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LP3990: Short-circuit protection

Carlo
Expert 7715 points
Part Number: LP3990

Hi team,

Good day.

It was mentioned in the datasheet that the LP3990 has thermal-overload and short-circuit protection. Regarding the short-circuit protection, our customer has the following questions.


1.To what time LDO output is ON to detect short circuit?
2.How frequently LDO output is ON to know short is removed?
3. By any reason if the output capacitor shorts then will LDO get damaged if they do not remove the short for few hours?


Regards,
Carlo

  • 0
    TI__Mastermind 24135 points

    Hi Carlo,

    For question 3, we do not recommend operating for long term in a protection mode as that can degrade the operating life of the part.  In this case the device will operate in current limit and that will result in power dissipation directly across the device.  This will cause the device to heat up, and may trip the thermal protection circuit.  At a minimum it will operate in elevated junction temperature.  While we don't anticipate the device will be damaged, operating in current limit will degrade the operating life of the device a little bit each time it enters this mode for a long duration.

    I took some measurements on an EVM to help answer your other questions.  First, I slowly ramped Iout to find where the current limit takes effect on this particular device.  It measured 449 mA.  Then I used an electronic load to rapidly slew the load current, and I measured the time it took the LDO to turn off Vout after Iout exceeded 449mA.  It measures about 15.5us.  When the short is removed, I did not notice any appreciable amount of time that it took the LDO to react.  The LDO is simply charging up Cout at the same time the short circuit is being removed.

    Channel 1: Vin = 3V
    Channel 2: Vout = 1.8V
    Channel 3: Iout

    Image 1: Locating the short circuit current limit threshold

    Image 2: Measuring the time from exceeding the short circuit threshold to disabling Vout

    Image 3: Measuring Vout when the short is removed

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 7715 points

    Hello Stephen,

    Thank you for a very detailed explanation. For question 2, how will LDO identify its output is removed from the Short circuit? It has to check by switching ON the output and see if still short exists or not? If so, then at what interval it will switch ON the output to see whether output short exists or not?

    For example, at 'T1' time LDO out is short-circuited, then within 20uSec LDO output is shut down to 0V. Now at some point in time, 'T2' LDO has to switch ON the output and check if the output is still short or not, what is the delta (T2-T1)?


    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 24135 points

    Hi Carlo,

    The operation of the LDO when the short circuit is removed is a little different than what you are imagining.  Let me try to explain, and let me know if you still have questions.

    When the LDO enters current limit, it limits the current to a specific value regardless of the load the output is trying to draw.  Here that value is just above 400 mA according to the measurement (the yellow waveform in the first oscilloscope plot).  Once the short circuit is removed, the LDO continues providing this current to charge up Cout.  You can see that happening in the last oscilloscope plot.  Once Cout is charged up to the setpoint voltage, the LDO current limit is disengaged and the LDO operates under normal operating conditions.

    Thanks,

    - Stephen