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LM555: Linearity Range of the LM555 Timer

Khoi Ly10
Intellectual 620 points
Part Number: LM555
Other Parts Discussed in Thread: TLC551, NE555, , LMC555, TLC555

Hi TI,

The LM555CN timer works based on the following linear relationship: F = 1/T = 1/(C*(R1+2R2)*ln(2)). I just wonder what the linearity range of this relationship is? In other words, what is the maximum and minimum capacitance and resistor values that allow the above equation to hold.

Sincerely,

Khoi Ly

  • 0
    TI__Guru**** 156686 points

    Khio,

    That formula works well for frequencies below 10 kHz. Above that will result in an error that increases with frequency mostly due to propagation delay from timing cap (1/3, 2/3 Vcc) to discharge output changing state.

    This formula and text from TLC551 data sheet is more accurate / informative. Ron = discharge pin on resistance.

  • 0 in reply to Ron Michallick
    TI__Guru**** 156686 points

    Khio,

    I start with this simplified equation for NE555 or LM555 astable timers to pick initial resistors and capacitor values.

    For application above 80 kHz or those that need more accuracy, I suggest TLC555, LMC555 (or TLC551).

  • 0 in reply to Ron Michallick
    Intellectual 620 points
    Thank you very much for your reply, does the timers you recommended me work with a very large range (about 100Hz to 10kHz)?
  • 0 in reply to Ron Michallick
    Intellectual 620 points

    Also, does it matter which capacitor or resistors I choose? Does it mean that as long as the frequency is within the timer's working frequency, any combination of C and Rs would give me a good result?

  • 0 in reply to Khoi Ly10
    TI__Guru**** 156686 points
    Khoi,

    All the timers listed will work well in the 100 Hz to 10kHz range and are pin compatible.
    The resistors used in "Figure 16. Free Running Frequency" chart are fine.
    Ra + 2*Rb total should be in the 1k to 10M range. In addition, I'd prefer that Ra > 470 ohms.
    Capacitors minimum over 100pF (>=1nF preferred) and no maximum.