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TPS797: Ensuring we understand the output accuracy correctly

Part Number: TPS797

Hi,

We are considering to use TPS797, or more specifically TPS79733. One of its tasks would then be to act as reference voltage for some ADC measurements. Therefore we would like to ensure that we understand the accuracy details correctly.

1. The datasheet states an output voltage accuracy of +/- 4%. Is the expected behavior that each LDO unit typically ends up at a static offset from 3.3V within this 4% range, so that one could calibrate away the offset once for each board after building? Or could/will the output voltage for an inidividual LDO vary within that range from time to time so that the (unloaded) output voltage one time could be for instance +1% and another time -1%?

2. The load regulation is stated as 5% in amperes. Should the unit volts, or how should the value be interpreted?

Thanks!

  • Hi,

    It is important to note that there is no long term drift spec for LDOs, that is a common spec for precision voltage references but it is not a standard industry spec for LDOs. Voltage references are designed, manufactured, and tested to minimize long term drift however LDOs are not, so the output can change over time.

    Now that I've covered that, we can discuss trends for long term drift. It is common for any voltage reference to change during the first few hours that it is powered on. Even for precision voltage references it takes between 100hrs-1000hrs of being biased for the voltage to settle (see the example plot from a voltage reference below) and an LDO’s internal reference will behave similarly.  

    So to address your first question, there will be some variation in the output voltage for an individual LDO (just like you can see some moment to moment variation in the plot below) but the general trend will show the output voltage trending to an offset value. There are a few things that you may be able to do in order to reduce that offset. 

    1. either calibrate periodically or run your calibration routine after the long term drift has stabilized (typically between 1000-2000hrs)
    2. After your boards are assembled you can run a burn-in procedure to help alleviate the normal mechanical stresses induced by soldering devices to a board. 
      1. This can reduce the magnitude of the large step seen in the first hundred hours or so, but it is important to note that it will not speed up the total time needed for the reference to stabilize. 

    To answer your second question, you are correct it should not have amps in the unit column. It should just be percent. Also looking at the data the load reg is not 5% typical that also seems to be a typo, it should be more like 0.5%.