LTC3789 buck boost converters


I am trying to design two buck boost converters based on the LTC3789 chips : Vin 10 - 35V, Vout 24V/3A and Vout 12V/2A

For the 12V converter : 

- The buck-boost works well (test with no load) but won't start in the boost region (buck region ok). I assumed first that it was related to Inrush current (output filter) so I changed Css to 100nF to slow down the start but it doesn't change anyting. Any thoughts about this ? 

For the 24V converter :

- The buck part works fine but not the boost part. Ith filter was initialy as stated in the LTC3789 typical application for 24V (extract from datasheet) :

I tried to change the Ith filter with values from LT power CAD but it doesn't work either. What/where should I look first ?

Thank you if you can bring some ideas on those 2 issues.

Best regards,


  • Hi,

    you have to supply at least your scematics and(!) / or scope-pictures of relevant points of the circuit.
    For the layout, it is highly reccomended, that you follow the example as closely as possible.
    You concentrated on the feedback, but most likely this is not the problem - as you would see ringing on outputs.
    When there is hardly any output, the problem is on the transistors side.
    Specify in more detail, what is happening in boost mode, and at which load current.

  • Hello,

    Thanks for your answer.

    I concentrated on the feedback because I encountered the same troubles (converter not working in the boost region) on the BuckBoost 12V that I manage to correct by acting on the ith filter. So I assumed troubles are more or less same on the buck boost 24V but could be wrong. In my opinion, issues are more related to values of components than routing.

    As I said, the buck mode on the BB24V is working as expected. More specificaly in the boost region, MOS QA and QB are driven as expected (steady states) but QC and QD are not. QC and QD are ramping up. There is no load on output.

    Best regards,


  • 0
    •  Analog Employees 
    on Sep 2, 2021 8:26 AM in reply to Sam23456

    I would also recommend to get hold of a demo board and perhaps modify to achieve to your output specs. PCB layout is tricky for these buck boost controllers.

  • Hello,

    Thanks for your feedback. The pb has been identified and solved : Mosfet QD wired gate resistor was 5K instead of 5R (BOM mistake).

    Best regards,