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# LM34914 maximum switching frequency

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?

• 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

• Fred,

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

Terry

• 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

• 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.

• 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.