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LM51551: Discontinuous outputs

Part Number: LM51551


I am using LM51551 in my project application. Please find the schematic below. When I am applying VBATT=20V at 1V/30ms, I see that the outputs are discontinuous as shown. I have connected 80mA load at P8V output and 430mA load at P15V. Why is this behavior observed?

Thank you,


  • Hello Anne, 

    Thanks for reaching out. Could you please share the schematics and the specs of your design (Vin, Vout, Iout max, switching frequency)? Is there any condition when this behavior is not seen (e.g. in no load)? Can you please share as well the waveform of the switch node and FB pin?

    Kind regards, 


  • Hello,

    Thanks for your reply

    Vin = 20V, Vout = 8V and 15V

    Yes there is no such behavior observed in case of no load.

    As you see in the below image, VBATT is the input voltage VIN which is 32V and the outputs are stable at 8V and 15V.

  • Hello Anne,

    LM51551 operates in pulse-skip mode if the load is low. This mode increases efficiency as the converter does not permanently switch and reduce the switching losses.

    Which of your outputs needs the highest output power? Normally it is best to use this one for regulation.

    Best regards,

  • Hi Brigitte,

    Thanks for your reply.

    But the load is high at 80mA for 8V output and 430mA for 15V output. This is under maximum load condition at the output.

    How does it account for pulse-skip mode due to low load?

    Thanks and Regards,


  • Hello Anne,

    The load on the regulated output defines the device mode. The other output cannot be seen by LM51551. As you are regulating the 8V output, only 80mA load are to be regulated which means Power save mode.

    You should regulate the 15V output instead.

    Best regards,