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LM3489 OCP application question

Other Parts Discussed in Thread: LM3489

Dear Sir


Our customer use programmable current limit IC LM3489, when our output current goes high and close OCP point, and vout will drop. but Vout can't reach to Zero.

If there have any way to cut Vout when we reach OCP point?

                                                                                       Best regards,

                                                                                                             Bogey

  • Hello Chen,

    Are you trying a hard short or just an overload condition?
    Depending on the overload condition the VOUT may/may not drop down to zero.
    Based on a preliminary viewing, one can probably design a latch up circuit which can pull down the Vout voltage based on the differential Rsense (or Rds,on) voltage, during an OCP condition, and with quite a few other external components. However I can not really think it will make much of a viable solution.

    Regards,
    Sourav
  • Dear Sir

     

    May we have formula to calculated how much over current will let Vout down  to Zero?

                                                                               Best regards,

                                                                                                  Bogey

  • Hi Sir,

    In overload condition, for example, if set Radj=14K ohm (Programming the current limit), the Iind_peak is about 2.8A (Vout is 12V). But when overload Iout is 2.9A and the Vout is 7.6V; Iout=3A ==> Vout=6.8V; Iout =3.1A ==> Vout=6.4V, is it normally at this condition? if want to set Vout =0V (turn off) at overload, how to do?

    thanks

    Regards,

    Driscoll Tsai

  • Hello Driscoll,

    When overloading the part, once the CL limit has been triggered, the output voltage should collapse to ~0V with a 9us CL off timer separating the SW node on time. However, before the CL is triggered, you should have a steady Vout. 

    Do you observe this 9us CL off-timer in the SW node at 2.8A? Is this the peak or the DC value? At what load do you see the Vout finally reaching ~0V? What is your CADJ values?

    It is possible for the exact current limit value to vary from part to part depending on the variance of the ICL parameter of the part and the Rds,on of the PFET / Rsense resistor.

    Is it possible for you to share the schematic or waveforms where you see this issue.

    On a sidenote, LM3489 there is an issue of CL foldback. Once the CL has been triggered, you may need to reduce the load current significantly before you have regulated output again.

    Regards,

    Sourav