LMR10515: Unstable output

Hi Steve,

I will look into your request.

Firstly, I would recommend to use one of our latest synchronous buck converters. LMR10515 is a very old device. Is there any specific reason to use this device? What is your application? 

Regards,

Febin

This has been used on other boards and has migrated to this board which has been in limited production for awhile.  I can re-design with one of your latest converters but i also need to support the boards that are in use.  What would you recommend for re-design?

I have attached the schematic of this converter (U4).  There are more sheets to the schematic that have components connected to the 3.3V supply.

Hi Steve,

I would like to suggest some of our newer smaller highly efficient buck converters (Assuming requirements as Vin-5V Vout-3.3V, Iout -1A): TPS62840, TPS6282x or TPS6280x. Please have a look at the datasheet and let me know if you need further help.

https://www.ti.com/lit/ds/symlink/tps62840.pdf

https://www.ti.com/lit/ds/symlink/tps62821.pdf

https://www.ti.com/lit/ds/symlink/tps62808.pdf

For the existing boards, Could you explain in detail what you observe?

What is the operating temperature? 

Have you swapped with a new IC and tested and does it behave the same way? 

Regards,

Febin

I have attached a scope shot of the 3.3V output when it is failing.  Once it fails on power up, it keeps cycling until power is removed.  It seems odd that the cycle rate is the same as the soft start rate.  I also have scope shots of the VIN if you would like them.  VIN looks the same to me when 3.3V is good or bad.  The enable pin input looks good also.

C31 on VIN is 220uF aluminum electrolytic, 10V 350mOhm.  C32 on VIN is 4.7uF, MLCC, 6.3V, X7R.  D12 is 30V, 1A, voltage drop of 0.5V max.  L3 is 2.7uH, 2.43A, 52mOhm (saturation at 3.5A).  C33 on the output is 10uF, 16V tantalum, 3 Ohm.

I'm not sure how we ended up with a tantalum cap on the output.  Could that be the issue.  Also not sure about the value.

While testing, this board is running at room temperature which is around 75 degrees F.  I have not tried a new IC yet.  We have a few other boards that have failed and swapped the IC but it did not fix the problem.  It will be next week before we can get to it but i will try a new IC on this board.

There are some items that might have a surge current.  If everything is on and drawing max current plus the surge current, it could be 1.4-1.5 amps.  I would think the soft start would help with that and it does appear to ramp up to 3.3V.  But then shuts off and soft start starts again?

Hi Steve,

I really appreciate for the detailed explanation.

From the datasheet, I understand that the aluminum electrolytic for input and tantalum for output are not good choices. Could you try to use Ceramics and 22uF each?

In a fully operational state, the load on LMR10515 is 1.4-1.5A? Could you please share the Vin, Iout, SW, IL waveforms for the above Vout occurrence?

Does the device re-start when this surge current occurs ? Can you ensure that the maximum Iout is not exceeded at all times?

Regards,

Febin

I removed the electrolytic cap.  I have tried input and output caps of 10uF ceramic (that is all I had) and so far it seems to have fixed the problem.  I will order 22uF caps and try them.

In worse case with power on inrush current, the load might reach 1.4 - 1.5 Amps.  The softstart function should reduce that some.  Normal full load current should be less than 1 Amp.  I don't have a way to measure current.

On the re-design I would like to stay with 1.5 to 2.0 Amp regulator.  Do you have any suggestions?

Thanks for you help on this issue!

Hi Steve,

Please note that the maximum load for LMR10515 should be always 1.5A.

For the re-design, I would like to propose TPS62825. Please have a look at the datasheet.

https://www.ti.com/lit/ds/symlink/tps62825.pdf.

Regards,

Febin