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TPS767: Inquiry on Recommended Output Capacitor

Jejomar Ildefonso
Expert 8590 points
Part Number: TPS767

Hi Team,

One of our customer was using the TPS767 (5V variant) for their design and they're encountering intermittent problems where noise was being introduced into the system that was traced back to the TPS767. The datasheet recommends the output capacitor specification, specifying that the Sprague and Kemet capacitors meets these requirements. They've looked at Kemet's spec sheets and was unable to find any claims made about the ESR of these devices.

Now the issue seems to be resolved with the use of Tantalum capacitors replacing the ceramic ones. They'd like to know if we have further information, where it states that ceramics have a known ESR value at certain frequencies and what the part numbers of these devices might be. They believed that it was originally chosen for the device but they had a number of the devices with ceramic showing the problem which was resolved by swapping with Tantalum. This knowledge may also be able to help trace down the root cause of the issue and fully resolve the problem they've encountered.

Thanks in advance!


Kind Regards,

Jejomar

  • 0
    TI__Mastermind 23361 points

    Hi Jejomar, 

    In general, the primary noise source from the LDO is the internal bandgap reference. The output cap could potentially affect the bandwidth of the LDO that could affect the noise performance of the LDO but only to a certain extend. In the meantime, the output cap creates another filtering at the output of the LDO that could reduce the noise. The resonating frequency of the cinematic and the Tantalum cap may sit at different frequencies.

    In the application you described, the Tantalum cap could happen to have a resonating frequency closer to the main noise frequency. Another thing that could happen here is whether the ceramic capacitor they used to meet the ESR requirement of the LDO, if not, the output of the LDO could see some oscillation and that small oscillation could be interpreted as excessive noise as well. Without actual data, it's very difficult for us to tell what exactly is happening in their application. 

    You may find this app note useful for the noise discussion. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    TI__Expert 8590 points

    Hi Jason,

    Many thanks for looking into this! We've shared the details of your response to the customer and they'd like to ask more information about the frequency at which the ESR must be between 0.05 and 1.5 Ohms when using the TPS76750QD. Looking at datasheets for different caps that have this parameter it is usually given as a curve over a range of frequencies.

    If you can help us identify the frequency of interest we would really appreciate it. Thank you!


    Kind Regards,

    Jejomar

     

  • 0 in reply to Jejomar Ildefonso
    TI__Mastermind 23361 points

    Hi Jejomar, 

    The ESR requirement is valid up to the bandwidth of the LDO which normally goes up to 1-2MHz and the curve plot on this is mostly common for older devices. 

    Regards, 
    Jason Song