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LMC555 operation above 100kHz

Ed Walker
Prodigy 5 points
Other Parts Discussed in Thread: LMC555

The LMC555 datasheet shows maximum frequency is 3MHz.
However Figure 10 Free Running Frequency stops at 100kHz.

We are seeing substantial error when trying to program using the ds equations:
tHigh 2.59us
tLow 90us
F=372kHz
Ra 7.68k, Rb 274, C=470pF
D=97%

In all cases we see substantial error, measured.
tHigh 3.75us
tLow 250ns
F=250kHz
D=93.75%

Based on the lack of a graph above 100kHz, I surmise that the device is very NON LINEAR above 100kHz and the equations do not accurately reflect the device operation.

Please provide some guideance for using this part above 100kHz, remember the ds claims 3Mhz operation.

 

  • 0
    TI__Guru 61796 points

    Hello Ed,

    Yes. The graphs and equations (and models to an extent) are "ideal" and assume operation at full voltage and less than 100KHz operation. Things fall apart quickly above 100KHz. Even the "3MHz test circuit" values do not calculate.

    The biggest contribution seems to be the asymmetrical output impedance, which gets much worse at low (<5V) supply voltages. Since the output is used to charge/discharge the timing capacitor, the output impedance becomes a series resistance with the timing resistor. This can be an extra 150 to 2Kohms. Sourcing is about twice as worse as sinking. Adding an additional output load (R and C) does not help things.

    Yes. It is capable of 3MHz operation - even more, but the output waveform is not symmetrical and is quite ugly if you have seen it (it is NOT square...). They never said it was pretty...

    So it comes down to trial and error selection for the values in the application, since there are so many variables involved (Load R, Load C, supply voltage, temperature)

    So the advice is to minimize the value of the timing capacitor and maximize the value of the timing resistor to keep the charge currents down. Keep the timing cap above 50pF (to swamp strays), and the timing resistor above 1K to swamp out as much of the output Z as possible.

    If possible, use the Discharge pin as the load output (as suggested in Fig 16). This isolates the load from the timing devices.

    Power supply needs to be as stable as supply variation has much more influence at the higher frequencies.

    Regards,