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LT3090 startup vs LT3094 startup

In the current application (symm voltages of LT3065 and LT3090 +/- 14V out output Cout 1000uF/ low ESR) , the LT3090 misbehaves during startup.

After recycling of input voltage, normal ops. I found this behaviour described in the LT3090 datasheet pg 21.

The more advanced LT3094 has a "fast start-up" feature.

Does this "fast start-up" feature prevent startup misbehaviours like the above mentioned?

Thanks, Frank

  • Hi Frank,

    Do you mean the "overload recovery" of LT3090?
    If this is the behavior you are referring to, seems like it will be the same with the LT3094.
    Since the LT3094 datasheet also indicates the same information referring to "overload recovery" (on pg.28).
    Regardless of the fast start-up feature, if your application will meet the conditions indicated on that section of the datasheet, that "misbehavior" you are referring will still be present.
    This is due to the fold-back current limit protection.
    I think that the high value of Cout (1000uF as you've written) caused this behavior and triggering the current limit protection during start-up. A very high cap on the output will caused an inrush during start-up triggering the fold-back current limit protection of the LT3090.
    As indicated in the datasheet, a minimum of 4.7uF output capacitance is enough for stability.


    Thanks!
    -Ash 

  • Hi Ash

    thanks for your reply.

    Just wondering , if there is a way to keep the capacitance in the circuit.

    It s not fully clear to me , if the fold-back current limit protection kicks in and then always  latches til a power recycle.

    I was thinking of an antiparallel Zener (5V) between IN and OUT, that would load the caps to 5V below V in.

    So, there is an initial voltage difference of 20V , but the main loading current for the caps would initially run via the Zener/TVS diode. Not sure if this is enough to prevent the protection from kicking in ...

    According to LTspice simulation, this works. But  not sure if the SPICE moddeling is correct.

    Best

    Frank

  • Hi Frank,

    Since your output caps have a very high capacitance, that will translate into a higher inrush current during start-up.
    That cap will be charge in your initial turn ON that will pull excessive inrush current.
    Due to that inrush current, LT3090 protection will be activated.
    Since the LT3090 has a fold back current-limit protection, you will need to recycle the input voltage for another start-up.
    Now, since your output cap is already charge from the initial turn ON, the next start-up/recycling of input voltage will not activate the protection and will have a normal operation.
    Ic=C*dVc/dt : The higher your capacitance is, the higher the current on the caps will be (Inrush).

    You may want to see this article for reference on Inrush current. This is on switching converters but you can refer to it to further understand the effects of inrush.
    https://www.analog.com/en/analog-dialogue/articles/preventing-start-up-issues-due-to-output-inrush-in-switching-converters.html

    I really think that your output capacitance is causing this and is inevitable to your design.
    If you want to avoid this recycling process during start-up, you may want to lessen your output cap value as advised in the datasheet.
    OR if you prefer to keep your caps, think of a solution to charge the output caps BEFORE the start-up of the regulator to prevent the protection from activating.
    This is a protection inherent to LT3090 and other linear regulators to avoid any damage on the IC when such condition/phenomenon occurs.

    Thanks!
    -Ash