LM25116: Humming sound from Power section

Vishnu K N

Part Number: LM25116

LM25116MHX used as 5V Buck. Power supply section produces a Humming sound when load current increases. How can I avoid this issue?

over 7 years ago

0 Gautam Hari over 7 years ago

TI__Prodigy 360 points

Hello Vishnu,

It is possible that the power system is unstable. Can you share your schematic?

Regards,
Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Power section schematics attached.

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

Just to clarify, what is the max load current in this application?

Regards,
Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

About 16 to 20 A at 5V

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

Upon Analysis, it was clear that the phase margin of the system was low.

Attached is a quickstart tool with requirements from the schematic you provided. The bandwidth of the system is usually place between 1/5 to 1/10 of the Switching frequency. Based on an assumed ESR of the output capacitance (50mohm), the calculator provides recommended values for compensation. This should solve any instability in the system.

Hope this helps.

-Gautam

LM5116_quickstart 5Vout 20A 80kHz.xls

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Hi Gautam,

I checked the system with recommended compensation values. But still, exist the issue.

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

Can you provide me the following info to better debug this problem:

1) At what conditions does the humming occur? - Vin, Vout, Load current

2) SW node waveform and Vout ripple (ac-coupled)

Thanks,

Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Humming occur at 20V input 5.04V ,2A output.

SW node waveform attached

Output waveform:

The humming sound generates from Inductor (p/n : HA55L-2223088LF). Could you recommend any other inductor?

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

Based on the waveform you provided we don't see any instabiltiy. The humming could be a result of a poor inductor/ouptut cap. Just to confirm our theory that it is not IC related, can you provide me the following:

1) SW node waveform in a 1ms timescale

2) Inductor current in a 5us timescale

3) AC-coupled Vout (only ripple at Vout- not the DC value)

thanks,

Gautam

0 Gautam Hari over 7 years ago in reply to Gautam Hari

TI__Prodigy 360 points

Hello Vishnu,

Were you able to collect the waveforms? It is common to have such a problem when the device is in critical conduction mode. Every 3rd or 4th pulse may ring due to zero current and this frequency (Every 3rd/4th pulse) is in the audible range.

Regards,

Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

we applied some, resin and hardener to the inductor, so the sound reduced considerably.

0 Vishnu K N over 7 years ago in reply to Vishnu K N

Prodigy 190 points

Hi Gautam,

In this board inductor used is HA55L-2223088LF from TT Electronics. It is an unshielded inductor. After applying resin the sound is reduced. So we replaced the inductor with Vishay P/N: IHTH1125MZEB100M5A and Wurth Electronics: P/N: 74436410 It is a 10uH inductor. But with the new inductor, the sound is low but there is a sound when close to the board. Is there any change need with new inductor value? or need to change any design?

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

The humming sound is due to the ring associated with DCM. This is a common occurrence when the SW node ring results in a frequency in the audible range.

Changing the Inductor value will shift the noise to another load if this is an issue at the tested load.

Regards,

Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Hi Gautam,

How can I solve this issue?

0 Gautam Hari over 7 years ago in reply to Vishnu K N

TI__Prodigy 360 points

Hello Vishnu,

Can you pull the DEMB pin to VCC and let me know if this eliminates the sound?

Regards,
Gautam

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Hi Gautam,
VCC means output 5V. Right?

0 Vishnu K N over 7 years ago in reply to Gautam Hari

Prodigy 190 points

Hi Gautam,

DEMB pin is connected to 5V. Buzzing sound is still there but the sound is reduced after this change.

0 David Baba over 7 years ago in reply to Vishnu K N

TI__Mastermind 40040 points

Hello Vishnu,

I was able to locate the datasheets for the TT and Vishay Inductors you mentioned above. I was not able to locate the datasheet for the Wurth Inductor.

Based on TT and Vishay inductors, I note that they are wound with many turns and the turns are wound relatively loosely on the bobbin/core. I believe the issue is relating to the construction of the inductor due to mechanical resonance (mechanical vibration).
The windings can vibrate at a sub frequency, lower than switching frequency; that you are hearing?
Spraying a coating on the windings can reduce noise, but may not completely eradicate? I suggest using an inductor that has fewer turns and that is much more rigid to reduce noise.

For example, can you try a flat wire construction like the SER2915 from Colcraft? I believe this will drastically reduce audible noise

Hope this helps?

David.

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LM25116: Humming sound from Power section