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TPS75433 -- Power up question

Will Jarrett
Expert 3425 points
Other Parts Discussed in Thread: TPS3898

Hello, My customer is seeing an issue with the output voltage dropping upon start up when using the TPS75433.  This dip upon start up is causing an issue for his circuits downstream.  It appears as through the LDO is transitioning through the minimum operating threshold, then below the minimum operating threshold, then finally back into the operating region.  This seems to be causing the output voltage to rise, then fall, then rise up to 3.3V regulated output. In the screen shots below the Yellow is the output voltage and the Blue is the input voltage.  Can anyone confirm what is happening here? 

We are considering a solution using the Enable pin and an TPS3808Gxx to resolve the issue.  Thanks for you help.

  • 0
    TI__Guru 55940 points
    Hi Jarrett,

    The second scope shot in particular looks interesting as it shows what appears to be a load transient on the LDO input and a line transient with the LDO. Is there anything else being powered off of the LDO supply that could be switching at the same time?

    Delaying /EN to after the LDO supply reaches its value should yield a cleaner startup similar to Figure 18 in the datasheet. Please keep in mind the logic of the /EN for this LDO is opposite of most of our devices. The output is enabled when /EN is low. As such, an Active-high SVS like TPS3898 is more appropriate.

    Very Respectfully,
    Ryan