TPS62825EVM-794: When near Vin=Vout, the output waveform looks like oscillating

Hello,

did you use the same inductor of the evm? Can you tell me how much is the output ripple? From the scope plot I have problem to evaluate it.

Thank you for your reply, and sorry my informattion was not enough.

Coil is changed for "TFM160808ALC-R47MTAA ".

When Iout = 1A,

  Vout ripple = 50mV

  Iin current ripple = 150mA

Best regards.

Hello,

for the selection of the LC combination you should refer to the table number 4 of the d/s. You have the same inductance and higher output capacitance that brings the converter close to instability. Can you test the evm using the recommended values? 

Hello,

Thanks, I will test with one of the Recommended Inductors in d/s.

Best Regards.

Hello,

your selected inductance is 0.47uH (this value is reported in the table in the d/s), so should be fine. You should decrease the output capacitance and test the evm with the value of inductance and output capacitance reported in the table.

I am going to close this post and you can re-open it whatever you want in case your new test is not going fine.

Hello,

Sorry for delay, I tested with inductance and capacitance in the d/s.
the values are below,

Inductor  0.47uH
Output capacitance 10uF x2

But, this phenomenon occurred.
I can watch this wave when Vin=3.67~3.47V with Vout setting=3.53V, Iout=0.5A

Hello,

it seems that the device is entering in 100%mode and going out, in this situation you can have higher ripple as shown in your scope plot. Can you test gain the evm, including more input cap (to stabilize your input) and take scope plots of sw node to determine in which mode the device is running?

Hello,

Thanks for your advise.
I added the input capacitor 100uF.
But it seems to be no change.

By the way, is this phenomenon normal?
Should I think of avoiding near VIN=VOUT?

Hello,

currently I am out of the office for training and next week I will test one evm with your condition. In the mean time, can you please specify which kind of load do you have? Can I have a picture of your setup? I want to check the input, output connections and how do you test the output ripple and sw node.

Hello.

Thank you very much for your support.
Yesterday was our holiday, sorry.

My setup is below,

Oscilloscope ; Tektronix TDS3054

C1=22u
C2=10u
C4=10u
C5=no
C6=47u
C7=100u
C8=no
R1=680k
R2=82k+56k
C3=100pF
Vout-GND = 7ohm (0.5A output)

JP1=1-2
JP2=close
JP3=open

Best regards.

Hello,

I have some suggestion regarding the setup that you used.

1) use short and twisted cable for the input and output (avoid crocodile and use different connector or solder directly the cable)

2) tip and barrel method to meas the output ripple (meas across the closest output cap to the IC)

3) test the sw node in DC coupling

From the pic I don't see well if you removed the input cap and placed somewhere else (if you did it is not ok).

Do you have another clean board to test? If not try to clean it a bit (remove the excess of tin).

Which power supply do you use? (maybe you can place at the input a big electrolytic cap to stabilize the input).

I would like to see your input voltage, output voltage and switching node.

I test the evm with you condition and this is my result: