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Improving accuracy of TPS7A4700 with Opamp

Shalom Kattan
Prodigy 80 points
Other Parts Discussed in Thread: TPS7A37, TPS7A47

I would like to improve on the output voltage accuracy of the TPS7A4700 using an external reference.  I am thinking of using an opamp to control one of the voltage setting pins.  Is this OK?  I will use a very low bandwidth control, below the bandwidth of the noise reduction node which I assume has an RC constant of 100k Ohm and the NR capacitor.

For example, if the output is set to 10 volts, the specified accuracy of 2.5% means a variation of 250 mV.  I can use the voltage setting pin 0P8V which has an 800 mV range.

 

Is this OK?

  • 0
    TI__Expert 3300 points

    Shalom,

    Would it be possible for you to share a schematic. This will enable us to understand your circuit better. Thanks

    Regards,

    Kartik

  • 0 in reply to Kartik Sinha
    Prodigy 80 points

    I attach a PDF of the schematic from TINA for the proposed circuit.  The generator VG1 is simulating the voltage reference which normally would be DC, but for the simulation I have some sinewave on it to see the frequency response.

    8371.TPS7A4700 Higher Accuracy.pdf

     

  • 0 in reply to Shalom Kattan
    TI__Expert 3300 points

    • The functional block diagram shown in the PDS for this part is only for the purpose of the user's better understanding of the operation of this device. It is not an accurate representation of the actual design. Thus the highlighted portion below shows us how the output of the regulator is decided when we ground the various voltage option pins but in reality the resistors shown are only a virtual representation. Thus biasing the voltage selection pins will not get the result desired by you.

    Can you please provide more inputs on your requirements so that I can help you further? 

    Vin?

    Vout?

    Iout?

    Accuracy required?

    max noise tolerance?

    Package size?

    Thanks and Regards,

    Kartik

  • 0 in reply to Kartik Sinha
    Prodigy 80 points

    I have two applications.  One has Vin of 12V and Vout of 11V, one is Vin of 5V and Vout of 3.3V

    The current is 750 mA

    I would like very high stability, like 1 mV, and the lowest noise possible.  There are regulators on the market with 40 uV of noise.

    Package can be as big as necessary.

    Thanks

  • 0 in reply to Shalom Kattan
    TI__Expert 3300 points

    Shalom,

    The accuracy in the PDS for these LDO's is specified for the entire temperature range for max load conditions. If you need that much accuracy at the boundary values of the temperature range, then for Vout =3.3V, you could use the TPS7A37. This will offer 1% accuracy over its full temp rage of -40C to 125C. This part should be available by July 2013.

    For  11V output, the only available options are Adjustable LDO's, and the best accuracy over temp is the TPS7A47 with +/-2.5% over temp. Again if you are not operating  at the extreme ends of the temp range over max load, then accuracy will be better. 

    I also recommend that you visit TI's LDO page and  filter by your requirements in case you can compromise further on any of your requirements.

    Please let me know how I can help further.

    Regards,

    Kartik