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TPS7A49: Test results is strange

Penn Zhang
Intellectual 1180 points
Part Number: TPS7A49

Hi Sir,

In my TPS7A4901 test, I got some strange results.  

As we all know, Load regulation of LDO(TPS7A4901), as well as Vout, is increasing along with Iout rising, and Line regulation of LDO(TPS7A4901), as well as Vout, is also increasing along with Vin rising. We can see from the datasheet of TPS7A4901 also.

But my test results are just on the opposite. Vout is decreasing along with Iout increasing or Vin increasing.

I have a guess, but I don't know it's right or wrong. As we can see, Vout is decreasing with the temperature rising, so I think the different Vin and Iout will cause different temperature, especially junction temperature. When Vin or Iout are high, power dissipation will be big, so the junction temperature will be high, this will cause Vout decrease, and this effect is much bigger than Vin or Iout increasing, so the Vout decrease a little.

I want to hear your opinion about this case.

Hope for your reply.

Thanks

  • 0
    TI__Guru 55940 points
    Hi Penn,

    Can you help me understand your test setup? Are you using an EVM for the measurement?

    Unfortunately the TPS7A49 EVM does not have sense test points away from the main current path; however, it is important to measure the voltage at a sense point so that the current does not cause an artificial voltage drop across the wire. You can add a sense point by adding a wire to the input and output capacitors for the measurement.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1180 points

    Hi Ryan,

    Thanks for your reply.

    I am using TI-PMLK TPS7A4901 board, and test setup like the picture below. I think this is the test method you mentioned, but still have this problem.

    Hope for your reply.

  • 0 in reply to Penn Zhang
    TI__Guru 55940 points
    Hi Penn,

    Unfortunately the test points that you are using are not true sense test points. The current still will impact the measurements due to the force current flowing across the trace generating a voltage drop. The higher the current, the higher the voltage drop across the trace will be. Ideally you would be able to measure the voltage at the LDO input and output through a path that does not have current flowing.

    Another possibility to explain the difference in measurement is that line and load regulation are DC measurements. If you take the measurements too quickly, then you may measure the transient response (ac) rather than the regulation (dc).

    You described the trend of your measurements in your first post. Could you provide a graph of your measurements?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1180 points

    Hi Ryan,

    Many thanks for your reply.

    Sorry for the delay.

    I did another experiment according to your suggestions. Test the input and output voltage from at the LDO pin. The results are as follows, and I wait for the results stable then read it, so I think there will not be a transient response problem.

    During my test. I find when Vin=6V and 9V, the Vout is stable for a long time. Then Vin =12V, Vout begin to decrease after LDO working for a while.Vin=15V, Vout decrease 0.0001V after about 60 seconds. Vin=21V, Vout decrease 0.0001V after about every 5 seconds. Before every different Vin test, I wait for the LDO cool down to room temperature, then change the Vin and test. So do you think this is a prove for temperature affect the LDO Vout? The higher Vin, the lower output efficiency, so more heat will produce in LDO.

    Hope for your opinion, it will be very helpful.

    Regards,

    Penn

  • 0 in reply to Penn Zhang
    TI__Guru 55940 points
    Hi Penn,

    You are correct that increased temperature will impact the measurements. While we want to make sure that we are not measuring a transient response by measuring too quickly after a large change in Vin or Iout, we also want to take the measurement quickly so that the junction temperature is approximately equal to the ambient temperature and thermal dissipation does not impact our measurements. The way we counteract this in our lab is we use an automated slow ramp.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1180 points

    Hi Ryan,

    Thank you for your patient explanation. This problem has been solved now.

    One more question, the automated slow ramp is a good way to test, and do you think this may also need a fan or thermostat to help LDO keep the junction temperature stable, because when Vin is high enough,  even if the change is a slow ramp, its own heat dissipation is not enough to keep junction temperature approximately equal to the ambient temperature, right?

  • 0 in reply to Penn Zhang
    TI__Guru 55940 points
    Hi Penn,

    Correct. We generally place the device in a thermally controlled environment so that we can control the ambient temperature during our characterization.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Intellectual 1180 points

    Hi Ryan,

    Thank you very much.

    Thanks for your patient.

    Regards,

    Penn