LM25118: not work in buck-boost mode

Hello Evengii,

Would you please check if the behavior changes when you remove R3? Please keep everything else like the above schematic and check how the behavior changes when you remove R3.

Brigitte, Nothing has changed.

Hello Evgenii,

Please reduce C6 to 100pF and check the behavior again.

No result. Please see plots. 

Input Voltage

Ripple Input Voltage

Output Voltage

On RAMP

On COMP

On FB

Hello Evgenii,

Could you please measure the FB voltage again with reference to the AGND pin of the IC?

Do you already have load on the output? If yes, please check without load.

Hello, Brigitte, 

Yes, I already have load on the output. It is correct without load in any mode. 

It is ripple voltage on transistor VT1 input. May be this is the problem in it. But I don't know how to remove these oscillations. (Vp-p ~ 4V)

Could you check my PCB routing please?

Hello Evgenii,

Are you using a 2 layer board or are there more GND layers in the middle? I am asking because with 2 layers, the GND connection is relatively long, especially for the low side transistor on the boost side.

Can you please check at which load the output voltage starts to drop? Please as well compare the real output voltage with the calculated value from the feedback resistors. Is there already a difference at no load?

Hello Evgenii,

As I did not hear back from you, I expect that you have been able to solve the issue and will close the thread now. If this is not the case, please post below or create a new thread if this one is already locked.