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TPS7A47-Q1: Output accuracy and output limits

William Thomas1
Prodigy 150 points
Part Number: TPS7A47-Q1

When using the adjustable feature on the TPS7A47-Q1, is the output accuracy determined solely by the accuracy of the set point resistors or do I need to consider overall accuracy, line regulation, load regulation specs?

Also, in looking at the overall accuracy, the test conditions state that Vo is at least 1V lower than Vin, is this a constraint in that the output voltage will always be lower than the input? If so, it seems to conflict with the 95% condition listed for line/load regulation. Color me slightly confused.

  • 0
    TI__Genius 13085 points

    Hi William,

    Let's start with your last question, yes the output of an LDO will always be lower than the input voltage unless the input voltage suddenly drops and the output has a light load and large output cap holding it up (this can be perilous to the LDO since it can result in reverse current  and damage the LDO but that is a different topic). If you need an output voltage that is higher than your input voltage then you need a boost switching regulator. That being said, the output will not always be 1V below the input. The dropout of an LDO (the minimum Vin - Vout voltage achievable) is load current dependent for TPS7A47-Q1 at 1A that has a max value of 450mV. So at full load to keep the LDO out of dropout you will need at Vin-Vout>450mV. 

    Like almost all our LDOs the accuracy listed in the datasheet does not account for the external resistors used to set the output voltage. The EC table Spec listed "Overall Vo accuracy" is +-2.5% for listed test conditions which are across the full load range (0 to 1A), the whole junction temperature range (-40C to 145C) and an input voltage which is from 1V above Vin to 25V. 

    I think the 95% you mention regarding line/load regulation is actually referring to the dropout spec. When we test dropout we need to make sure the device is actually in dropout, there are several ways to do this but for this device we set the input voltage to be equal to 95% of the regulated voltage set by the resistor divider from Vout to FB, since the input is less than the nominal output this ensures the LDO will be in dropout. 

    I hope this helps, let me know if you have any other questions. 

  • 0 in reply to Kyle Van Renterghem
    Prodigy 150 points

    Kyle, thank you for the quick response. Apologies about the 95% confusion, makes me feel I'm getting cross-eyed. One additional question ...

    I'm performing a worst case analysis on a circuit that uses the LDO, the resistors they specified are not tight tolerance, so when I use the formula provided, it returns a worst case output voltage above the input voltage. I think what you are telling me is that I can never achieve that and it will stay below Vin based on the dropout voltage .. correct? Or does this present a problem because the feedback is trying to force it to a higher voltage?

    Regardless of your answer, my recommendation to the company will be to use tighter tolerance resistors to minimize the output variation.

  • 0 in reply to William Thomas1
    TI__Genius 13085 points

    Hey William,

    You are correct the output will never go above the input voltage, but the problem with the situation you describe is that in the worst case scenario the LDO will be in dropout since the feedback is trying to force it to a higher regulation point than can be achieved such that Vout=Vin-Vdo. When an LDO is in dropout the output follows the input so the PSRR will be 0db and their will be no line/load regulation.

  • 0 in reply to Kyle Van Renterghem
    Prodigy 150 points

    Again, thank you for the rapid response, answer is exactly what I needed.