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LM61460-Q1: why high side current limit is higher than low side current limit

Elec Cheng
Expert 7055 points
Part Number: LM61460-Q1

Hi Expert,

I have a confusion in LM6146--Q1 datasheet. why high side current limit is higher than low side current limit. I thought the current flow the same between high side and low side. Am I understanding wrong? Thanks.

BR,

Elec Cheng

  • 0
    TI__Mastermind 28660 points

    Hello Eric,

    The HS current limit senses the peak inductor current and the LS current limit senses the valley of the inductor current. 

    When the HS limit is engaged it turns off the HS FET and turns on the LS FET.  The LS FET will remain on until the LS valley limit has been reached. The current in the HS FET is different than the current in the LS FET.  Current in the HS FET is typically a positive going trapezoidal waveform and the current in the LS FET is a negative going trapezoidal.

    Hope this helps?

    David.

  • 0 in reply to David Baba
    TI__Expert 7055 points

    Hi David,

    So the high side current limit means the max current, and the low side current limit means the min current on the inductor, right? I understand how the how the max current limit works, but I don't really understand how the min current works. What if the output current is 1A, which is far below the min current 7.1A. As I learn, 61460 use peak current control. May you please explain more about the low side current limit? Thanks.

    BR,

    Elec Cheng

  • 0 in reply to Elec Cheng
    TI__Mastermind 28660 points

    Hello Elec,

    That is correct, the HS and LS current limit levels relate to the peak and valley currents in the inductor respectively.  this relates to the output current as shown below.

    Output current = [HS Limit - LS limit]/2 + LS Limit 

    Hypothetically, from the typical values in the table above assuming hiccup and additional duty cycle protection are not implemented, if the HS and LS limits were to engage the output current would be 

    (10.3A-7.1A)/2 + 7.1A = 8.7A. 

    as you can see, if you only had 1A output current the current limit circuits would not engage.

    Hope this helps?

    David.