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Original question:

TPS7A94: VSET/MODE, FIXED OR ADJUSTABLE OUTPUT VOLTAGE

TPS7A94: Is Vout Adjustable through the NR/SS pin? Is NR/SS continuously sampled?

Tom Martin
Prodigy 100 points
Part Number: TPS7A94
Other Parts Discussed in Thread: TPS7A96

Hi,

On a current design, I would like to move from our simple LDO to the TPS7A94. We are concerned with noise, and the TPS7A94 appears to have excellent PSRR.

Our requirements for Vout include a 1.8V nominal output, with Vout,min ~1.4V and Vout,max ~2.2V.

On the "simple" LDO (in fact it is the TLV75801 from TI), we are successfully able to adjust the voltage via the FB pin and an external digital potentiometer which adjusts the pull-down resistance Rpd. After reading the TPS7A94 datasheet, it appears we can use a similar method, although we will be adjusting the resistance on the NR/SS pin.

Specifically, this is my calculation of the necessary resistance: Vout/I,nrss=[2.2,1.4]/150u=[14.7k,9.3k], where I choose a [max,min] target voltage and calculate the corresponding resistance.

My biggest concern as to whether this should be possible is this...is the NR/SS pin continuously sampled or just at startup? If the voltage is continuously sampled, I assume an externally adjusted resistance at the NR/SS pin will inform the part to adjust the voltage.

Please help with this concern.

Thanks,

Tom

  • +1
    TI__Mastermind 33625 points

    Hi Tom,

    This should work just fine.  The internal circuitry is analog in nature.  You can dynamically change the NR/SS resistance and see a corresponding effect on Vout as you have indicated.  Please be aware that the NR/SS capacitor is necessary for the lowest noise performance however charging or discharging this capacitance will slow down the change you will see on the output when you slew from one Vout to the next Vout using your digital potentiometer.  Also be aware that the NR/SS pin is noise sensitive so please select your components carefully and lay out your PCB such that unwanted noise does not couple into this pin.

    If you should ever need more than 1A of current you can consider the TPS7A96 which is a drop in replacement to the TPS7A94, but the TPS7A96 is capable of 2A while the TPS7A94 is capable of 1A.

    Thanks,

    Stephen