TPS62080: Vout measures 3V though feedback is setup for 1.5V

Vivek Sikri

Part Number: TPS62080

We are using the TPS62080 to go from 5V input to 1.5V output for a DDR supply. Feedback resistors are 80.6K between Vout and FB and 34K from FB to GND.

When we power on ~some~ boards we see the output voltage is 3V.

We have a ferrite bead on the output of the part (after the inductor and output cap) to reduce EMI. When I short this bead the power supply regulates fine.

I checked and reduced the total capacitance on the output rail to less than 100uF. Was originally about 120uF, and is now about 40uF. This had no effect.

I used some power resistors to increase the static load on the supply (added 5 ohm resistor across 1.5V so 300mA draw) and that changed the 3V to 2.5V but didn't actually fix the problem.

There is the following case that looks similar to what I'm seeing, but there is no resolution for it:

http://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/397441?tisearch=e2e-sitesearch&keymatch=tps62080%20output%20inductance

Any help would be greatly appreciated.

over 7 years ago

0 Chris Glaser over 7 years ago

TI__Guru*** 145360 points

Can you post a waveform of Vin, Vout, SW, and FB when Vout is too high at 3V?

What is the part number of the ferrite bead?

0 Vivek Sikri over 7 years ago in reply to Chris Glaser

Intellectual 300 points

Hi Chris,

Thanks for your quick response. Please find the requested information in the attached debug record.

- Vivek20170516_REX200C_Debug_V1p5.pdf

0 Chris Glaser over 7 years ago in reply to Vivek Sikri

TI__Guru*** 145360 points

Thanks for these details. Yes, please replace that output cap with a ceramic. That tantalum has 10 Ohm ESR! This makes quite a lot of ripple voltage.

0 Vivek Sikri over 7 years ago in reply to Chris Glaser

Intellectual 300 points

Hi Chris,

Sure I'll swap out the output cap for a ceramic. I can see how that would increase ripple but do you think that is the root cause for the output voltage being too high? It seems that has more to do with the ferrite bead.

I'll swap the output cap and report back, meanwhile if you have any other suggestions please share and I'll try those as well.

Thanks,
Vivek

0 Vivek Sikri over 7 years ago in reply to Vivek Sikri

Intellectual 300 points

Hi Chris,
Changing to a ceramic did the trick in all regards. I guess the ripple was so high that it made the whole thing go bezerk!
Many thanks for your help.
- Vivek

0 Chris Glaser over 7 years ago in reply to Vivek Sikri

TI__Guru*** 145360 points

Thanks for sharing your solution.

Yes, and shorting the ferrite bead reduced the output impedance and ripple by adding the ceramics downstream to the output.

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TPS62080: Vout measures 3V though feedback is setup for 1.5V