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LM34914 maximum switching frequency

Fred P
Expert 2050 points

Datasheet states in the FEATURES section : « Maximum Switching Frequency: 1.3MHz ».  However, equations 5, 6 and 9 can yield a switching frequency much higher than 1.3MHz while respecting the minimum off-time of  265ns.

 Moreover, Quick Calculator spreadsheet for this part gives a maximum frequency that can go beyond 2MHz depending on input voltage. 

 What’s the matter with that 1.3MHz maximum frequency?

  • 0
    TI__Mastermind 23825 points

    Hi Fred, 

    Yes, you are correct. After looking at the datasheet, the maximum frequency seems to be able to reach beyond 2MHz. By calculating the frequency based on the minimum on time at 40V (~200ns) and minimum off time (~265ns). We'll forward this question to our internal apps time to find the discrepancies. 

    Thanks

    -Arief

  • 0
    TI__Expert 3535 points

    Fred,

    Lets start out with you telling me what is your minimum and maximum VIN, and your desired output voltage?.

    Terry

  • 0 in reply to Terry Allinder
    TI__Expert 3535 points

    Fred,

    At the minimum input voltage, the maximum switching frequency  is restricted by the forced minimum

    (Toffmin) time.

    fswmax=1-Dmax/Toffmin

    At maximum input voltage, the maximum switching frequency  is restricted by the minimum (Tonmin) time.

    fswmax= Dmin/Tonmin

    Terry

  • 0 in reply to Arief Hernadi
    TI__Intellectual 2320 points

    Hello Fred,

    If you take into account the min off time and  the min on time in the same equation (1/(Ton,min + Toff,min)) you will get the ideal internal oscillator frequency much higher than defined in the DS.

    But, in our practical application, we need to take into account the minimum input voltage (8V), Toff,min (265ns) and the minimum regulated output voltage possible (~5V) which constrains the maximum achievable operating switching frequency.

  • 0 in reply to Sourav.Sen
    Expert 2050 points

    That corner case gives something like 1.4MHz, which can be considered near enough the 1.3MHz...

    That being said, if one respects the minimum on and off time constraints, please confirm that everything will work fine for any resulting frequency.

    Regards,

    Fred P.