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TPS799: Approach of using External Vbias for TPS79901DDCR

Shyam Daram
Expert 1255 points
Part Number: TPS799
Other Parts Discussed in Thread: TPS7A14,

Hi,

I have a design where i was using TPS79901DDCR for generating 1.2V for ADC from 1.5V

the 1.5v dcdc takes directly 12v rail as input, which is beyond limit of VBIAS 

can i take VBIAS from alternative rail ? kindly let me know will there be any issues with this kind of approach

my 2.5v bias voltage is coming from rail of dcdc shown below, which is a dcdc output that is being used for generating 1.8V for dcdc.

  • +1
    TI__Genius 12511 points

    HI Shyam,

    I see here in your schematic that you are using the TPS7A14, not the TPS799, the TPS799 does not have a bias rail so I think I am looking at the correct part in the TPS7A14, correct me if I am wrong,

    12V rail for the Bias is too high for this particular device, the in recommended operating condtiion is the larger of 2.2V or Vout +1.4V, you would need 2.6V on your Bias rail as a minimum and 5.5V as a maximum.

    In your current configuration your VBIAS is 100mV below recommended for operation, is there any way that you could step that higher, or if you have a rail between 2.6V - 5.5V?

    Regards,

    John

  • 0 in reply to John Schneider
    Expert 1255 points

    Thanks verymuch i did not see in detail about the point in datasheet "Greater of 2.2 or VOUT(NOM) + 1.4" 

    I have a 5V rail used for alterantive purpose, i used it for bias voltage.

    I hope a alternative rail independent of input is ok to be used, thanks.

  • 0 in reply to Shyam Daram
    TI__Genius 12511 points

    Hi Shyam,

    Yes, Bias rail is encouraged to be a separate rail from the input rail, utilizing the 5V rail will be sufficient to allow the part to operate correctly.

    Regards,

    John