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UCC2895: Rslc calculations

Dan Abbey
Prodigy 590 points
Part Number: UCC2895
Other Parts Discussed in Thread: , UCC1895

Hi Team, 

Please can you help with the below to support active project at a customer:

I'm looking for information relating to the calculations to support choosing Rslc in Figure 18 of the datasheet. 

The calculations in the datasheet  appear to be are based on the UCC28950 (Figure 23) , and they do not quite match up. I have cross-referenced most components but have not resolved Rslc.

On page 39 ‘Rrcs’ is stated as being 510R, and I believe this is actually Rslc.

Is my thinking correct, and how was this value arrived at?

 

Thank you for your help on this issue.

BR

Dan

  • 0
    TI__Guru 65945 points

    Hello,

    I am not sure why the UCC2895 data sheet is referencing the UCC28950 and looks like there are errors in it as well that you have pointed out.  I will need to dig into this deeper when I have a chance.  I am not sure why the author of the data sheet did this.  The UCC2895 is an older device and for some reason was updated with UCC28950 information.

    The UCC28950 is one of TI's newer PSFB controllers which has better documentation than the UC2895 and requires less components for slope compensation. It only requires one resistor.  It is also easier to use and setup compared to the older UCC2895.

    The following link will bring you to detailed application note that discusses how to design with the UCC28950 including setting up slope compensation.

    https://www.ti.com/lit/an/slua560c/slua560c.pdf

    The application note also has an excel design tool that can be used as well.  It is based on the application note previously referenced.

    https://www.ti.com/lit/zip/sluc222

    If this is a new design using a PSFB I would recommend using the UCC28950 instead of the UCC2895.  It requires less components and is easier to design with.

    Regards,,

  • 0 in reply to Mike O'
    TI__Prodigy 590 points

    Hi Mike, 

    Thanks for the above, as you say it's a bit unusual, not quite sure what the author was intending here!

    I had a quick talk with the customer and this is for a rework/reverse engineer of an older project that's seeing some issuers in production (avionics application). There's unfortunately no scope to switch away from the older UCC1895 as it would trigger the need for a requalification of the design. 

    I understand it's challenging at the moment bandwidth wise with the pandemic, what kind of time would you need to look into the Rslc value and i'll align with the customer accordingly?

    Best regards,

    Dan

  • 0 in reply to Dan Abbey
    TI__Guru 65945 points

    Hello Dan,

    I reviewed the data sheet and equation 118 sets the RSLC resistor.

    The following link will bring you to the UC2895 EVM and will show the configuration of the oscillator and slope compensation circuit.  That the data sheet should have referred too.

    https://www.ti.com/lit/ug/sluu109b/sluu109b.pdf

    Regards,

    Mike