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Change TPS23754EVM Vout

Richard Andres
Prodigy 30 points
Other Parts Discussed in Thread: TL431, TPS23754EVM-383

I just picked up one of these PoE PD eval boards, and have a question.  I don't see any information on the data sheet (for the eval board or PoE controller / DC-DC converter) for adjusting the output voltage.  I assume since max duty cycle of the DC-DC converter is nearly 80% a wide range of voltages should be realizable.  Usually there is an equation for calculating feedback resistances to set desired output voltage.  Am I missing something obvious?  While I actually want the 12V output the board is designed for, I'd like to know about this.  I have read the threads on both the 23754 and 23756 here without finding what I'm looking for.  

Thanks for your help.

  • 0
    TI__Mastermind 42795 points

    Hi Richard,

    Thanks for your input. The output voltage setpoint for a design like this is set at the input of the TLV (or TL for a 12V output) 431 error amplifier. In general, the equation can be found as shown below from http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slva305&fileType=pdf section 2.8.2, EQ 29

    where Vref is the TL431 internal reference voltage and Rfbu and Rfbl are the upper and lower feedback resistors.

    For TPS23754EVM-383 (the 12V output EVM), Rfbl=R25=11k and Rfbu=R23=41.2k. So, Vout=(2.495V x 41.2)/11+2.495 = 11.84V.

    The actual output voltage on this EVM is closer to 12V but the EQ is a bit low due to the additional drop across R18 (the 50 ohm loop injection resistor) which is about 200mV.