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
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