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LM5123EVM-BST: The LM5123-Q1 EVM takes too long to restart after power off

Jeff Yun
Intellectual 2505 points
Part Number: LM5123EVM-BST
Other Parts Discussed in Thread: LM5123-Q1

Hi, Team

Hope you are doing well!

My customer used our LM5123-Q1 EVM to test the restart time after power off and found that the recovery time was too long.

Working condition: Vin = 11V, Vout = 15V.

Customer lowered Vin to 0V for 100us, then powered on again, and observed the time it took for Vout to return to the set value. It was found that it was as long as 4ms.

Is it possible that the output capacitance on the EVM board is too large causing this problem?

How to shorten the power-off restart time? 

Best Regards

Jeff

  • 0
    TI__Genius 10385 points

    Hi Jeff,

    Thank you for using E2E forums, regarding your query yes it is possible that due to the output capacitance on the EVM board being large it takes longer for the output to return to the set value. Also, the SS capacitor's size can be the main cause for such a long time, on the EVM this is set at 0.33uF, which brings the soft-start time to around 4ms as you said, so that is why you are seeing that much time for it to get back to the set value, if you want to reduce this time, I think SS cap is the right place to start with.

    One question, when the customer lowered the Vin to 0V, was the system still connected to a load or not?

    Hope this helps.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    TI__Intellectual 2505 points

    Hi, Haroon

    Yes, when the customer lowered the Vin to 0V, the system was still connected to a load.

    The load is lighting, customer hopes that users will not observe the light flickering during the power-off and restart process.

    Best Regards

    Jeff

  • 0 in reply to Jeff Yun
    TI__Genius 10385 points

    Hi Jeff,

    In that case yes this is more to do with the soft-start capacitor than the output caps, when you first startup the device it also takes 4ms for Vout to get to the set value, right?

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    TI__Intellectual 2505 points

    Hi, Haroon

    Customer didn‘t test the first startup time. I will let customer test it to confirm this speculation.

    Best Regards

    Jeff

  • 0 in reply to Jeff Yun
    TI__Genius 10385 points

    Hi Jeff,

    Ok, let me know what they find.

    BR,

    Haroon