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LM5180EVM-S05: frequency and mode of operation

Part Number: LM5180EVM-S05
Other Parts Discussed in Thread: LM5181, LM5180

Tool/software:

Dear support,

i used calculator to help me with the design (see attachment) and then i did some tests on EVB.

As you will see, i have 5V input voltage and i need 3.3V: i changed flyback transformer and i mounted 750319117 (from Wurth Elektronik, i worked with their experts in order to select the best PN according to my application). According to calculator, when the load is 120 mA, switching frequency should be 350 kHz ( so i should work in DCM) but i am measuring 209 kHz actually (see picture below, where i show you the swithcing voltage on TP3).

Can you help me with this? 

Thank you.

BR,

Giorgio

 

0336.LM(2)518x PSR flyback converter quickstart tool revB.xlsx

  • Hi Giorgio,

    Thank you for considering our product.  In you calculator you choose the LM5181, while the EVM has LM5180.  According the the calculator, the EVM will switch in FFM at about 230kHz typical, and your 209kHz is within the tolerances. 

    Hope this clarifies. 

    Regards,

    Youhao

  • Hi Youhao,

    i forgot that EVB has LM5180 insted of LM5181. 

    I changed that in calculator and i obtained the following current vs frequency graph:

    In my setup i set Iload = 160 mA and i measured Vsw frequency: it is about 243 kHz and i would expect about 300 kHz as shown in the picture.

    Is this still between tolerances? If so, where can i get info about frequency tolerance? 

    Thank you.

    BR

    Giorgio

  • Hi Giorgio,

    I think it is within the tolerances. First, the main focus of the device is the output regulation, and the frequency is an intermediate parameter and it varies accordingly to regulate the output voltage.  This is why we characterize the Vref with min and  max limits, but not the switching frequency.  Regarding the frequency itself, it can easily have a +/-10% or maybe slighter higher tolerances.  On top of that is the transformer inductance tolerances of +/-10%, and I believe you noticed that the frequency change with inductance in the FFM and BCM, right?  To me, your frequency is not a sign of something very wrong.  243k is reasonable. 

    Regards,

    Youhao

  • Ok, i understood. 

    Thank you for the support.

    BR

    Giorgio