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Original question:

LM555: Asking for the edge detection and delay fucntion IC by LM555

LM555: LM555 application question.

Chen Bogey8
Mastermind 9990 points
Part Number: LM555
Other Parts Discussed in Thread: LMC555, TLC555

Hi Sir 

1.Attached circuit RA(R34) / RB(R39) is suit for our application?，Is there having limitation in resistor value?

2.Frequency =56KHz ，RB = 520R our resistor value will increase a little bit.is there have any impact?

3.The LM555 Pin5 floating is ok?or how to connect this pin?

                 e-rack_proximity detector_LM555 calc.xlsx                                                         

Bogey

  • 0
    TI__Guru**** 156686 points

    Bogey,

    The maximum sink current of the timer (discharge pin, output too) varies with supply voltage. Data sheet has a Pin 7 Sat spec 4.5 mA is 80mA typical, 200 mV max. So resistor RB should be greater than 2/3*5V / 4.5mA = 740 ohms.  For frequencies above 10kHz the data sheet formulas will result in slower than expected performance because the propagation delay is not included in the formula. 

    This is a better starting point to chose C, Ra, Rb: (2*RB+RA)*C = 1.44 / [ F * (1+F/350 kHz) ]. Even so the final value may need to be adjusted.

    TLC555 and LMC555 are faster device so the frequency error will be less. TLC555 even has a design tool that does include the prop delay. TLC555 Component Calculator