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LM5021: LM5021_PWM_frequency issue

Joey Lin94
Intellectual 360 points
Part Number: LM5021
Other Parts Discussed in Thread: PMP7917, PMP20278

Hi 

I has used the LM5021 to design a 10W DC / DC module, but the PWM frequency unstable.

Do you need to pay attention to the place（PCB Layout)?

 no loading ripple over spec, has any solution?

test point is Q1 Drain

Vin=12V Vout=5V Load=300mA 

Vin=12V Vout=5V Load=600mA 

  • 0
    TI__Genius 9130 points
    Joey,

    Can you include more schematic details, showing all the component values around the LM5021? What is the rated output current or power?

    In particular, what value are you using for Rt?

    Can you also include your PCB layout around the LM5021?

    Also the power stage, I assume this is a buck converter? What are the values of the inductance and output capacitance?


    It looks like the freq is ~460 kHz at 300 mA load, and is 2x larger ~910 kHz at 600 mA load. What happens if you increase the load slowly between 300 mA & 600 mA? Does the freq suddenly change at some load level?


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Intellectual 360 points

    Bernard

    sorry, because it is the company's products so inconvenience to provide a complete schematic.

    RT is 13.7k

     I assume this is a buck converter? it's Clamp Forward 1/4 Brick

    What are the values of the inductance and output capacitance? transformer is 6.2uH output cap is 47u//47u//100u

    What happens if you increase the load slowly between 300 mA & 600 mA?  is unstable not i set freq

    i has 2 project one is like i show freq too high one is too low i set 400khz, but practically is 200kHZ.

  • 0 in reply to Joey Lin94
    TI__Genius 9130 points
    Joey,

    From the LM5021-1 datasheet Fig. 7 & eqn. 1, with Rt= 13k7, Fsw should be ~ 484 kHz - which tallies with the waveform you show above for 300-mA load.

    At 600 mA, the freq appears to be doubled, which does not make sense.

    In between these load levels, you say the freq is unstable, what do you mean? Does it vary continuously with load, or is it random, not a stable fixed freq, at any given load? Can you add waveforms?

    There may be noise on the RT or VIN or VCC pins.


    If you want to send me the schematic and PCB layout outside this forum, you can email to me at bernardkeogh@ti.com


    I also had a look and there are 2 TI reference designs that may be useful to look at – PMP20278 & PMP7917 – both are low DC input, but targeted more at 36 V, and both are Flyback rather than Active-Clamp Forward.

    There are also many Active-Clamp Fwd TI Designs, but they would use one of the ACFwd control IC’s rather than LM5021.


    Thanks,
    Bernard