LM5642: Output oscillation problem

Asaf Abraham

Prodigy 140 points

Part Number: LM5642
Other Parts Discussed in Thread: LM2675,

Hello,

We encountered a problem when using the LM2675.

Attached the schematic of the power supply.

We are using this design for years.

Recently we had to change the output capacitors (C41 and C30) from Aluminum capacitors (470uF/30mΩ) to Polymer capacitors (470uF/20mΩ) due to capacitors shortage.

The problem we encountered was that when we connected SD card to our circuit, the 3.3V started to oscillate at 6.2kHz and 300mVp-p amplitude (the SD supply is connected to the 3.3V).

Attached two plots which show the moment the SD card is inserted, and the afterward oscillations.

The problem was solved when C45 (Cc2) is removed.

Since the Cc2 capacitor is proposed to increase stability, we don’t really understand why its' dis-assemble helped with the oscillations.

Can you suggest why removing Cc2 helped?

Can you tell if the compensation network is correct?

Best regards,

Asaf Abraham5226.PS.pdf

over 2 years ago

0 Timothy Hegarty over 2 years ago

TI__Mastermind 49887 points

Hi Asaf,

The Cc2 cap provide a compensator pole (=1/(2*Pi*Rcomp*Cc2). the value of 1nF is quite high, so it likely eroded phase margin, especially with the change to a lower ESR output cap (the ESR zero increased in frequency, so the related phase boost around the loop crossover frequency was lower).

Try placing the high-frequeny pole with Cc2 at Fsw/2 or ~100kHz so that it provides less phase lag at corssover. For example, 18.1k and 1nF puts the pole at 8.8kHz (where the phase lag contribution is 45 degrees) -- this is very low. Also, please use the LM5642 Excel quickstart calculator to assist with component selection.

Regards,

Tim

0 Asaf Abraham over 2 years ago in reply to Timothy Hegarty

Prodigy 140 points

Hi Tim,

Thanks for your fast reply.

Where can I find the quickstart excel? I didn't find it.

Also, I attached the PS schematic, and noted that the feedback it taken after a bead.

Can the bead change the feedback compensation? How can I calculate the compensation in this situation?

Best regards,

Asaf Abraham

+1 Timothy Hegarty over 2 years ago in reply to Asaf Abraham

TI__Mastermind 49887 points

Hi Asaf,

Looks like we only have MATHCAD available for this controller: https://www.ti.com/product/LM5642#design-tools-simulation

The ferrite bead creates a high-frequency LC double pole on the output, but it is not in the loop if the feedback sense point is taken before the bead.

Regards,

Tim

0 Asaf Abraham over 2 years ago in reply to Timothy Hegarty

Prodigy 140 points

Thanks. I'll se if I can do this change.

Best regards,

Asaf

0 Timothy Hegarty over 2 years ago in reply to Asaf Abraham

TI__Mastermind 49887 points

Ok, thanks Asaf.

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LM5642: Output oscillation problem