LM5122: LM5122 Booster 12 to 35v 100A UVLO Noize

Hello Vasilis,

Without schematic it is hard to understand what is what on the layout.

To make UVLO more noise robust, you could add some small capacitance close to the IC pin to AGND.

Best regards,
Brigitte

Thank you for the fast reply

I have add some more close photos of the shematic and board.

You mean capacitor on VIN , AGND pins ?

Thank you in advance

Hi Vasilis,

To minimize the noise of the measurement,

1. Make sure you are measuring with a ground springing.

2. Use high sample rate (<1ns/pt) and >200MHz bandwidth

3. Use scope full Y-axis scale

You may add small capacitors (~1nF) to UVLO and GND if necessary.

Best Regards,

Feng

Thank you for your responce 

I change the uvlo resisotrs and capacitor 

RUV2 to 68K
RUV1 to 20k
Capacitor to 1nf

I measure the capacitor with ground spring and i have attached a photo.

It seams that the uvlo is ok until suddenly drop down and starting enable/disable the output continuously.

Is any other reason that the lm5122 will disable the output?

Thank you in advance

Hi Vasilis,

Can you share the waveform (Vin, Vout, inductor current and SW) when the issue happens?

Best Regards,

Feng

Hello,

i have attached some photos 
in all photos the 
-yellow is input 12v
-blue is output 32v
-violet is sw


No Load


Everything ok with 18A on Input





here is when the problem happen (stop at 30A input)


The resitors if i measure it with multimiter i have 5mv

Thank you in advance

Hi Vasilis,

I think 18A input is already showing something abnormal.

The switching node waveform indicates the current is not stable.

I think the converter is restarting. Can you check the RES pin voltage?

Best Regards,
Feng

Hello



Blue is output 
Violet is SW
and green is the reset

In addition i notice that the efficiency is very low.

One example
Input 12v 20 Amps
Output 33.5v 5.5 Amps
so about 80 %

Hi Vasilis,

Can you please check Vin and UVLO?

Vin dropping can caused shut down.

Best Regards,
Feng