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Soft-start for SN6501

Tapajit Mondal
Prodigy 160 points
Other Parts Discussed in Thread: SN6501

My application is powered from a battery and requires to be switched off periodically to conserve charge. I am using SN6501, push-pull transformer to provide an isolated rail to a CAN transceiver.

I would like to have a controlled start-up, a soft-start is unfortunately not provided on-chip, please give  suggestions on how to implement soft-start for SN6501 ?

- Tapajit

  • 0
    TI__Mastermind 28265 points

    Hello Tapajit,

    Commonly the SN6501 receives its input voltage, Vin, from a regulated output of a linear regulator (LDO). The soft-start is therefore applied to the output voltage of this  LDO. As there are various options on how to design a soft-start circuit, I have attached an application note describing its design procedure.

    I hope this information is what you are looking for.

    Best regards, Thomas

     

    slyt096-Soft-start Circuit for LDOs.pdf
  • 0 in reply to Thomas Kugelstadt
    Prodigy 160 points

    Hi Thomas,

    Thankyou for the reply, but if I use a Buck regulator with a built-in Soft-start instead of an LDO to step down the input voltage, will it work ??  I am concerned about putting two switching regulators in a succession.

    I am using a Buck regulator to step down from 12V from a Battery, to power an application and using the SN6501 to create an isolated rail for the CAN transceiver.

    - Tapajit

  • 0 in reply to Tapajit Mondal
    TI__Mastermind 28265 points

    Hello Tapajit,

    your concern is absolutely correct. Having two switching frequencies in a system bares always the risk of beat frequencies (fbeat-low = f1 - f2 and fbeat-high = f1 + f2) and their modulation with other signal frequencies. This is a complex matter as frequencies and amplitudes depend on the design location, system layout, and the actual switching frequencies. If both switching frequencies are close together, then your lower beat frequency will reach a very low value and as such requiring large L-C filter components to accomplish an effective low-pass filter.

    Unfortunately the SN6501 has never been tested in combination with a buck converter and I believe the only reliable way to determine the interactions between two switching supplies is to build and test the entire system in real field environment.

    An additional source of advise I recommend is our power-management department's  E2E forum which will be able to help with regards to cascaded switched-mode supply systems.

    Best regards, Thomas