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LM51581-Q1: LM51581-Q1 IQ is too high

Part Number: LM51581-Q1
Other Parts Discussed in Thread: LM51581

I remade the PCB, but the problem still exists,When Input voltage is 12V,input current is 62mA,Its still very high。the SW node wave is below,Fs=2.2M。

It looks like do not input pulse-skipping mode  when load is light. So something is wrong,

  • I have collected waveforms for a longer  time and still haven't found any missing pulses

  • As a comparison, I also attach the image of TPS62932DRL who is entering the mode. When under light load, the waveform of SW only appears once for a long time.

  • The interval time between two SW wave exceeds 5ms

  • Hi Wenhe,

    Pulse skipping starts when the output voltage is still to high when the next switching cycle starts.
    As long as the output side is consuming energy more then with the minimum duty cycle does deliver on Energy, no pulse skipping is required.
    Load on the output is the some of: load, current through Feedback resistor divider, output cap leakage.

    So as long as the output is not shifted above the set target level the controller continuous to pulse the MOSFET.
    So the energy consumed by the feedback divider, leakage of the output Cap and the load if connected is larger the then the energy shifted to the output by one pulse. Where the minimum on time for 2.2 MHz is 80ns.

    Best regards,


  • I know what you mean, but now I don't think that the leakage current of the capacitor is too large to enter the Pulse skipping.becuese its below 1mA or lower. Please tell me how much current is less than when it will enter the Pulse skipping.

    thans so much!

  • By the way, LM51581 is a bit hot, which is not normal for no loads

  • And I found that the SW node is very sensitive. When I use the X10 probe to observe, the input current will increase by 3mA. When I use the X1 , the input current will increase by 26mA

  • Do you have any idea?

  • Hi Wenhe,

    The LM51581 should not get hot but as it is still switching it consumes the power to drive the MSOFET.

    In the case of the LM51581 there is no difference (as long as there is no skip mode) about the power consumption between light load and full load. It needs to drive the MOSFET and this load is determined by the switching frequency and the Gate Capacitance of the MSOFET.

    If just power consumption is the highest criteria - you might need to go to lower switching frequencies.

    The probe on the SW node will consume ~ 5.5mA with a 8pF probe. Just make the math.

    Best regards,


  • hi,stefan

    It makes me confuse that when there is no load or the load is very light, it should enter the  pulse skiping mode,Only enter PWM mode when the load is heavy.

    LM51581 Enters PWM mode with light load in my case so it is abnormal

  • Hi Wenhe,

    if with the shortest pulse the device does support (~ 80ns @ 2.2 MHz) more energy is delivered from the input to the output as the output side is consuming  in the time to the next pulse. The output voltage would increase above the set target.

    If this happens the pulse skip mode gets enabled.

    So if the leakage and load is able to consume this amount of energy then the device stays in normal mode.

    Just a rough math:

    12V *80ns *1/2 *1/100uH = Ii  = 4.8mA 

    Ii *12V /50V = Io = 1.15mA

    So if your output consumes > 1.15mA the device will not enter skip mode.

    (Note ~ 10% are already consumed by the FB divider)

    Best regards,