DC/DC buck-boost converter LT8705

[rado8705]

HI everyone,
I work on my project buck-boost converter with IC LT8705.
Parameters:
Uin = 9 - 30 V DC
Uout = 19 V DC
Iout = 5 A

Scheme:


When I set frequency - fsw = 100 kHz and use inductance L = 10uH, everything work properly at full load 5A in buck region and boost region. This is waveform form switch-node:

and high-side and low-side gate:


But when I set frequency fsw = 200 khz in buck region, everything is work properly at full load 5A, but when converter work in boost region with load above 1,5 A, it start oscillate like this:


I think, that it is something like sub-harmonic oscillations. I read, that SLOPE compensation prevent these oscillations, but LT8705 has internal slope compensation, so it can´t be set by external resistor. In datasheet is defined the minimal inductance which is given by equation and result is 27uH. I use 32 uH but the problem is not solved. Or is it caused by wrong design of PCB? I have 2 layer pcb. Despite of these oscillatons, on the output is still 19,3 V with full load 5A.



I don´t know where is problem. Thanks for every advice and sorry for my English.

 

[D.A.(Tony)Stewart]

Invalid Attachment specified.

Can you verify your L with rated DC flow?

Perhaps more margin is required on L_min or it has SRF value too close to operation.

 

[rado8705]

Invalid Attachment specified.

Can you verify your L with rated DC flow?

Perhaps more margin is required on L_min or it has SRF value too close to operation.

Are You think, measure inductance at full load ? Firstly, I use (https://katalog.we-online.de/pbs/datasheet/74435572200.pdf), then I use 22uH + 10 uH (https://katalog.we-online.de/pbs/datasheet/74435561100.pdf).
SRF of 22uH inductance is 10,5 MHz and for 10uH is 14 MHz.