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LM2587: About the current limit when FBpin is GND.

Kengo.Y
Guru 11165 points
Part Number: LM2587
Other Parts Discussed in Thread: LM51551

Hi team.

I'd like to know about the current limit for switch pin.

My customer refer to snva005b(p.7) and configure the flyback.

When FB pin is GND, does the NPN Switch(4pin) over current protection work?

According to snva005b, it says that the short circuit protection function does not work.

Sincerely.

Kengo.

  • 0
    TI__Mastermind 31015 points

    Hi Kengo,

    The short-circuit protection for LM258x parts is dependent on the part first exiting soft-start mode, and then entering into short-circuit protection mode. Because the part is in flyback configuration, the feedback pin is already pulled to ground, so the part will be in soft-start mode upon start-up and never exit this mode. Therefore, short-circuit protection mode is never entered.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Guru 11165 points

    Hi Richard.

    Thank you for your reply.

    I understand about the short-circuit protection.

    I'd like to know about the over current protection for NPN transistor.

    When FB pin is GND, does the NPN Switch(4pin) over current protection work?

    Maybe overcurrent protection and short circuit protection are the same?

    Sincerely.

    Kengo.

  • 0 in reply to Kengo.Y
    TI__Mastermind 31015 points

    Hi Kengo,

    Sorry for the misunderstanding. 

    Per the datasheet, in a short-circuit condition, the device is save since the energy in the transformer will be dissipated before the switch turns back on when the frequency is reduced. Therefore, cycle-by-cycle current limit will limit the peak of the inductor current. 

    If FB is grounded, there is no frequency reduction mechanism, and the inductor current will be limited by cycle-by-cycle current limit. 

    This may not be too helpful since components may heat up very quickly and become damaged. In this case, I would recommend the LM51551, which has hiccup mode enabled. Please refer to the LM5155-flyback EVM for evaluation if needed.

    Thanks,

    Richard