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LM5155-Q1: On time and light load confirmation

Songzhen Guo
Intellectual 2840 points
Part Number: LM5155-Q1
Other Parts Discussed in Thread: LM5155

Hi Expert,

LM5155-Q1 is under evaluation on customer side. There are two parameter need confirm with you:

1. for the minimum on-time, there is a typical value, but it doesn't show the max and min value for minimum on-time. Could you check it for us?\

2. For the light load application, datasheet mention that LM5155 will enter pulse skipping mode for light load. Could you share more information or test result for this function?

For the test result on customer side, the PWM frequency is keep changing in light load mode. Thanks

Best Regards

Songzhen Guo

  • +1
    TI__Mastermind 22510 points

    Hi Songzhen,

    Thank you for using the e2e forum.

    The minimum on-time is strongly dependent on the switching frequency of the device, which is set via the Rt resistor. If the switching frequency varies between its min and max value, this also has a direct effect on the minimum on-time, even if Rt is set to a constant resistance.
    I would recommend to check the frequency (and their margins), and compare the minimum on-time values accordingly.
    A graph in the datasheet on page 9 shows the minimum on-time versus switching frequency.

    Additionally the fsw can be calculated as described on page 21.

    Regarding the pulse skipping function:
    In general, when the load of the device gets so low, that even minimum on-time pushes too much power to the output, the devices starts to skip pulse cycles, so the output voltage stays constant and does not rise.
    The number of pulses that are skipped, is not fixed and can vary depending on the load. Maybe this is the 'change in PWM frequency', the customer mentioned.
    During pulse skipping mode, the output voltage ripple may increase, as the device only triggers single pulses, if the output voltage falls to much.

    I hope this explanation is helpful.

    Please let me know if there are additional questions.

    Best regards,
    Niklas