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TPS76350 inrush current

Marcus Bennett
Prodigy 20 points

In my design, I have 6V as input to the TPS76350 5V LDO. When I enable the LDO after the 6V is already ramped up, I'm finding that the output voltage ramps up to 2V, stays there for around 50us, then continues to ramp up to 5V. Simultaneously, the input voltage dips down to around 5.4V , ramps up again, drops again, then ramps back up to 6V. Using i = C * dv/dt, I find that the inrush current must be well over 1A for a short amount of time. Could the LDO be damaged over time if there is high inrush current every time the LDO is enabled?

  • 0
    TI__Guru 55940 points

    Hi Marcus,

    During startup, the current limit is briefly disabled to allow for inrush current.  So long as the inrush current does not exceed the maximum current limit in the Electrical Characteristics Table, the device should not be damaged; however, it may not be a bad idea to try to limit the inrush current.  Is it possible to reduce the output capacitance in your application?

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Thanks for the quick response, Ryan. 

    It seems this maximum current limit of 1.5A is exceeded for a short amount of time. From this screenshot showing the output voltage, you can see the cursors show a slew rate of 488kV/s for a portion of the waveform. With 4.7uF of output capacitance, this would correspond to about 2.3A. Note: the voltage was probed at the output capacitor.

    Not all rail ramp ups look alike. Here's one with an even higher slew rate. This screenshot shows the input voltage as well.

    Given that the TPS76350 requires a minimum of 4.7uF output capacitor for stability, it does not seem like I can reduce the inrush current by reducing the output capacitance. Has TI heard from any customers about device failure due to inrush current like what I've seen?

  • 0 in reply to Marcus Bennett
    TI__Guru 55940 points

    Hi Marcus,

    Current limit is designed to protect the device from brief occurrences of excessive output current.  Consistently running a device into current limit may degrade the lifespan for the device.  When your output waveform flattens before continuing to rise, it appears that you have run into current limit.

    The interesting thing about your application is that you are using the minimum capacitance already; therefore, I would not have expected you to have nearly this high of inrush current.  Could you share what your load is? 

    Very Respectfully,

    Ryan