LM27762: LM27762, Ringing and output will work only if input is momentarily shorted

anand2623

Prodigy 180 points

Part Number: LM27762

Hi, I am trying to use the LM27762 as a +5 and -5 LDO.

PFA my Schematic and layout.

The device keeps ringing when connected to a known input voltage of 5.2V.

The +5 out goes to 3.8 and the -5 goes to -1.8 V. When the input is momentarily shorted, the output goes to +5 and -5 V

I am using a 2 layer board and the bottom layer is a ground plane with two small traces. There is a ground plane (RED) on the top layer also.

Please do have a look and advice.

over 6 years ago

0 Chris Glaser over 6 years ago

TI__Guru**** 150770 points

Hi Anand,

Can you post waveforms of Vin, Vout+, Vout-, and the EN line when it is not working and when it is?

If you have any load connected, please remove it and see if it works.

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

PFB the waveforms, all of them are at 2v, - 1ms.

Waveforms for Vin EN+ EN- are similar

Not_working

VIN EN+ EN-

Vout+

Vout-

Working

Vin EN+ EN-

Vout+

Vout-

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru**** 150770 points

Thanks for sharing. Is it possible to get all 3 waveforms on one scope shot?

Is the IC hot when it is not working? What is the input current draw for each case? Is there a load connected?

In your layout, do you have GND vias under the IC?

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

Sorry dont have multiple channels.
No, IC is not hot when not working. Its the same case when load is connected/not connected.
Only difference in waveforms between load and no load is, Pgood signal goes low only for a moment when there is no load.
Yes i have GND vias under the IC.

input current:
Load:
Not working 37mA, working 28mA
No Load:
Not working 8mA, working 2mA

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru**** 150770 points

Thanks for confirming. Does this happen on 1 board or all boards?

Are all the caps ceramic with good dielectrics (X5R, for example)?

Your FB resistors are very large. You could try making each of them 1/10th of the value to see if this is noise related.

You can always order the EVM to obtain a known working circuit.

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

I have tested three boards till now, this is the case on them all.

All the caps are ceramic with X5R except CFLY which is film capacitor.

The FB resistors used were the ones generated with webench.

I have tried using (80.6k, 261k) and (83.5k,261k) for +5 and -5 respectively.

This did not change anything for me.

Do you think adding an RC between Vin and EN+ and EN- would do anything?

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru**** 150770 points

Thanks for confirming all items.

Yes, you can try taking EN high after Vin is present and see if that helps. Then, you could add an RC delay to mimic that behavior.

As well, can you replace Cfly with a normal ceramic cap?

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

Hi Chris,

Replacing Cfly did help, i am getting +5 and -5V out now.

But the PGOOD signal is not getting pulled down, and remains at VIN levels.

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru**** 150770 points

Great!

Can you measure each voltage at the IC: Vin, Vout, -Vout? If you have a load, the -5V may not be within 95% of your target with lower Vins due to the output impedance.

0 anand2623 over 6 years ago in reply to Chris Glaser

Prodigy 180 points

Vin is 5.203V, Vout+ is 5.02 and Vout- is -4.947V which should be more than 95%.

These values are with load.

And is there a minimum capacitance needed for the fly capacitor?

0 Chris Glaser over 6 years ago in reply to anand2623

TI__Guru**** 150770 points

Hi Anand,

Does PG work normally when either increasing Vin to ~5.4V or removing the load?

Yes, the D/S discussed the Cfly value. You might try increasing this to a 2.2uF to see it makes any impact.

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LM27762: LM27762, Ringing and output will work only if input is momentarily shorted