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LM5122ZA: about restart

Guru 11170 points
Part Number: LM5122ZA

Hello E2E,

Our customer is evaluating that the 2-phase boost converter using the LM5122ZA.
When a low signal of about 80-usec is input to UVLO-pin, the circuit does not restart and remains stopped.
The datasheet said ”At the beginning of this soft-start sequence, allow VSS to fall down below 25 mV using the internal SS pulldown switch”.

Is it correct to recognize that Vss needs to be reduced to less than 25 mV for a reboot?
Is it okay to force the voltage down with an external circuit?

Regards,
ACGUY

  • Hi Acguy,

    yes, the voltage on the Softstart pin (softstart Cap) needs to be reduced to be below 25mV to ensure a softstart sequence. This can also be done with an external circuit.

    See datasheet: section 7.3.7 Soft Start

    Best regards,

     Stefan

  • Hello Stefan,

    Thank you for your reply. If Vss has voltage exceed 25mV when restart, could you let me know the reason why the circuit does not restart and remains stopped?

    Best regards,
    ACGUY

  • Hi Acguy,

    my assumption (without having additional measurements):

    When the converter starts up with the output caps discharged, a larger current needs to be provided by the output of the power supply to ramp up the application. This can then trigger an over current condition and enforcing the power stage to shut down again.

    Therefore the softstart circuit is required to ramp up the output voltage slowly and allow to ramp up the application with a limited current for the power stage.

    Best regards,

     Stefan

  • Hi Acguy,

    I have not seen an update on this thread for the last 23 days.

    If the issue is solved it would be great to press the is solved button.

    If there is still something open, just let me know with the new details.

    Best regards,

     Stefan