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TPS63700: TPS63700 - problem with voltages lower than ~-9.0 V

user5332766
Prodigy 50 points
Part Number: TPS63700
Other Parts Discussed in Thread: TPS65131

I'm trying to use TPS63700 IC to produce ~-14 - 15 V output voltage. Depending on feedback devider I can produce output voltages down to ~-9V (under light or no load conditions). Any further decrease results in large current supplied from power source and output voltage close to -2V. A FB pin is no longer  at ground level.

This is stable condition with PWM on state ~250 ns. Schematic used is shown below:

I've used 4.7 uH and 10 uH coils, and different feedback drivers - with no result. Also I've used feed-forward capacitor with RC conficguration as described in datasheet and in forums but with no result.

All other measurable signals are within normal parameters. I have had similar problem with TPS65131 negative voltage output with the same symptoms.

Sins this looks like problem with converter stability I'm attaching footprint below for reference:

All test I've decribed were made at light lodad or no load. I could use some advice what can cause this effects. This circuit should work with light loads with current consumption not larger than ~130 mA.

  • 0
    Prodigy 50 points

    Schematic and footprint for problem above:

  • 0 in reply to user5332766
    TI__Guru 58225 points

    Hello user5332766,

    1. I think the biggest problem is the connection of the power input pin. Most probable the input voltage directly at the input pin of the IC is dropping due to the bad connection. Please review the EVM layout and if possible order an EVM to check that the part is able to deliver the energy.

    2. Please increase the resistor from FB to REF to at least 120k. With 100k you are drawing too much current from the reference.

    3. Please check if you output capacitor has 12uF at -15V. The DC bias effect reduces the available capacitance in ceramic caps heavily.

    4. Remove thermals. These are nowadays normally not necessary as reflow ovens are used for soldering and even if you are using a soldering iron, it is better to have no thermals and increase the solder iron power than having thermals in a power supply.

  • 0 in reply to Brigitte
    Prodigy 50 points

    Hello Brigitte

    Thank you for your answer. I'll try to apply fixes and check the result.

    Best regards,
    Krystian

  • 0 in reply to Brigitte
    Prodigy 50 points

    Hello Brigitte

    I'v emanaged to rule out sugestions 1-3 and I don really think thermals can be repsponsible for such effect.

    Whatever I do I can't force IC to have output voltage grater tha ~- 9.0 -  -10 V. If any other, greater divider is set only visible effect is that Ufb is no longer at 0 voltage. Can you explain what can be the couse of such behaviour.

    Best regards,

    Krystian

  • 0 in reply to user5332766
    Prodigy 50 points

    OK

    I've solved the problem. It turned out that the diode monted on the board had too small breakdown voltage - only 10 V.  Problem solved, subject can be closed.

    Thanks for help.

    Best regards

    Krystian

  • 0 in reply to user5332766
    TI__Expert 7925 points

    Hi Krystian,

    Thanks for the feedback. We will close this thread, if you have any questions feel free to come back here or open a new thread if this gets locked.

    Best regards,
    Milos