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TPS23525: Bulk capacitor incase of input power fail

Sriman Ojjapally
Prodigy 70 points
Part Number: TPS23525

Hi,

Incase of power failure, TPS23525 can deliver power to the load for certain amount of time using bulk capacitor. This diagram can be seen in design calculator excel sheet  (brown out section). After unhiding the sheets for background calculations, I am unable to find any calculations regarding Cbulk. May I know how do I determine the value of Cbulk to deliver the power to the load incase of power failure.

Thanks

  • 0
    TI__Guru** 105361 points

    Hi Sriman,

    Please use below expression

    Cbulk > (2*Pload*Thold)/ (Vnom^2 - Vmin^2)

    Pload is the amount of power your need to support for the interval Thold. 

    Vnom is the nominal operating voltage and Vmin is the minimum output voltage you desired to maintain during the power failure

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    Prodigy 70 points

    Hi,

    Based on the above equation I am getting Cbulk as 3.49mF, which will take more area on a PCB. Any suggestions on this?? Moreover, how do they define liming resistor as 100 Ohm ??

    Thanks

  • 0 in reply to Sriman Ojjapally
    TI__Guru** 105361 points

    Hi Sriman,

    Cbulk is function of load power and Thold. One option is to reduce Cbulk is to disable the non-critical loads during 'Thold' interval. 

    Regarding liming resistor.... Higher value of limiting resistor (ex 100 Ohm) helps to pick small FET in the parallel path. The selection would be a trade-of between the inrush current through it, the series FET size to handle the stress during charging the Cbulk.

    Best Regards, Rakesh