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Original question:

TLC555: Output frequency not fix

LMC555: astable mode doesn't work exactly,

Sizhen Bian1
Prodigy 35 points
Part Number: LMC555
Other Parts Discussed in Thread: TLC555

Hi, I'm using LMS555 in astable mode. There are three questions confusing me:

1,  In the datasheet, Figure 7 astable waveforms, according to the given resistors and capacitors, the output frequency should be 6.58 kHz, namely 152 us per period. But from the Figure 7, each period takes around 68 us, so is the figure here wrong, or I missed something by calculating.

2, I use Ra = 10K ohm, Rb = 100K ohm, C = 10 pF, hoping to get an 685.7kHz output, but indeed I get an output of  around 324.5 kHz, So where is the problem?

3, When I remove the C, put nothing between thres pin and ground, there is still an output of around 707 kHz, so is there still a charging and discharging object even I remove the C?  

Interesting is that, if we calculate this "virtual" charging/discharging object, it seems the value is around 9.7 pF, this seems an "answer" to the question 2, when the true capacitor(10pf) and this "virtual" capacitor(9.7 pf) are combined. Is there something to do with the stray capacitance? (which I think usually at level of fF on a PCB board). 

Can someone give me some clue? 

BR,

Bian

  • 0
    TI__Guru**** 164586 points

    Hello Bian,

    The threshold and trigger inputs have some capacitance. I'm not sure of the exact value but the similar TLC555 states typical is 2.1pF. So LMC555 inputs are likely close to that; about 2pF each.

    690 kHz is high frequency so it needs more current, try 2k,10k, 100pf instead. The simple formulas do not include the effects of propagation delay. For high frequency, some fine tuning of the component values may be needed to get desired frequency.

     Also see the a table timer FAQ.  https://e2e.ti.com/support/clock-and-timing/f/48/t/879112?tisearch=e2e-sitesearch&keymatch=lmc555

  • 0 in reply to Ron Michallick
    Prodigy 35 points

    Hallo, Ronald

    Thanks for your information.

    1, I didn't take the current into consideration, will try resistors with smaller value tomorrow. 

    2, I choose LMC555 since the datasheet says it can reach the industry's fastest astable frequency of 3 MHz, so 690 kHz shouldn't be a large output. Let me try different resistors' value, and let's see what will happen. 

    3, The Figure 7 in the datasheet shows an inexact waveform, right? Just want to make sure that I didn't make a wrong calculation. 

    The link you cited gave me a lot help. Many Thanks.

    I will update later. 

    BR,

    Bian

  • 0 in reply to Sizhen Bian1
    TI__Guru**** 164586 points

    Bian,

    I never noticed before, but the figure 7 waveform is incorrect. TH should be 89.4us and TL 62.4us

  • 0 in reply to Ron Michallick
    Prodigy 35 points

    Hi, Ronald

    Today I tried the resistor values like 10k and 1k, and capacitor value like 100pf, unfortunately the output is 455 kHz, still not the expected 685 kHz. 

    So I guess the reason for this inexact reason is either the input capacitance of Thresh and Triger pins ( around 2 pF for each), or the propagation delay. As you pointed out yesterday.

    To get an expected output, a fine tuning is really needed.

    Thanks for your help.

    BR,

    Bian