DC/DC Supply Failure

Discussion in 'The Projects Forum' started by aws505, Oct 1, 2013.

  1. aws505

    Thread Starter Member

    Mar 11, 2013
    59
    7
    Hey All,

    I've got a board with a DC/DC converter on it (an LT1931). It's being arranged such that the output is -9.2VDC from a 5V source. My circuit is almost exactly the arrangement suggested on the bottom of Page 9 on the datasheet, except that I've altered R1 to 54k to achieve -9.2VDC out. When I draw any amount of current from the circuit, though, my output voltage seems to drop substantially. For example, I connected a 1kΩ resistor in parallel with the output and measured only -3VDC across the resistor. As another example, when I use the power supply to power an voltage follower op-amp with low output current, the output from the supply drops to -5VDC!

    This is all surface-mount on a board, so it's hard to make changes, but here are the things I've looked at so far:
    - Obvious: The +5V supply is present. Yes!
    - The diode is not backward nor blown out. Seems good.
    - The resistor values are accurate. Seems good.
    - The cap values are right. Seems good.
    - Haven't quite found a meter to check the inductance of L1 and L2, but I've checked that they are, in fact, inductos (not caps or resistors, or at least they have low resistance) and their DCR is similar (suggesting they're similar inductance values)
    - I don't seem to have any other shorts on the board, that I have found.

    Can anyone think of some more tests to run on this board? I'm quite confident in the design (especially because it came right off the data sheet), but have fried my brain testing this board. Please keep in mind that this is a surface mount board and so some tests are difficult to accomplish (depopulation is hard with a large iron!). Any and all help is always appreciated.

    Thanks!
    - Aws
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,018
    3,235
    Do you have part numbers for L1 and L2?
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507

    Did you design the PCB layout? For very HF switchers, layout problems are almost guaranteed.

    Other typical gotchas:

    using wrong type of caps. ONLY high Q ceramics can be used such as X5R or X7R types. tantalums and electrolytics won't work.

    Coil saturating. The coil has to be good for frequencies of 1 - 2 MHz with this part.
     
  4. aws505

    Thread Starter Member

    Mar 11, 2013
    59
    7
    Thanks for the replies crutschow and bountyhunter! I didn't actually design the boards myself: This is a project for a company that I've just joined. Part of my role is to help in debugging boards and this is a helluva first problem. Apparently, this same board layout has been used before with a different R1 yielding (approximately) -7VDC without issue. I believe in that instance, the same inductors and caps were used, along with the same board layout. I'll double-check this, though.

    Once I get into the office, I'll double-check the part numbers and verify that the inductors aren't saturating and that the caps are high Q ceramics. I am sure, from a cursory glance, that the caps are not electrolytics. I'll post part numbers in an hour or so.

    Thanks for your thoughts!
    - Aws
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    This is load collapse. It means there is not enough energy getting through to feed the load. The inductor is what stores the energy, so that is the first suspect but too little capacitance will give low voltage readings too.
     
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