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TPS84250: External Output Capacitance max spec

Sean Rauchfuss
Intellectual 1025 points
Part Number: TPS84250
Other Parts Discussed in Thread: TPS84259, LMZ14203H, LMZ31710

Posting for a customer:

In the datasheets for the TPS84250 and TPS84259 there is a specification for a max external output capacitance of 430uF. Due to system design requirements (to minimize bus fluctuation due to load variation) our customer would like to have lots of capacitance on this line (greater than 430uF).


What is driving this specification?  Is it due to inrush current?

Is there any way around the spec so our customer can have additional output capacitance?

 

Thanks!

  • 0
    TI__Mastermind 25590 points
    Sean,

    Inrush current should not be an issue since it has a soft-start pin. The output capacitance spec has more to do with stability. As always you may build the circuit any way you wish and validate that the solution works for your application.

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Expert 7055 points

    Sam- thanks for this response, I think we're looking for a little more detail. 

    Is the 430uF a value we've tested?  Is that the stability limit, or do we have any other data?  Do we know it oscillates at 600uF?  1000uF?

  • 0 in reply to Jake Salmi
    TI__Guru 50120 points

    Hi Jake,

    The 430uF shown in the electrical characteristics table of the datasheet is a value that has been tested on the EVM. 430uF is the recommended maximum output capacitance for the device. As Sam stated anything over this spec may result in unstable design. The reason behind this is that with high output capacitance during startup, the output voltage will take some time to charge up to the final regulated output value. This will result in a load fault condition and push the TPS84250 into cycle-by-cycle current limiting. Because of this, it is recommended to stay below the maximum output capacitance of 430uF. 

    An alternative to this can be the LMZ14203H which does not have an output capacitance limitation. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Intellectual 1025 points

    Jimmy,

    What device would you recommend that does not have an output capacitance limit and has a freq sync feature?  Unfortunately LMZ14203H doesn't seem to have the sync feature so we are looking for another option.

    Thanks!

  • 0 in reply to Sean Rauchfuss
    TI__Guru 50120 points

    Hi Sean,

    What is your application (VIN,VOUT,IOUT)? I can look through the power module portfolio and recommend a device accordingly. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Intellectual 1025 points

    Application is a Medical Capital Equipment

    Vin: 12V

    Vout: 3.3V

    Iout: <4A

    Thanks!

  • 0 in reply to Sean Rauchfuss
    TI__Guru 50120 points

    Hi Sean,

    It might be worth looking into the LMZ31710. I've just emailed Jake offline but to keep it transparent here, the LMZ31710 has the following features:

    • VIN range: 2.95V – 17V 
    • VOUT range: 0.6V- 5.5V 
    • IOUT = 10A max
    • Frequency SYNC
    • 1500uF external output capacitance maximum.

    Regards,

    Jimmy