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LM3481-Q1: Frequency setting tolerance

Part Number: LM3481-Q1
Other Parts Discussed in Thread: LM3481


I'm considering LM3481 as controller for my flyback application and my query is with the frequency setting tolerance over temperature.

From datasheet, I see the frequency can vary by +15% over the operating temperature range -40C to 125C.

My desired frequency is 140kHz and the Rfa is chosen as 151 k.Ohm. During tests (on same PCBA, with same test conditions), I measure the switching frequency as 123kHz & 136kHz with two different LM3481 parts, at room temperature which has deviation of about -12% & -3% respectively from the desired value.

Is this normal behaviour from LM3481? In other words, is that the frequency setting tolerance + 15% not distributed (or spread) across the operating temperature range and it can occur at 25C as well?

Awaiting your quick feedback!! Thanks in advance!!

- Chaitanya K

  • Hello Chaitanya,

    I expect you used the same board and exchanged only the device. Otherwise the resistor value itself would influence the accuracy of the frequency for example.

    Please check out Figure 8 in the datasheet. As you can observe there the temperature behavior of the frequency is relatively stable.

  • Good Morning Brigitte,

    Thank you very much for your quick response!!

    Here I confirm that only the device is exchanged between the tests and rest of the test conditions (like test PCBA, input voltage, load, ambient etc) remain same.

    From datasheet, Figure 8 (on pg. 8) shows that the frequency setting is flat over the temperature, but the section 6.6 Electrical characteristics (on pg.5) tells that "Nominal switching frequency" varies over the temperature : (in fact I've calculated 15% tolerance from these numbers only)

    It seems confusing that both the statements are contradicting each other. Could you please help me  understand the frequency deviation if I'm missing some other details? 


    Chaitanya K

  • Hello Chaitanya,

    Section 6.6 might be misleading because one line only gives the typical value and the other one gives the min and max values.

    Based on Figure 8, the part to part variation seems to be higher than the temperature variation. I will try to get more information, but will need some days for it.

  • Hi Brigitte,

    Thanks for your efforts. I'll await your answer.

    - Chaitanya K

  • Hello Chaitanya,

    As mentioned above, the temperature variation is limited, but the frequency range is given in the electrical characteristics table with roughly +/-15%.

    +/-15% should not fully be seen at 25C, but the spread you have seen is within the tolerance given.

  • Hello Brigitte,

    Does this mean that frequency variation is due to the internal tolerance of each individual part and will be with in the +15% (approx.) even at the extreme operating temperatures? Please confirm.

    If yes, Can I ask that which factor(s) will cause this variation?


    Chaitanya K

  • Hello Chaitanya,

    The frequency variation is due to the accuracy of the internal oscillator, the input voltage as well as the temperature.

    If you need a more accurate frequency I recommend to use a synchronization signal from external.

  • Thanks a lot for all the support to my queries.