, the CMOS4049 is overglorified for the purpose that it serves.

    Outputs are as follows:

    Pin10 = !pin9
    pin6 = !pin7

    The usage has power input at pin 1 and 10, and powergood signal comes out from pin 6. Pin 7 is connected directly to pin 10.

    So basically pin6=!pin7=!pin10=!!pin9=pin9.

    So why is it there?

    I thought for a delay, however it's only 8ns propogation time, and it would be a lot simpler to just use a cap and a resistor (as other people have done, and it works).
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  6. #6
    Raw Wave Rob Withey's Avatar
    Join Date
    Apr 2000
    Location
    Bedfordshire, UK
    Posts
    2,139
    Some answers:

    - Relay: why are we using an SPDT instead of an SPST? The SPST's are cheaper and smaller for the same current load.
    An SPST will do just fine.

    - are the 10uH inductors needed for the output of the MAX787/788? According to the spec sheet they are not (but the spec sheet also says to use a 50uH main inductor)
    Two reasons. They add smoothing, and they prevent one of the regulators doing all the work on the 5v line since otherwise you would have two sense connections wired directly together, and due to manufacturing tolerances, one of the regulators would be set slightly higher than the other.

    - are the second 470uF caps (after the 10uH inductor on the voltage output for 5.0 and 3.3) needed? Spec sheet does not seem to think so either.
    Same reason, smoother outputs.

    - what are the MBR745's used for and why do they need to be heatsinked? Since this is a diode to ground, one would not think that there would be a lot of current flowing through there...
    They are very necessary. Don't forget this is a switching regulator, not a linear one. The concepts involved here are reactive, not passive. The output of the MAX787 and MAX788 is switched at a high frequency. The inductor acts as a flywheel and the diode lets current flow from ground as needed. I recommend finding a book about switched mode power supplies and reading it.

    - do the caps need to be 35V or will 25V do? Why are some 25V and others 35V? From what I understand, the voltage is unimportant provided that it is below the max that you will charge it with?
    25v will probably do. I don't see any capacitors that are near the switched outputs on the MAX chips - they are always on the regulated/smoothed side of the inductors. However, I'm not totally certain of the high frequency components in the switched signal here - get a second opinion.

    - why do people keep saying that you need low-ESR caps, when regular ones will (as people have shown) work?
    Low ESR capacitors are better at dealing with high frequencies. With high ESR capacitors, you'll end up with a hotter, less efficicient power supply. Again, some reading about SMPSUs is recommended.

    - Can the T6 or equiv transformer be ordered from digi-key?
    Obvious answer - certainly, if they sell it. I'm not familiar with their product line.

    - If I understand this correctly, the CMOS4049 is overglorified for the purpose that it serves.
    The 4049 acts as a buffer, and prevents the switched load affecting the time set by the RC circuit. It also acts as a switch to give a nice clean voltage transition on the power good line (otherwise you'd be feeding an exponential voltage which can be argued as a bad thing in logic circuits).



    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •