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TPS7A8001DRBEVM: Bode Plot measurement

Jerry Chen
Expert 5110 points
Part Number: TPS7A8001DRBEVM

Hi,

My customer measured the bode plot of TPS7A8001EVM when Vin=3.3V and Vout=1.8V. They found the bandwidth is only 100Hz. But LDO is supposed to have larger bandwidth. Could you help check it?

Thanks.

  • 0
    TI__Mastermind 23361 points

    Hi Jerry,

    What's your concern for running this test on this LDO? Are you worried about stability at certain conditions?

    If you would like to measure the phase margin of the LDO at customer’s conditions, here are the things that are important. The designs of the LDO contains multiple control loops and it’s not very practical to break all the loops and the stability of the LDO involves all internal loops combined. The load current will also change the phase margin. One practical one that can help to estimate the phase margin of the LDO is to create a fast load transients to see how all control loops combined react to that disturbance. Here is an application that covers this topic. Please note, since the phase margin will change depending on the load currents, it is important to do the fast load transients according to the application note using different loading conditions that may exist in customer’s application.

    Regarsd,
    Jason Song

  • 0 in reply to Jason Song
    TI__Expert 5110 points

    Thank you, Jason.

    This is not a question of stability. Communication customer would like to qualify TPS7A8001 as ripply/noise filter. They checked the loop because wider bandwidth helps PSRR, so they measure the loop of TPS7A8001 with a traditional DC/DC way, finding the bandwidth is about 100Hz. If TPS7A8001 has multiple control loops, could you pls show a functional block diagram of the loops and how to measure the bode plot correctly. To qualify our device in this communication customer, we need to explain it.

  • 0 in reply to Jerry Chen
    TI__Mastermind 23361 points

    Hi Jerry,

    Thanks for the explanations, however, for linear regulator, the traditional DC/DC way may not work well to measure the bandwidth of the LDO. Part of the reason is due to the complexity of the design creating mutiple control loops; when these loops work together, it's impossible to isolate them from each other and even you managed to break the main control loop from the feedback pin to the output, the data you got does not relect the true perpormance of the device. 

    If you do need to have bandwidth and phase margin at different Cout and Iload, I would suggest you still use the fast load transient way. Besides counting the rings to estimate the phase,  you can also get the bandwidth information of the LDO by reading how fast LDO's output starting to responde to the same disturbance due to sudden load transients. To make it simple, it's the time from the overshoot on the Vout starting to happen to the time that Vout starting to recover(dropping). 

    Let me know what your thoughts are and if you have any questions on using the way I suggested to estimate phase and bandwidth of the LDO.

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Jerry,

    Since it has been a week without a reply, we assume that the issue has been resolved. If this is not the case, please feel free to reply. 

    Regards,
    Jason