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LM5150-Q1: LM5150's irush input current is too high when start-up

Fandy
Expert 2285 points
Part Number: LM5150-Q1

Dears

Vin=6V ,Vout=7.5V ,when start-up, there is transient drop in the input volatge of LM5150 Vin,so is the Vout;   meanwhile there is a comparator to compare Vin and 4V, when Vin decrease to 4V,then power supply will be replaced by Back-up Battery ,But this tranformation of power supply,will lead to my system re-start,and this is not allowable.

I find there is very big irush current flow into Vin when LM5150 boost start-up,maybe this is the reason lead to Vin and Vout dropping. the waveform is show as follow picture. 3ch(Blue) is Vin, 2ch (Red) is Vout, 1ch(Yellow) is Iin.

Question:A).If this is normal? : big inrush current Iin when LM5150 start-up,

               B)Is there any way to limit this inrush current? if it is valid by seting the RS resister?

               C).is there some way to make LM5150 start-up without transient drop of VIN/VOUT ?

  • 0
    TI__Guru 58391 points
    Hi
    A) I guess your power supply current limit is too low. It should be higher than ~12A to handle peak current when the LM5150 waking up.
    B) If you limit the peak current when waking up, you will see a big undershoot. It is a trade-off.
    C) Please check your power supply and increase its current limit to > 12A. Also, add a big bulk capacitor at the output of your supply if needed. It will help to minimize the inrush current which the power supply can see.
    Regards,
    EL
  • 0 in reply to EricLee
    Expert 2285 points

    Hi

    1、I set the power supply current limit exceed 12A , its also have big drop

    2、Even I added 300uF/470uF in the input of LM5150, it have no improvement

    what's the relation with RS and Limit Current? and how to set the limit curren ? by change RS ?

  • 0 in reply to Fandy
    TI__Guru 58391 points

    Hi

    Current limit and Rs relationship is described in the section 7.3.7 of LM5150 datasheet. Please look at eq6 and eq 7.

    Another way you can try is increasing the Rs. 

    Regards,
    Eric Lee 

  • 0 in reply to EricLee
    Expert 2285 points
    Hi
    For LM5150EVM ,set the 7m ohm of Rs for output 8.5v/3A ,I calculated the Ipeak_CL=16.7A , and EVM user guide adviced 20A limited current of power supply , so why reference design advised as low as 7m Ohm for Rs, why set the limited current for high up to 16.7A ?
    If I can set the 12m Ohm of Rs for limited the current lower than 10A ? and this change will affect any others ?
  • 0 in reply to Fandy
    TI__Guru 58391 points
    Hi
    You can find the C/L equations in the datasheet (See equation 27)
    User guide recommended 20A to have enough margin. Assuming peak current is 17A, 20A current limit setting will give you ~3A margin.
    If you increase Rs to 12mOhm, your maximum output current capability will be limited. Go to www.ti.com/.../snvc214 and input your requirements.
    Regards,
    EL