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Original question:

LM74930-Q1: Fails to activate output if input/output capacitance ratio is insufficient

LM74930-Q1: instability issue during starting

Manuel Calmaestra
Prodigy 20 points
Part Number: LM74930-Q1
Other Parts Discussed in Thread: LM74930

Tool/software:

Hello,

we are using LM74930 on several designs and we are experiencing similar issues as explained on:  LM74930-Q1: Fails to activate output if input/output capacitance ratio is insufficient .

Weeks ago we open a case to share the issue and no solution was defined. This is the case we opened:  LM74930-Q1: Inrush Current / Voltage Instability During Startup 

It is very important for us to solve this instability issue as we are using this IC on several design for protection purposes, and we are experiencing similar issues on all the designs. If hold up capacitor is large enough, the system is not able to turn on because of it is on unstable operation during the starting transient.

It is crucial for us to find a solution, please, support us to define actions which lead us to find a solution.

Thanks in advance.

  • 0
    TI__Guru 54700 points

    Hi Manuel,

    I see VA is falling low which can mean DGATE is toggling and cause charge pump to fail.

    OR

    If VA is falling, maybe VIN is falling as well and hence causing UVLO to hit.

    Can we confirm if any of these is happening?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 20 points

    Hi Shiven,

    Finally we have found a solution. A couple of considerations to be taken into account:

    - It is very important Cin, Cout relationship. Cin needs to be several times greater than Cout. With Cin = 10uF and Cout = 2.2uF seems to be enough.

    - We have increased mosfet gate resistor to 100 ohms.

    This instability issue appears when hold up capacitance has a high value (in our design around 18mF).

    With these changes we have removed the instability and now the inrush current is correctly controlled. I think it is important to include these considerations in datasheet or design guideline, it was quite difficult and finding a solution took long time.

  • 0 in reply to Manuel Calmaestra
    TI__Guru 54700 points

    Hi Manuel,

    Thanks for letting me know.

    This hasnt been included in datasheet since it is a fundamental MOSFET issue and not related to controller. 

    But it will help me in future that CIN>COUT helps solve this.

    Regards,

    Shiven Dhir