LMR14050: Audible noise observing

pls also upload your Layout

Hi,

Do you use Rfbt=150K and Cff=560pF? pls remove Cff to see if improvement.

Dear Mr. Andy Chen

we are using Rfbt = 150 Kohm

Cff removed and checked still we are getting humming noise.

Regards

Shaithanya C

Dear Mr. Gavin Wang

Vout and SW waveforms attached at "humming noise" situation

Regards

Shaithanya C

Dear Mr. Gavin Wang

we don't have 4A load and source. since we have captured all waveform in IO=3A situation.

1. When Vin = 30V, IO=3A, SW waveform

1. When Vin = 30V, IO=3A, Vin waveform

2. When Vin = 22V, IO = 3A, SW waveform

2. When Vin = 22V, IO = 3A, Vin waveform

3. When Vin = 13V, IO = 3A, SW waveform

3. When Vin = 13V, IO = 3A, Vin waveform

3.1. Two Cout=47uF, Vin = 30V, IO = 3A, SW Wave

3.1. Two Cout=47uF, Vin = 30V, IO = 3A, Vin Wave

3.2. Two Cout=47uF, Vin = 22V, IO = 3A, SW Wave

3.2. Two Cout=47uF, Vin = 22V, IO = 3A, Vin Wave

3.3. Two Cout=47uF, Vin = 13V, IO = 3A, SW Wave

3.3. Two Cout=47uF, Vin = 13V, IO = 3A, Vin Wave

Regards

Shaithanya C

We are observing noise when our system is drawing below 250ma from 3.3V convertor section.

Here are the answers to your questions:

1. Where does your Vin come from? (DC power supply, Keithley 30V, 3 Amps.)

2. Why does the spike for Vin reach about 10V in your second waveform ? (not known exactly how that peak reached 10V)

3. Which situation does audio noise happen for the three cases? (The audio noise happened only in low load condition less than 250 ma approx.)

4. Only put two 47uF output cap does no effect for audio noise, right? (No effect)

5. Put two 0.1uF Ceramic capacitors in Vin side, see any improvement?  (No effect)

Please do find the attached power section for your reference. In that we're observing noise in 3V3 converter section when it draws below 250ma (approx.)

Power section.pdf

You can try add a bead and cao before your inout cap(Cin) as below picture

But, i think the root cause is your bad Layout , you can optimize your layout as WEBENCH or Datasheet. Thank you!

Hi 

    we tried with following bead values. 

1. 200ohm@100Mhz, 5A

2. 300ohm@100Mhz, 7A

3. 180ohm@100Mhz, 5A

4. 150ohm@100Mhz, 5A

5. 56ohm@100Mhz, 10A

6. 42ohm@100Mhz, 6A

Still we are getting humming noise.

Regards

Shaithanya C

Dear Mr. Gavin

1. If you disconnect 12V_VBAT, just use E-load to generate 250mA to LMR14050, is there any noise?  If there is no noise, I think this is mainly because of the second stage. No relationship with LMR14050.

we disconnected 12V_VBAT, and used 250mA load, we didn't observed any noise.  you are right Noise is from second stage.

2. What is the meaning of audible noise?  Pls upload the inductor for L2 and L11 when audible noise happens.

it's like low volume buzzer sound coming from board.

L2 input (LMR14050SDDA) side wave 

L2 output (12V_VBAT)side 

L11 Input (TPS563200DDC) side

L11 output (+3V3) side

3. Only 3.3V section has noise? How about any section?

we are observing only from 3.3V section.

Regards

Shaithanya C