according to the website, the 555 timer has a maximum frequency of 2MHz. Why is it not less or more? What property of the timer makes the limit of the frequency?
Website:http://groups.ist.utl.pt/lee/SUBA/Suba_files/555/555ncsu.htm
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2\$\begingroup\$ Not a single property. What does your car not go faster than x km/h? Is it the design of your motor pistons? Or is it the aerodynamics? Or is it the maximum throughout of the carburetor? Or is it the weight of the car? The friction in the bearings? The resistance of your tires? It's all factors combined. \$\endgroup\$– mmmmApr 5, 2021 at 9:22
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3\$\begingroup\$ The most straightforward one for CMOS 555 timers is the maximum rise/fall times of 75ns/60ns, being approximately 1/10th the 500ns period associated with 2MHz. Under those conditions ~1/5th of your output pulse is rise, ~1/5th is fall, and only 3/5ths is the high level. \$\endgroup\$– DKNguyenApr 5, 2021 at 9:24
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1\$\begingroup\$ Only newish 555 timers can do 2MHz. The fanciest ones can get over 3 MHz. Older ones can't even manage 1MHz. \$\endgroup\$– K HApr 5, 2021 at 9:24
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1\$\begingroup\$ This one can do 3 MHz. Next year one may come out that can do 5 MHz. \$\endgroup\$– Andy akaApr 5, 2021 at 9:26
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3\$\begingroup\$ Because it was designed in the 1960s using genuine 1960s transistors. \$\endgroup\$– user16324Apr 5, 2021 at 11:52
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1 Answer
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There's no single answer to your question. It's a mix of the maximum speed of the transistors involved, limits put by maximum currents you want to carry on an IC, the parasitics you can't wish away in the olden packages that 555s are often sold in, the fact that for higher speeds, you'll need smaller capacitors, which means they are less large compared to inherent capacitors.
Also, the fact that if you're building something very fast, you probably won't be happy with what the 555 can do, anyway, so there's little demand backed by actual money that would justify spending millions on devising a higher-speed 555, doing the analog design, taping out that chip, and finally manufacturing it. Where there is no real demand, no products emerge.
In short: the 555 idea is limited by many things, and what it is in your specific use case will depend. If you need higher frequencies than 2 MHz, my bet is that you should be asking a different question, namely how to achieve what you want to achieve through the 555, without t trying to use a 555.
