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LMC555: Timer

Charles T
Intellectual 335 points
Part Number: LMC555
Other Parts Discussed in Thread: TLC555

I try to make this timer circuit in bread board but i am not able to achieve the frequency. According to my calculation frequency comes around 1.2 MHZ . But i am able to achieve only 240 KHZ  in my bread board design. I am attaching the reference schematics . Astable mode in LMC555 Datasheet is different from what they have used here. Please help me to figure this out. One more this is that C306 - 4.7uF capacitor used is electrolytic one due to the unavailability of MLCC.

  • 0
    TI__Guru**** 165236 points

    Charles,

    If the diode was perfect, the LMC555 propagation delay was zero, OUT and DISCH were zero ohms when on, and the bread board had no parasitics then output would be 1.21 MHz . In other words, the simple math formulas indicate 1.2MHz. If it was a little slow and output low time was correct then I'd simply blame the diode voltage loss increasing the charge time (high time too long).

    240 kHz is a big miss so I suspect there is a bigger problem. Did you take reasonable care keep leads / wires short and direct. Double check R308, R309, C307 values.

    Measure OUT pin's: VOH, high width, VOL, and low width to help locate the issue.

  • 0 in reply to Ron Michallick
    Intellectual 335 points

    The values used are very close to the specified values in the schematics.  R308 = 4.4K ( 2 X 2.2K) , R309= 1.1K. and C307= 220Pf ( 4 x 56 pf in parallel). Checked all the wires and leads still the frequency is not improving. I have a reference brought out PCB board with same circuit, The only difference is that there they have used SOIC package IC. In that board the timer frequency is coming upto 725 Khz, but still not achieving the calculated value of 1.2MHz. 

    I am attaching the output waveform obtained in my bread board design. I suspect some other problem other than connection issues. 

  • 0 in reply to Charles T
    TI__Guru**** 165236 points

    Charles,

    In a perfect world your high time would be a short 183 ns and low time a long 643 ns. 1.2MHz 22% duty cycle.

    I put together a real world spreadsheet to select working values. I have a decent formula to do that taken from the previous version of TLC555 (a very similar product) data sheet. However the diode voltage loss and weak VOH drive (compared to DISCH drive) greatly complicates things. Nevertheless I believe I have a decent formula to get close.

    At the speed used, the propagation delay matters and CT will overcharge the 1/3 and 2/3 thresholds. So simple formulas are no good. charging voltage is far short of VCC and that changes timing significantly.

    Rows 1 - 30 show the modified formula and sanity checks the circuit board that achieved 750 kHz.  My math based on several assumptions predicts 878 kHz. That is 17% error, which may be in my assumption (and) or the circuit board capacitor is  higher than stamped 220pf claim, 275 pF actual.

    Row 33 - 46 are used to get desired timing. I picked a new 80pF capacitor. The resistor calculation depend on each other so I copied output resistor values into the assumed value in the other formula. After a few rounds, the numbers stopped changing.

    The end result is C307 = 80pF, R309 = 993 ohms, R308 = 7254.

    This will get you closer, one more round of changes may be needed after this.

    LMC555 is good to 3MHz however, tuning is difficult well below that frequency. Practical maximum is half of actual maximum.

    LMC555 circular.xlsx

  • 0 in reply to Ron Michallick
    TI__Guru**** 165236 points
    Charles,

    While breadboard seems like a good way to tune the LMC555 it won't be the same.