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LMC555: Maximum Timer frequency at 3.3V supply

Parin Chheda
Prodigy 100 points
Part Number: LMC555
Other Parts Discussed in Thread: SN74LVC1GX04

Hello,

I am using LMC555 in astable configuration to generate a 2MHz square wave. However, I observed that the maximum I am able to go is approximately 1.6 Mhz, with a very low (~50pf) timing capacitor.

I am working at 3.3V and the output sees a maximum load capacitance of 10nf. I have made the circuit on pcb (used a bypass cap across Vcc and Gnd).

I want it to know whether at 3.3V with light capacitive load, the IC will be able to generate a 2Mhz wave?

Also, please point out if you see any particular issue why am I not getting a frequency output of 2Mhz. 

  • 0
    TI__Guru**** 156686 points
    Parin,

    Are you using a similar setup as figure 2 in the data sheet. What value did you chose for the resistors.
    Try 390pF with figure 2 resistors, but you may need to lower the timing cap closer to 200pF

    3MHz is the typical maximum a-stable frequency. So 2MHz is really close to the maximum which is only a typical spec.
    If a high accuracy 2MHz is needed or duty cycle is much different than figure 2 provides then LMC555 is not the right choice.
  • 0 in reply to Ron Michallick
    Prodigy 100 points

    Thank you for your reply.

    I was using RA= 1.5 k and RB = 2.7K, after setting up the circuit as shown in the data sheet, I am indeed getting a 2Mhz squarewave. However, I find that there is some slewrate issue near sharp high transitions.

    if I use the circuit config mentioned in figure 2 for 2 Mhz with precision ressitors, will I be able to get relatively accurate frequency (say within +/- 50ppm or so).

    If not, what will you suggest as an alternative. I was also looking at using a crystal along with TI's crystal oscillator driver as an alternative.

  • 0 in reply to Parin Chheda
    TI__Guru**** 156686 points
    Parin,

    Can you provide the output waveform so I can see slew rate issue. The on resistance of the positive (source) driver is weaker than the on resistance of the negative (sink) driver. So I would expect a slower P_slew while driving capacitance.
  • 0 in reply to Ron Michallick
    Prodigy 100 points
    Sure, I will go to the lab and post it tomorrow. Thank you for your assistance appreciated!
  • 0 in reply to Ron Michallick
    Prodigy 100 points

    The probe capacitance is the only load. The probe has a bandwidth of 60mhz while the scope 200 Mhz

  • 0 in reply to Parin Chheda
    TI__Guru**** 156686 points

    This waveform seems to slew slowly. The VOH level looks odd. Does 3.3V supply have 1.6MHz ripple?

    Here is my waveform.

    Load is about 30pF total and 10 Mohm

  • 0 in reply to Ron Michallick
    Prodigy 100 points

    Oh!

    Can you tell me the resistor values you used?

    The source may have ripple! I will try with a battery supply and post here.

  • 0 in reply to Parin Chheda
    TI__Guru**** 156686 points
    Parin,

    I didn't check the values, I somewhat randomly tried some of the passive in my 555 parts bin and I got 2 MHz quickly. The cap was just the test board and socket , roughly 20 pF.

    For variance in frequency once the value are picked will be capacitor tolerance and shift in propagation delay which is not a data sheet parameter. Clearly a crystal based oscillator much tighter frequency variance.
  • 0 in reply to Ron Michallick
    Prodigy 100 points

    Hmm, I see.

    I am making a pcb for both 555 and Crsytal. I am using a crystal along with SN74LVC1GX04 (crystal oscillator driver), works fine.

    By the way, I also used the same crystal oscillator driver in another project which required a 20Mhz clock, worked flawlessly. 

    I will post the results of the 555 frequency once I am done. Thank you for your help!

     

  • 0 in reply to Parin Chheda
    TI__Guru**** 156686 points
    Parin,

    I will mark your post as "TI thinks resolved". Feel free to post your results.
    The SN74LVC1GX04 is cool device that is optimized for crystals. Previously many have used the regulator inverter for crystals but the SN74LVC1GX04 is better suited.