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LM5069: Capacitor value in the live backplane for Hot-swap chip protection

Nikhilesh Karanam
Prodigy 40 points
Part Number: LM5069

Hi,

I need some technical information regarding the part LM5069. It is mentioned in the datasheet "The capacitor in the Live Back plane section is necessary to absorb the transient generated whenever the hotswap circuit shuts off the load current. If the capacitance is not present, inductance in the supply lines generate a voltage transient at shut-off which can exceed the absolute maximum rating of the LM5069, resulting in its destruction."

Considering the maximum inductive load in our case is 24V 3A, What capacitor value are we talking here in the datasheet?

Thank you,

Regards,

Nik

  • 0
    TI__Mastermind 39460 points

    Hi Nik,

    Thanks for reaching out. 

    Minimum 1 μF ceramic capacitor should be placed along with a TVS diode (SMBJ30A or equivalent) at the input. 

    Please click on "This resolved my issue" if this post answers your query.

  • 0 in reply to Avishek Pal
    Prodigy 40 points

    Hi Avishek,

    Firstly thanks a lot for the response. But I still have the question uncleared.

    Is there a formula or a procedure to deduce the value of the capacitor on the live backplane? minimum 1uF is suggested by you. But how do we come to an ideal number of capacitance?

    and another one,

    For eg., consider there is a PCB-A which is the live backplane and a PCB-B with the Hot-swap controller on it.

    Here is the Hot-swap circuit that is on PCB-B.

    In this case we already have the 24V TVS diode on the PCB-B.

    Do you still recommend to have the capacitor (1 uF suggested by you) along with the SMBJ30A  TVS diode again on the PCB-A (Live backplane) ?

    Hope i am clear. Please let me know. Thank you

     

    Regards,

    Nik

  • 0 in reply to Nikhilesh Karanam
    TI__Mastermind 39460 points

    Hi Nik,

    Please see the below screenshot regarding the input capacitor selection. 

    TVS diode and the decoupling capacitor should be placed as close as possible to the VIN pin of the hot swap controller. 

  • 0 in reply to Avishek Pal
    Prodigy 40 points

    Hi Avishek, 

    Thanks for the details. So C IN Input capacitor alone serves the transient absorption function? or please check the below figure 

    should a capacitor as shown above is necessary on the live backplane too? 

    My actual intended question was regarding this capacitor highlighted in the figure on the live back plane but not the input capacitor of the hotswap controller itself. Thanks. 

    Regards,

    Nik 

  • 0 in reply to Nikhilesh Karanam
    TI__Mastermind 39460 points

    Hi Nik,

    1μF ceramic capacitor needs to be placed at the input of the LM5069.

  • 0 in reply to Avishek Pal
    Prodigy 40 points

    Hi Avishek,

    All right. I get your point. The C-IN and TVS diode Z1 are to be placed as shown in the picture you added.

    But my question was regarding the capacitor on the live backplane. Please check this screenshot below from the datasheet and respond. Thank you.

    Regards,

    Nik

  • 0 in reply to Nikhilesh Karanam
    TI__Mastermind 39460 points

    Hi Nik,

    Yes, a bulk capacitor should be there on the back plane. The value is decided by the PSU or back plane manufacturers.