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LM5022: LM5022 Over current action

Yuchang Zhang
Mastermind 18510 points
Part Number: LM5022

Hi Team,

Could you please help check..

1, Follow DS description, when CS voltage gets over 0.5V, 5022 will get into over current protection, right?

But one customer found before CS got 0.5V, Vcomp got ~5.4V(max?) in advance, and then Vout goes down to lower, such as normal is 60Vout, but now is 58V.

and continue to increase loading, Vcomp is still ~5.4V, Vout will goes down to 57V..

Is this action right?

2, what will LM5022 do while Over current happen? decrease Vout to get into power limitation, or shut down power mosfet to make Vout off and then hiccup?

Thanks

  • 0
    TI__Guru 68495 points
    You forget to take into account the internal slope compensation on the peak limit. Please refer to the block diagram of the IC, and see there is internal slop compensation that is superimposed onto the sensed current signal at the input of the peak current limit comparator. The longer the duty cycle, the more is superimposed, reducing the effective peak current limit. The result is reduced duty cycle and hence Vout.
  • 0
    TI__Mastermind 21720 points
    Hello Yuchang,

    Can you please tell the input voltage specification for this application? I'm guessing that it is a large step up ratio. Please not that the maximum duty cycle for the device is 90%. Also the maximum COMP voltage is 5.2V typical. When at this voltage the device is already hitting maximum duty cycle and supplying all the energy to the output that it can. This means that the output voltage will decrease as load increases.

    When current limit threshold is achieved the MOSFET is turned off for the rest of the clock cycle. This is true cycle by cycle peak current limiting with out any hiccup mode.

    If the customer cannot share the schematic on E2E please send it to me directly though email.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    TI__Mastermind 18510 points

    Hi Guys,

    Thanks for your reply.

    Yes, I know current sense circuit will also affect slope compension as well, customer follow equations 36~40 at page21 to set RS1/2 and Rsense, but can't get OC happen normally. and The application is 24Vin to ~60V, so duty cycle will not over the max duty, that's why customer wonder why comp gets max value ahead of VCS gets 0.5V when big Iout happen.

    Do you have OC test waveform to share? include VCS, VCOMP, VOUT, IOUT, OR IL...?

    I will check with customer if can share schematic.

    Thanks,

    Yuchang

  • 0 in reply to Yuchang Zhang
    TI__Mastermind 18510 points

    Hi Garrett,

    Attached schematic, please help review if there is any value need to change.

    and also could you please help reply my questions in last post?

    thanks.

    Yuchang

  • 0 in reply to Yuchang Zhang
    TI__Mastermind 21720 points
    Yuchang,

    Please provide the maximum load current the that application requires. I want to double check the calculations.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    TI__Mastermind 18510 points

    Hi Garrett,

    Application is 60V 0.4A, maybe I think OC can be set at around 0.8A.

    Thanks

    Yuchang

  • 0 in reply to Yuchang Zhang
    TI__Mastermind 18510 points
    Hi Garrett,

    Sorry to bother you,
    Could you please provide your comment for schematic?
    and also is there any OCP test waveform to share?

    Regards,
    Yuchang