LMZ31506: Buzzing sound when load increases

Hi Ibram,

The schematic image seems pixeled, therefore I can't see the components used. Can you re-upload it so I can take a look at the design. 

Eileen Hernandez

Hi Eileen,

Here is the pdf, let me know if you can view this file.

P4V2.pdf

Regards,

Ibram

Hi Ibram,

I'm able to see the schematic now. Let me take a look at it and I will get back to you as soon as I have an answer. 

Eileen Hernandez

Hi Eileen,

Have you got any feedback for me on this?

Regards,

Ibram

Hi Ibram,

Audible noises can occur due to different reasons, this include stability issues, current limiting and possibly broken inductor or other parts in the system.

Does the module regulate well at those high currents? If not regulating then it can be an indication to one of the problems above. Does noise happen with multiple modules or just one?

 Also, in the second condition were the load is switched continuously with the delay of 100ms and 500 ms (equivalent to 100 Hz to 20 Hz) which is in the audible frequency range. Can your system do this every 700 ms instead? in order to get out of audible range.

Eileen Hernandez

Eileen Hernandez Hernandez said:

Does the module regulate well at those high currents? If not regulating then it can be an indication to one of the problems above. Does noise happen with multiple modules or just one?

This happens with all the module.

Eileen Hernandez Hernandez said:

Also, in the second condition were the load is switched continuously with the delay of 100ms and 500 ms (equivalent to 100 Hz to 20 Hz) which is in the audible frequency range. Can your system do this every 700 ms instead? in order to get out of audible range.

As I mentioned earlier when the constant load goes above 5.4A (where we are not switching the load), I still hear the sound. I can set it up to switch at 700ms for an experiment but that's not going to help to solve the problem.

Eileen Hernandez Hernandez said:

Audible noises can occur due to different reasons, this include stability issues, current limiting and possibly broken inductor or other parts in the system.

Do you have any suggestion to narrow down any of the above issues?

Ibram,

Is the module regulating correctly at those high currents? Or is there any abnormal behavior besides the buzzing/humming? 

Eileen

You might have seen the waveform I attached in the beginning where it shows lots of fluctuation (peak to peak of 475mV) at higher load current.

Here is the image taken at 4.8A of load to show the ripple. It looks acceptable to me, let me know if you think otherwise.

/cfs-file/__key/communityserver-discussions-components-files/196/P4V2_5F00_SENSE_2800_4.8A-Load_2900_.bmp

Hi Ibram

Eileen is looking into this and will feedback to you soon.

BR

Ruby

Hi Ibram

     The switching frequency ripple is outside  the audible band and is not of concern to me. However if your load is switching at a rate that could be in the audible band, then it can create a noise (as with any converter) that is audible.

To identify the root cause in this specific application, can you please increase the load rate to see if the noise goes away? Giving us clues under what circumstances the noise goes away will help us identify the problem.

Another alternative is to order an EVM (evaluation board) and test the EVM to see if you are able to reproduce the issue with the load tests that you have.

Regards,

Gerold

Hi Gerold,

Thanks for getting back on this.

Gerold Dhanabalan - High Performance Radar said:

To identify the root cause in this specific application, can you please increase the load rate to see if the noise goes away? Giving us clues under what circumstances the noise goes away will help us identify the problem.

The test results posted above were not in the audible range (100ms = 10Hz, 500ms = 2Hz).

However, I went ahead and reduced the switching speed of the load further to 1second (1Hz) and I still hear the sound. I wasn't able to go above 20KHz due to DC load limitation.

Also I tried few more things as described below.

Audible sound stops when I changed the output voltage to 3.3V (i.e changed R557 to 237ohm). Waveform 1) shows 3A and 6A of load switched at 1 Hz, 2) shows 3A and 6A of load switched at 100 Hz 3

Then I increased the output voltage to 3.4V, still no audible sound heard. See waveform 3 and 4 for ripple

Then I increased the output voltage to 3.7V, No audible sound heard as soon as powered up (see waveform 5) and started to hear the sound after ~1 min of power up (see waveform 6)

1) /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.3V_5F00_switching-at-1Hz_2C00_-6A-max.bmp

2) /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.3V_5F00_switching-at-100Hz_2C00_-6A-max_5F00_2.bmp

3. /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.4V_5F00_switching-at-100Hz_2C00_-6A-max.bmp

4. /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.4V_5F00_switching-at-1Hz_2C00_-6A-max.bmp

5. /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.7V_5F00_switching-at-1Hz_2C00_-6A-max.bmp

6. /cfs-file/__key/communityserver-discussions-components-files/196/P4V2-changed-to-3.7V_5F00_switching-at-1Hz_2C00_-6A-max-after-1mins-of-power-up.bmp

My observations on this issue:

1. Reducing the output voltage to 3.3V or 3.4 helps to get rid of the audible noise. 

2. Switching the load at different speed (both audible band and out of audible band) does not seem to contribute to this problem.

3. Peak to peak is too high when load increases.

I modified my test set up in such a way that 1) bench power supply is used to power the module directly, 2) only DC load connected to this rail

Any suggestion to resolve this issue ASAP would be appreciated.

Regards,

Ibram

Thanks Ibram. Have you tried it on TI's Evaluation board to replicate the issue with your load?

Regards,

Gerold

I do not have TI's evaluation module with me to test it, we will try to order one for testing.

Is there anything else, I could try to resolve this problem?

Do we have to maintain any specific difference between input and output voltages?

Regards,

Ibram

Hi Ibram

    Can you please take a scope shot of the SW node - to have a zoomed in look at the switching frequency? As Vin=5V, Vout=4.2V, if the min-toff of the device is not met, the device will hold on the HS FET for multiple switching cycles, essentially lowering the switching frequency.

Regards,

Gerold

Hi Gerald,

I took the captures on SW node under various conditions, please take a look at them.

1. Measured at U109_PH at No load condition

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-No-load.bmp

2.Measured at U109_PH at 6A of constant load  

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-6A-constant_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-6A-constant_5F00_2.bmp

3. Measured at U109_PH at 3.9A of constant load  

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3.9A-constant_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3.9A-constant_5F00_2.bmp

4. Measured at U109_PH at 3A of constant load  

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3A-constant.bmp

5. Measured at U109_PH at 1A of constant load  

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-1A-constant.bmp

6. Measured at U109_PH when 3A and 4A of load switching at 1Hz  

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3A-and-4A-switching-at-1Hz_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3A-and-4A-switching-at-1Hz_5F00_2.bmp

7. Measured at U109_PH when 3A and 6A of load switching at 1Hz

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3A-and-6A-switching-at-1Hz_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/U109_5F00_PH_2C00_-RRT_2D00_95.3K-_4000_730KHz_2C00_-4.2V-output_2C00_-3A-and-6A-switching-at-1Hz_5F00_2.bmp

One other thing I noticed is that when the constant load goes over 4A, output voltage briefly cut off at regular interval. This was the reason I had take so many captures.

Let me know if you need more input to come to a conclusion.

Regards,

Ibram

Hi Ibram

Gerold is looking into this and will feedback to you soon.

BR

Ruby

Hi Gerold,

We received LMZ31506EVM-692 today and tested it using its default setting except changed the jumper setting on J8 to be between pin2 and 3.

I am able to reproduce the problem even with EVM operating at 3.3V. See the waveforms below and let me know if you need more info.

We urgently need some input from you guys on how to solve this as we have already made 300 boards with this design.

1. Vin 5V, Vout 3.3V, No load condition

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-No-load.bmp

2. Vin 5V, Vout 3.3V, 6A constant load

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-6A-constant-load_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-6A-constant-load_5F00_2.bmp

3. Vin 5V, Vout 3.3V, 5A constant load

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-5A-constant-load_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-5A-constant-load_5F00_2.bmp

4. Vin 5V, Vout 3.3V, 4A constant load

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-4A-constant-load_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-4A-constant-load_5F00_2.bmp

5. Vin 5V, Vout 3.3V, 4A constant load after few mins of power ON

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-4A-constant-load_5F00_3_2C00_-after-a-few-mins.bmp

6. Vin 5V, Vout 3.3V, 3.2A constant load - Seems to be stable at this condition.

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-3.2A-constant-load_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-3.3V_2C00_-3.2A-constant-load_5F00_2.bmp

Regards,

Ibram

Hi Ibram

     Thanks for sharing these results. for the condition where it is failing, can you please try increasing the Vin from 5V to something higher to see if that solves the problem? 

Regards,

Gerold

Sure, I am going to try that. Even If I find it working at higher input voltage, its not going to be helpful for us as we share that 5V input with other 5V devices on the system.

Meanwhile, do you have any test data on EVM with 5V input?

Regards,

Ibram

Hi Ibram

    The datasheet has curves that show VIN=5V, Vout=3.3V upto 6A. So the device is capable of supporting this application. The EVM shows Vin=5V, Vout =2.5V and does not have Vout=3.3V. Not sure if this was intentional. We will check this out on the EVM on our side.

Also the original issue on your board was audible noise. Now the issue being reported on EVM is the converter shutting down. Am I understanding this correct?

Also, do you have long cables from power supply to VIn or Vout to load?

Regards,

Gerold

Please check and update for 5V input and 4.2V output as this is our expected operating con

Gerold Dhanabalan - High Performance Radar said:

Also the original issue on your board was audible noise. Now the issue being reported on EVM is the converter shutting down. Am I understanding this correct?

Yes, converter is shutting down at higher load and also I can still hear the audible noise with EVM.

Gerold Dhanabalan - High Performance Radar said:

Also, do you have long cables from power supply to VIn or Vout to load?

Vout to load : 12" long cable

VIn : ~ 30" long cable

Regards,

Ibram

So I modified the EVM to get 4.2V output. Here is the test results,

1. Vin 5V, Vout 4.2V, 3A and 6A load switched at 1Hz - Noise very loud and clear

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-Vout-4.2V_2C00_-3A-and-6A-load-switched-at-1Hz_5F00_1.bmp

2. Vin 6V, Vout 4.2V, 3A and 6A load switched at 1Hz : Output looks to be stable when the input is increased to 6V. There is no or very low audible noise.

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-6V_2C00_-Vout-4.2V_2C00_-3A-and-6A-load-switched-at-1Hz_5F00_1.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-6V_2C00_-Vout-4.2V_2C00_-3A-and-6A-load-switched-at-1Hz_5F00_2.bmp

Let me know your suggestion on how to deal with this issue now as we can not increase the input voltage or reduce the output voltage.

Regards,

Ibram

Hi Ibram

     That does not give much options as your Vout is very close to Vin and the drop across the FET and the inductor is large enough causing device to shutdown and restart. Can you change the output of the previous stage to provide a larger Vin?

Regards,

Gerold

This is unfortunate that this details was not provided in the datasheet.

Were you able to test the EVM to see if it matches our test results? Just want to make sure there is no measurement error.

Regards,

Ibram

Hi Ibram

     We are double confirming on the EVM.

In the mean time, have you tried lowering the Fsw? 

Regards,

Gerold

Yes, here is waveform with 5V input, 4.2V output, Switching frequency 250KHz, Constant load of 6A.

/cfs-file/__key/communityserver-discussions-components-files/196/Vin-5V_2C00_-SW-Fq-250KHz_2C00_-Vout-4.2V_2C00_-6A-constant-load_5F00_2.bmp

Output is still bad.

Regards,

Ibram

Hi Ibram

    Can you please confirm that your input power supply is not current limited?

Regards,

Gerold

Bench power supply that I am using to test EVM is capable of delivering upto 20A of current. During my test, I was testing it with 7A of current limited at the input.

Hi Ibram

    Can you try increasing the current limit and check with limit of 10A or higher? 7A should be sufficient - however, I am not sure about the equipment. So want to remove that out of the suspect list.

Regards,

Gerold

I bumped upto 10A and converter still shutdown at 5V input, 4.2V output, Constant load of 6A.

Regards,

Ibram

Hi Ibram

     We were able to confirm that the module operates with 5V Vin, 4.2V output upto 6A.

Can you please check the Vin close to the module pins, to confirm that there is no drop across the cables?

Can you try using a resistive load directly on the board - to avoid setup issues?

Regards,

Gerold

Hi Ibram

    Also previously we faced an issue because our input power supply was current limited. Now correcting that ,we are able to deliver 6A at Vin=5V, Vout=4.2V.

Regards,

Gerold

Hi Ibram,

Please make sure you monitor the input voltage using an oscilloscope and very close to the module itself.

I have seen similar issues/waveforms to the ones you are reporting, but that was due to the input source hitting the current limit that was set too low.

The interesting thing is that I was monitoring the input voltage with a multi-meter and the meter did not show any drop or any fluctuations in the 5V!

The drops in the input voltage can be very short in time and not noticeable with a volt meter.

Regards,

Yitzhak Bolurian

Hi Ibram

    Any updates from your side? Was the issue resolved?

Regards,

Gerold

Thanks for the input. I tried below,

1. Old set up, 5 Vin, 4.2 Vout, 780KHz, and 6A constant load

/cfs-file/__key/communityserver-discussions-components-files/196/Old-setup_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-6A-constant-load.bmp

2. Old set up, 5 Vin, 4.2 Vout, 780KHz, and No load

/cfs-file/__key/communityserver-discussions-components-files/196/Old-setup_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-No-load.bmp

3. Reduced the wire length and Used 18 AWG wires, 5 Vin, 4.2 Vout, 780KHz, and No load

/cfs-file/__key/communityserver-discussions-components-files/196/Changed-the-input-wire-length-and-20AWG_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-No-load.bmp

4.Reduced the wire length and Used 18 AWG wires, 5 Vin, 4.2 Vout, 780KHz, and 6A constant load

/cfs-file/__key/communityserver-discussions-components-files/196/Changed-the-input-wire-length-and-20AWG_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-6A-constant-load_5F00_2.bmp

/cfs-file/__key/communityserver-discussions-components-files/196/Changed-the-input-wire-length-and-20AWG_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-6A-constant-load_5F00_4.bmp

5. Reduced the wire length and Used 18 AWG wires, 5 Vin, 4.2 Vout, 780KHz, and 3A and 6A at 1Hz switching load

/cfs-file/__key/communityserver-discussions-components-files/196/Changed-the-input-wire-length-and-20AWG_5F00_5Vin_2C00_-4.2Vout_2C00_-780KHz_2C00_-3A-and-6A-SW-load-at-1Hz.bmp

It looks like it was the set up issue as reducing the cable length and changing the wire gauge (18AWG) helps to eliminate the converter shutting down problem.

Since you guys tested the EVM with 4.2V and 6A load, could you provide the expected ripple at full load?

Let me go back to the original issue I had in our PCB. I still hear the audible sound even when whole PCB consumes ~9W. I am going to probe both input and output and share the waveform. Meanwhile, could you think of possible reason for this scenario?

Regards,

Ibram

Hi Gerald,

It looked like there were some issue at the 24V input section. Audible noise greatly reduced after adding some fix to the input side but I still hear very small amount of noise. I looked at the switching node and output of 4.2V output, it looks similar to EVM.

Waveform below taken at the input of LMZ31506RUQ. Do you think that ripple is in acceptable range? If not, what is the max allowed ripple?

/cfs-file/__key/communityserver-discussions-components-files/196/scope_5F00_223.bmp

About the beat frequency, could you share some app note regarding this? We do have power converters on board that operates at 750KHz, 730KHz, 630KHz, 400KHz, 1.2MHz. 

Regards,

Ibram