LMR36520: Audible noise on GSM module during call

Hi Danilo,

Do you know the load current during the noise? Is the device operating in PFM mode during this noise?

Can you send a waveform of the VIN, VOUT, and SW pin?

I would also look at the ceramic capacitors as they may be reacting to the noise.

Regards, Jason

Hi all and good year,

I'm going to provide the waveforms requested ASAP.

Regards.

Giancarlo

Hi,

During the GSM burst (in calling), there are current peak of about 1600mA. Here is the modem current (IBAT) waveform:

1) Vin during GSM burst:

2) Vsw during GSM burst:

3) Vout during GSM burst:

4) Vsw with resistive fixed load (about 500mA output current):

5) Vsw with resistive fixed load (about 1600mA output current):

When the current load is fixed (1600mA) there is not audible sound!

Do you have any suggestion?

Regards.

Giancarlo

Hi Giancarlo,

Thanks for sending the waveforms. These give me a lot of good info.

When the current load is increased it ensures that the device is operating in PWM mode. At lighter loads, the device enters PFM mode, which reduces the frequency of the pulses. Also, the GSM burst occurs at a very low frequency which is in the audible range.

Have you tried adding additional input capacitance? It looks like the input voltage really gets hit hard by the GSM burst.

Are your input capacitors placed near the VIN pins? 

Regards, Jason

Hi Jason,

thank you for support.

I tried adding both input and output capacitance without any inproovment!

We have a lot of products with GSM modules but this is the first time that I ear this audible noise.

Maybe, due to the waveforms, we have to use FPWM version?

This is our layout:

This is recommende one:

Any suggestion?

Thank you again.

Regards.

Giancarlo

Hi Giancarlo, 

The layout looks pretty good. 

It's possible that the noise could be coming from the ceramic caps or inductor. 

I would start with ceramic caps. Sometimes they vibrate at just the right frequency on the PCB. Try replacing them with caps from a different manufacturer. If you can get soft-termination caps, they might be best. You may want to try the same thing with the inductor if you can.

.

Regards, Jason

Hi Giancarlo,

Have you been able to replace the components and see any improvement?

Let me know, otherwise I will close the post. 

Regards, Jason

Hi Jason,

thank you for support. I tried to change capacitors with out improvement.

I tried to supply my GSM module with your evaluation board LMR36520AEVM.

During the GSM call I still hear audible noise (little lower but audible!).

Regards:

Hi Jason,

I attached you the audible sound!

I wait for your considerations!

Regards.

Giancarlo20210119_173242.m4a

Hi Jason,

I attached another audible noise sound.

In the first part you can hear the typical noise during the ringing.

in the second part you can hear the noise after the answer

Regards.

Giancarlo20210120_140711.m4a

Sorry Giancarlo,

Either the audio file didn't pick it up or my old ears can't hear it.

Have you tried replacing the caps with soft-termination caps? How about the inductor?

Regards, Jason

Hi Giancarlo,

You might try touching the different components during the noise to determine which component is actually ringing. Touching the part may keep it from vibrating or at least dampen the noise and you'll know which component is the culprit.

Regards, Jason