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Original question:

CSD95490Q5MC: LSET resistor design & VOS pin layout suggestion

CSD95492QVM: Rlset resistor equation, and high value inductors

Cam Brown
Prodigy 20 points
Part Number: CSD95492QVM
Other Parts Discussed in Thread: CSD95490Q5MC,

Hi,

Similar to the question in: https://e2e.ti.com/support/power-management/f/196/t/663588 

1. I'm looking for the Rlset equation for CSD95492QVM, does it match the equation in the link above for CSD95490Q5MC?

2. Our design uses 330nH for a main power rail (278kOhm from above equation), and 4700nH for a lower current auxiliary rail (both using CSD95492QVM for part reuse). For the larger value inductor, the calculated Rlset is >700MOhm, I assume open circuit is the correct solution here?

Thanks for your help!

Regards,

Cam.

  • 0
    TI__Prodigy 510 points

    Hi Cam,

    Thanks for the question!

    Yes, the Rlset equation is the same as the CSD95490Q5MC.  The 330nH rail will work fine with the 278kOhm resistor.  Unfortunately, these devices cannot support inductances above 1000nH.  The current sense circuit goes out of range and will cause unpredictable behavior.  In addiion, there is open-pin protection on the LSET pin to help ensure that the system is designed properly with a valid resistor setting on LSET.  So if you leave the pin open the TAO pin will always be driven low as an indication to the controller that the power stage is not ready.

    Regards,

    Evan

  • 0 in reply to Evan Reutzel
    Prodigy 20 points

    Thanks Evan for the reply!

    I will use 330nH/280k for the primary supply, and 1000nH/1.8M for the auxiliary.

    As a follow up question, I noticed that most TI controllers (that are in evaluation designs with smart power stages) use a 1.7V Vref that supplies REFIN. However, none of these controllers support our use case. Will the CSD9549x range support a 1.2V REFIN voltage?

    Regards,

    Cam.

  • 0 in reply to Cam Brown
    TI__Prodigy 510 points

    Hi Cam,

    The valid REFIN input range is 0.8V to 2.5V.  So 1.2V will work fine.

    Regards,

    Evan