Question: NE555, TLC555 for company Zenitel, in Switzerland

Hello Mikhail,

Fundamentally, the NE555 and TLC555 relying upon the same threshold and trigger operating principles. The primary difference being the first is a bipolar transistor implementation and the latter a CMOS implementation. As expected operating current and node currents will be higher for the bipolar device. There may be some node current trade-offs at the highest operating temperatures. Also, the TLC555 will operate with the 3V3 supply level as required; whereas the NE555 minimum supply is 4V5 volts.

The TLC555 is supported by Precision Analog while the NE555 is supported by SLL. Therefore, I am not as familiar with the NE555's detailed operating characteristics. Since you intent to use the TLC555 in your application I can direct you to some helpful resources that will provide you with the most precise monostable operation.

About two years ago, a brand new, transistor-level simulation model was developed for the TLC555 by TI. The actual die layout sizings and process characteristics were incorporated in the simulation model. This has resulted in a precise model that well models the electrical behavior of the TLC555. You will find both a PSpice and TINA-TI Spice models available on TI's TLC555 web-page:

http://focus.ti.com/docs/prod/folders/print/tlc555.html

Also there, within the TINA-TI Mono Reference Design circuit, you will find a notes section that provides information for attaining the expected TLC555 monostable performance. You will find that the TI's TINA-TI resources are very complete in helping you set up and evealuate your application circuit's performance.

Please let me know if there is anything else I can provide you.

Regards, Thomas

PA-Linear Applications Engineering

Just a quick look at the 2 data sheets, I found this:

The NE555 is not specified to work at 3.3 Volts.

The trigger and threshold currents are considerably different between the 2 parts.

That would cause the 2 parts to not have the same timing.

They are pin compatible, but if you want the same timing, you have to change the R/C values.

Hope this helps,

Dave

Mikhail and all,

To clarify some points, the basic timing equations are the same for TLC555 and NE555. Timing equations are determined by ratios of the supply voltage, not absolute voltages or currents. Timing should be quite similar for most astable and monostable circuits. David is correct, the NE555 is not specified to operate at 3.3V. Though it may "operate" and provide pulses, it is probably very marginal. This is probably what is affecting timing in this application.

There are many ways that clever engineers have used the generic "555" that were never imagined. It is possible that some less standard circuits could behave differently.

Because of its CMOS fabrication, the input bias currents of the comparators on the TLC555 are much lower. This may allow a wider range of capacitance to be used.

Regards, Bruce.

One thing I'll add to Tom's post:

The TLC555 Timer will run nicely in TINA but if you are using it in the asynchronous mode, you must set the Transient simulation mode to "Zero Initial Values" or the oscillation will never start. A trigger signal can also be injected into the timing capacitor to start the oscillator but this is a rather clumsy approach.

Regards, Neil P. Albaugh   ex-Burr-Brown

Neil,

Thank you for the added information regarding simulating the TLC555 in asynchronous mode. Good to see you posting on the forum!

Thomas

PA-Linear Applications Engineering