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

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. Switch node waveform is on the picture.
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.

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.

 

Stray inductance or capacitance becomes more problematic as frequency increases.
If it works properly at 100kHz why do you want to go to 200kHz? Your English is good, by the way.

 

Stray inductance or capacitance becomes more problematic as frequency increases.
If it works properly at 100kHz why do you want to go to 200kHz? Your English is good, by the way.

Requairment is 200kHz. Is it possible that it can be caused by setting of loop compensation ? Or, it can be caused by PCB layout, because in datasheet and even in application note of Linear Technology, use 4 layers PCB with ground plane between power component and small signal. thanks

 

It could be due to PCB layout. I'd suggest you follow the datasheet/appnote advice.

 

@rado8705: Hi, I also believe it is due to the PCB layout and if 4 layer PCB is required you should go with the 4 layer PCB instead of 2 layer PCB and may be it is causing the oscillations. So you go with the application notes.

 

Why is 200kHz operation required? The main (good) reason to do that is to reduce the size of the inductor and filter capacitors. Are you planning on doing that?