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LM5155: Q1

Rakshith Manjunath
Prodigy 30 points
Part Number: LM5155
Other Parts Discussed in Thread: LM5156

Is there a way to force the controller LM5155 to work in constant PWM mode and avoid getting to pulse skipping mode operation? On various units having the same design with LM5155 I am noticing some units where the controller works in pulse skipping mode and some units working in constant PWM mode, input and load condition is the same. Vin = 14V , Vout = 55V, Iout = 1A , Cout = 4400uF, Iin(limit) =3A, Lm = 10uH and Fsw = 1.8MHz. What could be tweaked in the design to ensure constant PWM operation? We are facing EMC issue when the controller enters the pulse skipping mode. 

  • 0
    TI__Genius 12515 points

    Hi Rakshith,

    Constant PWM operation will lead to OVP at light load. And I do not think pulse skipping would cause EMI problem.

    pulse skipping should occur only at light load. You might be facing other problems. Please show some waveforms with operating condition.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 30 points

    Hi Feng, 

    What I find strange is that the controller works in constant PWM mode when the load is not applied on the output but when the load is applied the controller starts to work in the pulse skipping mode. Wouldn't the constant PWM when the load is disconnected lead to OVP? 

    There could very well be something else causing EMI issue. Nevertheless, we are trying to use Synchronization frequency signal coming from a MCU to do spread spectrum modulation to bring the peak emission down. I am wondering how well the controller keeps up with frequency shifts on the SYNC pin, we are planning to do +/- 40KHz at 10KHz steps every 10mS. Would you think this is feasible to do? My concern is if the controller locks on to one frequency sync signal and does not change the Fsw with change in the sync clock. Please advise.

    Thank you for your assistance on this.

  • 0 in reply to Rakshith Manjunath
    TI__Mastermind 45321 points

    Hi Rakshith,

    so the LM5156 would be another option as it has already DRSS included.

    For the LM5155 you can of course use the sync input and provide an clock signal from external.

    The controller will then use this signal as clock source if it stays within the specified limits.

    Best regards,

     Stefan