LMR36506: Coil Ring Issue

Egemen Aksoy

Intellectual 925 points

Part Number: LMR36506

Tool/software:

Hello,

In our project, we are using LMR36506

Vin: 8-36V

Vout : 5V, output current around max 400mA

Schematic is attached.

L2 is SRN6045TA-470M

The RT resistor (R7) is 16.5k, which shall be changed to 39.2k. (The RT resistance is currently 16.5k.)

For the output capacitors C2 and C3 (2x 22u-10V), coil ringing decreases significantly when we place two more on the output side (a total of 4x 22u-10V).

Could you please let us know if there is any mistake related to the schematic?

How can we solve the coil ringing issue?

Best regards,

Egemen Aksoy

9 months ago

0 Richard Fu22 9 months ago

TI__Mastermind 31315 points

Hi Egemen,

I think adding additional output capacitance is the right move. The output capacitance is getting derated, so it is possible that you have less output capacitance than you expect, resulting in oscillation.

The best way is to probe the output voltage and load current during a load transient and see the response- this may give insight to the stability of the converter

Thanks,

Richard

0 Egemen Aksoy 9 months ago in reply to Richard Fu22

Intellectual 925 points

Hello Richard,

Even when we use 2 capacitors for the 5V output of the SMPS, the 5V output is quite stable. Our only problem with the circuit is the coil ringing. The capacitor that we are using is CL31B226KPHNNNE, and its value drops to 15uF with the DC bias derating at 5V. In fact, the 2 capacitors we are using seem to be sufficient for the operation. In short, our only problem right now is coil ringing.

Best regards,

Egemen Aksoy

0 Richard Fu22 9 months ago in reply to Egemen Aksoy

TI__Mastermind 31315 points

Hi Egemen,

I see, is it like a physical ringing, or what you can see when probing the switch node?

Thanks,

Richard

0 Egemen Aksoy 9 months ago in reply to Richard Fu22

Intellectual 925 points

Hello Richard,

Measurements are attached.

Best regards,

Egemen Aksoy

0 Richard Fu22 9 months ago in reply to Egemen Aksoy

TI__Mastermind 31315 points

Hi Egemen,

If it is physical ringing, you may need to add extra support to the inductor connection to prevent the vibration. If it's electrical ringing, I suggest adding a snubber network at the switch node in order to reduce the spike. A simple RC in series may help. You may see the app note for help

/cfs-file/__key/communityserver-discussions-components-files/196/1665.RC-Snubber.pdf

Thanks,

Richard

0 Egemen Aksoy 9 months ago in reply to Richard Fu22

Intellectual 925 points

Hello,

We took measurements with and without snubbers.

Measurements are attached.

Without snubber

With Snubber (1nF + 2.2 ohm)

With snubber current consumption increased the coil ring continues.

Could you please comment on the measurements?

Best regards,

Egemen Aksoy

0 Richard Fu22 9 months ago in reply to Egemen Aksoy

TI__Mastermind 31315 points

Hi Egemen,

The switch waveform looks normal and that you're operating in DCM. It looks like the coil ring is physical. Is there a way you can enclose the coil to reduce the audible ringing?

Thanks,

Richard

0 Egemen Aksoy 9 months ago in reply to Richard Fu22

Intellectual 925 points

Hello Richard,

We can not enclose the coil.

So, should we choose a new SMPS?

Best regards,

Egemen

0 Richard Fu22 9 months ago in reply to Egemen Aksoy

TI__Mastermind 31315 points

Hi Egemen,

I don't think you will need a new SMPS yet.

Can you provide the layout and schematic file? I just want to check to see if the layout is done properly. Afterwards, I would suggest changing to another inductor and see if the issue follows.

Thanks,

Richard

0 Egemen Aksoy 9 months ago in reply to Richard Fu22

Intellectual 925 points

Hello Richard,

Since the schematic is attached, I attached the pcb layout.

The PCB is 4 layers and there is a solid GND layer under the attached signal layer.

Best regards,

Egemen

0 Richard Fu22 9 months ago in reply to Egemen Aksoy

TI__Mastermind 31315 points

Hi Egemen,

The layout in general looks ok to me, the one thing that could possibly improve is reducing the switch node area if you choose to re-spin the board.

I can think of two other fixes before re-spinning the board:
1) varnishing the inductor

2) Selecting another inductor with similar characteristics and see if performance improves.

Thanks,

Richard

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LMR36506: Coil Ring Issue