TPS56837: Loop stability high current

Georges Maccario

Prodigy 40 points

Part Number: TPS56837
Other Parts Discussed in Thread: TPS54821

Tool/software:

Hello,

I would like to use a TPS56837RPAR for our new design.

V in is 15V, V out between 3.5V to 12.5V, adjustable using a DC voltage between 0 (12.5V) and 3V (3.3V)

Required maximum current out 5/6A

I've made de board, trying to follow the PCB layout given by the DEV. KIT (4 layers).

Unfortunately, I am having problems with currents over 2.5A (3.3R load), the efficiency drops dramatically from about 80% down to 60% !

Lots of oscillations (30Khz), at 6.5V the buck sound (literally... inductor noise) unstable ?

I don't really understand the relation-ship in the control loop, with the output capacitors (I need a very low noise), load impedance and feedback resistors (+ small cap) ?

I've attached the my circuit; for your review, if possible please.

I use a 2.2uH, I dc 10A, I sat 15A. Fo 800Khz

thank you for your support

Best regards

over 1 year ago

0 Lucia Gao over 1 year ago

TI__Mastermind 30451 points

Georges Maccario

Thanks for reaching out! My colleague Bella will get back to you during these days.

BRs

Lucia

0 Gui He over 1 year ago

TI__Mastermind 27306 points

Hi Georges

Can you please share the layout images? And can you please share the waveform of the SW, Vo, Vin. You can measure the waveform under light load(normal operation) and higher load(issue operation).

Thanks and best regards.

0 Georges Maccario over 1 year ago in reply to Gui He

Prodigy 40 points

Hello,

Got a bit more stabilized by changing the // cap in the feedback resistor divider : 27pF to 100pF

The inductor still a bit noisy low load (?)

0 Lucia Gao over 1 year ago in reply to Georges Maccario

TI__Mastermind 30451 points

Georges Maccario

Thanks for your feedback. Bella will help check soon and get back to you during these days.

BRs

Lucia

0 Gui He over 1 year ago in reply to Georges Maccario

TI__Mastermind 27306 points

Hi Georges Maccario

The image is kind of blur, Can you please re-attach the image? but the vout and sw waveform seems normal. The iL does have oscillation and that shouldn't happen.

Can you please try removing the Fb2 (short circuit) and see if any improvements?

Thanks and best regards.

0 Georges Maccario over 1 year ago

Prodigy 40 points

Thank you for your reply.

@ Gui He: not sure what is Fb2 ? did you have a look at the schematic ?

@ Lucia Gao : I did not get any comments from Bella ?

Thank you

Regards

Georges

0 Georges Maccario over 1 year ago in reply to Lucia Gao

Prodigy 40 points

Hello,

I did not get any comments from Bella ?

Thank you

Regards

0 Georges Maccario over 1 year ago in reply to Gui He

Prodigy 40 points

Hello,

Not sure what is Fb2 ? did you have a look at the schematic ?

Thank you

Regards

0 Gui He over 1 year ago in reply to Georges Maccario

TI__Mastermind 27306 points

Hi Georges

Sorry for the confusion. I was meaning that can you please try removing the bead by short-circuit it. And see if any improvements?

Thanks and best regards.

0 Georges Maccario over 1 year ago

Prodigy 40 points

Hello,

I've changed C17 to 150pF and shorted Fb2, there is un improvement.

There are probably other issues with the prototype other than the PSU : HF noise due to the fast current loop, I need to get (5A/10Mhz).

I will leave it like that for now and got back to the PSU later.

I was also if I did an obvious error circuit or layout ?

Thank you

Best regards

Georges

0 Lucia Gao over 1 year ago in reply to Georges Maccario

TI__Mastermind 30451 points

Georges Maccario

Thanks for your feedback. Bella is out of office and she will be back on Friday. We will get back to you by end of this week.

Thanks for your understanding!

BRs

Lucia

0 Gui He over 1 year ago in reply to Georges Maccario

TI__Mastermind 27306 points

Hi Georges

Sorry for late reply. I checked the layout it looks ok to me.

The circuit is designed to operate for Vout=3.5V to 12.5V, There is Recommended component values in the Table 7-2 in the datasheet. Can you refer to the recommended BOM and see if the circuit can be stable?

Please paste the waveform of SW. Thanks and best regards.

0 Georges Maccario over 1 year ago in reply to Gui He

Prodigy 40 points

Hello,

Thank you for the check and your time.

Thank you for the advice.

At the moment the prototype is not too bad, difficult to work on, as small and packed, with 8 layers 1OZ copper and not thermal relief (we need the best thermal)

I will get the dev. kit and experiment a bit more with it, as we are looking for a replacement for the TPS54821 on most of our products and so far the TPS56837 does the job.

Thank you.

Best regards

Georges

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TPS56837: Loop stability high current