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Posted on behalf of Mikhail Gorin and George Becke:
Dear clocks and timers team,
Below is a question from the customer. I believe this will be an easy one:
***Hi TI, could you explain the difference between part TLC555 and NE555? (We are using the part monostable, 3n3 and 1k giving the pulse length at 3V3 operation. The length of the output pulse are different and they behave different in respect to trigpulse?) Thanks and regards***
That's what I've found in the product folder of TLC555:"Because of its high input impedance, this device uses smaller timing capacitors than those used by the NE555. As a result, more accurate time delays and oscillations are possible. Power consumption is low across the full range of power supply voltage."
Is there anything you could add to this sentence, considering the design parameters that customer has sent?
Thank you in advance for any input on this issue and best regards,
Customer Support Agent
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:
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.
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,
In reply to David Tiefenbrunn:
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.
In reply to Thomas Kuehl:
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
In reply to Neil Albaugh:
Thank you for the added information regarding simulating the TLC555 in asynchronous mode. Good to see you posting on the forum!
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