NE555P Monostable Shortest period possible

Felipe Neira

Other Parts Discussed in Thread: NE555

Hi all!

I am working on figuring out how an old circuit worked with very little information about why some of the components were implemented with respect to the final outcome.

On of the elements on the circuit is a NE555P part that is set up as a monostable timer with the following RC configuration:

R = 270 Ω

C = 33pF

According with some of my research found on basic electronics textbooks the period of the pulse is set by the following formula T = 1.1 x (RC)

However, I found online and and other sources recommend to use at least 1k resistors for this applications, and some of the data sheets and web information mention that the smallest period one can achieve with this IC is about 10 µs. My calculation show that the period I should get with the existing device is about 10 ns. Since we want to maintain the circuit intact I bread-boarded the same circuit, but what I get is a period of about 3 µs and nothing smaller than that.I tried different ICs but I get the same result. I tried to measure the existing circuit and I do not get a good read from the Oscope.

Any ideas why I do not get the theoretical value? and if I need to use that value how should I go about achieving that?

Thanks for your time and help on this question.

Best regards,

Felipe Neira

Here is the Picture of the circuit:

over 12 years ago

0 Ron Michallick over 12 years ago

TI__Guru**** 165236 points

Felipe,

Your picture did not insert correctly. I can't see it. The propagation delays of the NE555 will be much longer than the 10nS R and C formula. The results could vary device to device.

0 Felipe Neira over 12 years ago in reply to Ron Michallick

Prodigy 30 points

Ron,

Thanks for your replay. So you mean that period can not be 10ns with this setup. Then if this is the case what you would recommend.

- Felipe

0 Ron Michallick over 12 years ago in reply to Felipe Neira

TI__Guru**** 165236 points

Felipe,

Because you are using the RESET input as the 'trigger' you can get 10nS.
The formula won't be very accurate, but it is a good starting point.
If the pulse is too short, increase R2 or C4. If the pulse is too long, decrease R2 or C4.
There will be a lower limit when R and C don't make any difference (the 3uS you see now).

0 Ron Michallick over 12 years ago in reply to Felipe Neira

TI__Guru**** 165236 points

Felipe,

I'm sorry, I thought the circuit was a mono-stable. I now see that it is an astable oscillator gated (on/off) by RESET pin.
For this astable, Ra=270, Rb =0, C=33pF. I have never seen Rb zero before. It is allowed, however the frequency and duty cycle will have device to device variance.

Increasing R2 and C4 will increase the oscillation period.

0 Felipe Neira over 12 years ago in reply to Ron Michallick

Prodigy 30 points

Ron,

Thanks I will give it a shot and try a few settings. So far your answer makes sense to me. At the beginning I was wondering if it was originally an Astable, but I was not sure. It makes sense now.

Thanks again for your time.

Best regards,

Felipe

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NE555P Monostable Shortest period possible