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TPS82150: Reducing DC_DC output voltage stops negative rail from being generated

Giles Hutchison
Prodigy 230 points
Part Number: TPS82150

I have successfully used a TPS82150's to generate +5V (approx 100mA) from +15V, and then a second TPS82150 to generate -5V from the +5V.

I then realised I needed to reduce the voltage of the both the +5V and -5V to +3.5V and -3.5V, so simply changed the potential divider into the FB pin.

From 10K (R4/R10) Low side / 55K3 (R1/R7)High side to 10K (R4/R10)low side / 33K2 (R1/R7) High Side.

I have attached the circuit.

The +5V reduces to +3.5V, as expected however the -ve rail is no longer being generated and is sitting at approx 300mV instead of -3V5.

I have tried two instances of the circuit, and bothe perform the same.

I suspect I am missing something simple...any suggestions welcome.

Giles

 

  • 0
    Prodigy 230 points

    Please note GND & GND_BSE are the same net/ ground-plane.

  • 0
    TI__Guru*** 142680 points
    Hi Giles,

    I'm glad this circuit from earlier this year worked out for you.

    I would go back to the +/-5V case and get a waveform of the -5V at startup. It sometimes happens, when powering an op amp (for example) with +/-rails, that the positive rail leaks onto the negative one and pulls it positive. It would be interesting to see if this is the behavior you are seeing and if this was present before or not.

    On your schematic, it looks ok but please remove R5 and R6 on the negative converter. We don't want to add anything between the input and output caps and the IC, as this adds impedance where there should be none (ideally).
  • 0 in reply to Chris Glaser
    TI__Guru*** 142680 points
    Hi Giles,

    Did you have a chance to look at this circuit some more?
  • 0 in reply to Chris Glaser
    Prodigy 230 points
    Hi Chris,
    Apologies for the delayed response.

    The following changes resulted in the negative DC-DC starting reliably.

    1. Replacing the bulk capacitance on the +ve input reference point from -Vout (suggested by Page 17 of the data sheet) to GND (suggested, and in conflict with the data sheet, by Page 8 of the App note ) ;

    2. Enabling Soft start on the -ve rail but not on the +ve rail

    3. The +ve power rail (OPAMP) does indeed pull up the negative power rail before it starts because of leakage through the OPAMPS, making the -ve rail harder to start, so:

    3a. Adding a Schottky diode across the output of the negative rail DC-DC, and,

    4. Ensuring the OPAMP rails were > 3.5V and < 4.0V ( they are now set to 3.7V and were previously at 3.4V).

    I believe the main culprit was #1, but have not had a chance to back out other changes to confirm this yet.

    Thank you for providing input, and again apologies for taking some time to get back to you

    Regards
    Giles
  • 0 in reply to Giles Hutchison
    TI__Guru*** 142680 points
    Thanks for following up with your solution.

    Thanks for pointing out the error in Figure 30.