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Original question:

LM5176: Constant Current Loop ans SS pin for input/output Power Limiting

LM5176: Limit the input power

d_zero
Genius 4470 points
Part Number: LM5176

Dear Garrett,

we have been discussing on how to limit the input power of LM5176 with ISNS+ and ISNS-.

We want to achieve same limiting as with LT8390A and CTRL, ISP/ISN pins is this possible with LM5176?

After analyzing the block diagrams of booth controllers in order to achieve this feature we need to control the VILIM of the LM5176 is that correct, and is it possible to control the VILIM with ISNS+ and ISNS- or other pins?

Best Regards,

D.

  • 0
    TI__Guru 68645 points

    Hi d-zero,

    Thank you for reaching out.  If I understand your question correctly, you want to adjust the average current limit, right? VLIM is for cycle by cycle peak current limit. 

    For the averaged current limit, the threshold is internally fixed at 45mV for the LM5176.  Depending on what you really want to achieve, there should be an external patch circuit to change the effective average current limit with the LM5176.  Could you please clarify what do you really want to achieve?

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Genius 4470 points

    Dear Youhao,

    First we have adjustable Vout, and for every Vout we want have the same output power that is limited by input power of the source does that make sense to you?

    We designing a buck-boost that can give max output power, but if the power source is weaker than the buck-boost max out power, we want to limit the power of the buck-boost to the power that source can supply in order to avoid that the whole system shuts-down because of the weaker power source.

    Thus we think that if we additional adjusting the cycle by cycle is better than average, because the average has some delay to limit the power?

    Can you tell how many cycles does need the LM5176 to limit average power , and how is this dependent on input capacitance?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Guru 58225 points

    Hello David,

    If the drop across the sense resistor is greater than 50 mV, the gm amplifier gradually discharges the soft-start capacitor. When the soft-start capacitor discharges below the feedback reference voltage, VREF, the output voltage of the converter decreases to limit the input or output current.

    The gm of the pulldown amplifier is typically 1mS.

    So the time until the average current is limited does not depend on the input capacitance (besides the filtering effect these capacitors have on the inductor current), but on the capacitor you have connected to SS pin.