TLC 555 RC Time Constant minimum Resistance requirement.

Daniel Hartung said:

Hey Rhode,

I moved your question to a different forum better equipped to help you.

Thanks,
Daniel

Hi Daniel,

Thank you for helping.  I am a little confused as to how this process works.  Please excuse me for not understanding the process.

I worked TecSup for Ampex, and when a customer called in with a problem I answered it, or found out the answer and provided it to the customer, if not immediately, usually with a few hours.

My question is product specific regarding the technical inner workings of the device and would (I think) require a TI application Engineer familiar with the device to answer completely.  As mentioned the Data Sheet for other similar devices, like the 74LS121 give a specification for the minimum resistance required for the RC Time Constant inputs.  And that I could not find any similar specification for the TLC-555.

That may be something left out accidentally from the data sheet, or I may have not seen it, though I read it over several times, or there may not be a specification.  The last I don't think would be the case as this would allow raw DC voltage to be applied directly to the pin(s), which is not a good thing to do.

I would think that most of the community members in this forum may be very well versed in the correct usage of devices, but when it comes to the very tiny details like the question I am asking, I would think the answer can only come from a TI engineer.

I could be wrong on that, but my reasoning is the amount of time it is taking to get a simple answer from TI on one of their products.  A question, I feel, would be answered in a few minutes on the phone.   Please don't get me wrong, this is not a complaint but simply trying to understand how the system works.

Is there any other information you may be able to provide regarding my concerns?  

How long, typically, does a product question take to be answered?

I really appreciate the help you are giving, thank you.

Sincerely,

Rhode Roberts

I continued searching through the data sheet for the TLC-555 trying to find an answer for the minimum resistance requirement tied to the RC Time Consonant input(s). On page 8, 1st Table, there is mention of "Min Freq.", and gives values of 470 Ohm & 200 Ohm.

Though it does not mention these values are minimum for the RC Time Constant, I decided to try a value of 700 Ohm, just to be safe. I guess those resistance values are not what I am looking for, because it seems the 700 Ohm fried the TLC-555.

Does anyone have an answer as to what the minimum resistance value is for the RC Time Constant for the TLC-555 Pins 6&7?

I can keep on poking and hoping, trying different values until I find the value that doesn't fry the chip, but this is starting to get costly just buying chips.  Won't someone from TI please answer my question?

Thank you.

Sincerely,

Rhode Roberts

Hello Rhode,

Sorry you had difficulty finding the E2E where the TLC555 receives support. It actually resides here with us in the Precision Op amps group, and happens to fall in within our line up of op amps that use the TLC prefix. 

The TLC555 pin 6 threshold input connects the gate of an N-ch MOSFET. Pin 7 is related to the timing capacitor, Ct discharge function. It connects to the drain of an N-ch MOSFET. The limitation on Rt, VR1 + R3 in your circuit, comes down to how much current the discharge MOSFET can sink from Rt and Ct in the discharge mode when the pin goes low. The TLC555 datasheet on page 9, VDD = 5 V, lists the Discharge Switch On-stage Voltage with an IOL current of 10 mA. The maximum voltage for the discharge pin in that case is 0.5 V maximum. If the discharge pin only had to conduct current the current from Rt, the the minimum resistance would be (5 V -0.5 V) / 10 mA, or 450 ohms; however, it must also conduct the transient current as the timing capacitor is discharged. The transient current can be quite high but only lasts briefly and it is limited by the on resistance of the discharge MOSFET. The current from the capacitor can be tens of milliamps, or even over 100 mA, depending on how quickly the MOSFET discharges the capacitor. It is designed to handle that current.

In your circuit the minimum resistance for Rt is 2.2 k, so with a 5 V supply its contribution to the current flowing through the discharge pin is less than 2.3 mA. That should not be an issue. The rest of the current during discharge will be whatever C4 provides when the MOSFET conducts and again it is designed to handle a high, transient current. Figure 5, the Output Pulse Duration vs Capacitance graph, shows a minimum Rt of 1 k-Ohm which may be a practical limit.

I did find one problem with your schematic. It shows pin 8, the Vdd pin, connected through a 10 k-Ohm resistor to the +5 V supply. It is okay for pin 4, the Reset pin, to be connected through a resistance, but not the Vdd pin. It will cause too large of a voltage drop when the TLC555 is swicthing states. I would just connect pin 4 and  pin 8 directly to the +5 V supply. A larger power supply decoupling capacitor such as a 100 nF would be a better choice than 1 nF.

Regards, Thomas

PA - Linear Applications Engineering