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TPS7A8001DRBEVM: Question about startup time and current limit operation

Ella KIM
Expert 4212 points
Part Number: TPS7A8001DRBEVM
Other Parts Discussed in Thread: TPS7A80

Hi team, 

I'm evaluating TPS7A8001 EVM as the customer has an issue on the their board. I have several question about the basic operation of this device.

Please refer to the slide deck. Thank you for your quick response.

tps7a80evm.pptx

  • 0
    TI__Mastermind 37065 points

    The waveforms all meet our expectations but there are a few things happening here.

     

    1. The internal error amplifier common mode range is not rail to rail, like many error amplfiiers.  This shows up in the measurements as a delay from the rise of the NR pin voltage to the rise of the output voltage.  Then when the output kicks on, it rises rapidly until it catches up to the sensor gain times the NR pin voltage.  As an example, your second slide shows the NR pin voltage at 350mV when the Vout begins to rapidly rise.  The sensor gain is 2.8V/0.8V = 3.5.  350mV * 3.5 = 1.225V, which is roughly where Vout stops rapidly rising and continues to rise with the NR pin voltage.
    2. The UVLO block may not completely turn off the error amplifier in the TPS7A80.  This results in a small voltage being present on Vout, less than a volt, until Vout begins to ramp up.  This explains the small voltage seen on the waveforms in power point slide 13.
    3. The staircase effect during the rapid turn on is due to the output voltage overshooting, causing the feedback loop to stop the turn on of the output, following by the output voltage overshooting again when the error amp turns it back on. During this time the reference voltage does not stop ramping up, and the error amp is trying to ramp Vout with the NR pin voltage.
    4. In some cases the inrush is causing the input supply to droop, causing the LDO to enter dropout.  This can be seen in slide 20.

     

    Here are some remedies and comments:

     

    1. Ensure that at least the same amount of input capacitance exists as the output capacitance.  There would preferably be more Cin than Cout, which will prevent the Vin from drooping during turn on.
    2. Keep in mind that the small amount of voltage on Vout, due to the UVLO failing to turn off the error amplifier, is low and less than 1V.  This is not enough voltage to turn on most electronics and will likely not have an impact on the design.
    3. The customer is doing the right thing by increase Cnr if they want a longer turn on time.  However the TPS7A80 is a rapid turn on device (as implied by the fast charge switch time of 2ms).  The customer may wish to choose a different LDO if they need greater than 5ms of startup.  If the customer would like a different LDO, we can offer other solutions if we are provided the LDO requirements.

    Thanks,

    - Stephen