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NE555: Output frequency variation with respect to calculated value

DEBPRASAD DAS72
Prodigy 60 points
Part Number: NE555
Other Parts Discussed in Thread: LMC555, TLC555,

Hello,

I have selected NE555PSR for my project from TI. Basically application to generate 12V ,90 KHz frequency output with duty cycle 81.21.

To achieve the above condition I have used Ra=10K (1% tolerance), Rb=3.01K (1% tolerance) and C1= 1nF(5% tolerance).

As per theoretical calculation the output value is 90.075 KHz and duty cycle 81.21 with the said components value. But in real time in the circuit practically while I was verifying I got output frequency approx 65 KHz. and duty cycle approx 81. 

I am not understanding, why there is this much deviation of frequency output from theoretical value to practical values??

Kindly suggest me how can i achieve the expected output frequency.

Thanks in advance.

DEBPRASAD

  • 0
    TI__Guru**** 164436 points

    Debprasad, 

    90.075 kHz has a lot of digits which suggests the needs for a high accuracy clock. The NE555 timer and the improved TLC555 and LMC555 timers cannot provide this accuracy.

    The data sheet formula is simple and works well for low frequencies. 90kHz is close to the 100kHz maximum practical limit.

    For higher frequency, I get better first result using this modified formula for choosing resistors and capacitor.

    The denominator becomes 113 kHz target to get 90kHz result because propagation delays are not considered in simple datasheet formula.

  • 0 in reply to Ron Michallick
    Prodigy 60 points
    Hi Ron,

    Thanks for the reply..
    As per your suggestion, I have calculated the resistor values and implemented the same on the 5 boards and I am able to get approximate 90 KHz frequency output practically, which is fine for me.

    But Before implementing your suggestion, Just for curiosity I have replaced TI make NE555 with ST make NE555 and I have observed a huge difference in output frequency.

    In TI make NE555 the output frequency is approx 65 KHz instead of calculated value 90.075 KHz , whereas in ST make NE555 the output frequency is approx 82 KHz which is somewhat close to calculated value.

    What would be reason for this much difference in output frequency ?? Will different manufacturing batch component can give different frequency output??

    Thanks & Regards,

    DEBPRASAD
  • 0 in reply to DEBPRASAD DAS72
    TI__Guru**** 164436 points
    Debprasad,

    It is normal to have a large difference in frequency when switching manufacturers because the designs are different. However, for a 9 kHz application both NE555 would be nearly the same frequency. The difference becomes bigger when used near the maximum NE555 frequency.

    The device to device variance will be much less than the manufacturers to manufacturer variance. For the least variance, it is best to start with a faster (higher max freq) timer. For example the TLC555 (2 MHz) and the LMC555 (3 MHz) timers.
  • 0 in reply to Ron Michallick
    Prodigy 60 points
    Hi Ron,
    Thanks for the support :)