AnsweredAssumed Answered

TS556 not working in monostable operation with 3.3V power supply

Question asked by Paul-François DOUBLIEZ on May 31, 2018

Dear Sir or Madam,


I am trying to use the timer TS556 in monostable operation with 3.3V power supply.
I implemented the application schematics of figure 4 (page 13) of the TS556 Application information (DocID4078 Rev 3).
In my case, the VCC power supply is 3.3V.
My input signal connected on the trigger input (pin6) is a squared signal of maximum amplitude of 3.3V.
The reset pin of the TS556 is connected to 3.3V.

I expect to get in output of the TS556 (pin5) a short duration pulse at each falling edge of the input signal.
However, i have an issue, because i observe that the output pulse is not reset to zero when the tension of the condensator on the threshold input reaches (2/3)*VCC.
Instead of that, the output pulse is reset only when the trigger input becomes low. Hence, the output pulse lasts too long.

You can see in IMG_7334.jpg in attachement the mesurement of the output signal (in yellow) and the measurement of the "Discharge" and "Threshold" signals ( in red) (both pins 1 & 2 are connected together between C & R).
On IMG_7334, we can notice that the yellow ouput is not reset when the red signal ("Discharge" and "Threshold" pins) reaches 2.2V ((2/3)*VCC ), but it is reset only when the input trigger signal (not displayed) becomes low.

However, according to table 3 "Functions table" of DocID4078 Rev 3, if "reset" is high, and "trigger" is high, when the "threshold" is high (i.e. when the tension of the condensator on the threshold input becomes >= (2/3)*VCC ) , then the "Output" should be Low.
But this is not the case...

My value of R is 1 kOhm, and C is 100 nF. So, the expected output pulse duration should be t = 1.1 * R * C = 0.11 ms. But on IMG_7334, we can see that the pulse duration is about 0.4 ms, which is too long.

I also checked the control Voltage value: it is well at (2/3)*VCC = 2.2 V.
I also tried with R = 10 kOhm, and C= 10 nF, but i have the same kind of issue...

I do not understand at all why the Ouput does not become low when the threshold voltage becomes >= (2/3)*VCC.
I saw in the datasheet that the voltage range is 2 V to 16 V, so i think that it should work with a 3.3V power supply and signals.
I also tried to connect the reset pin to the trigger pin (monostable retrigerrrable operation), in this case i can just observe a very short 3.3V pulse in outpout ( duration is < 1 microsecond ). But this pulse is too short for my application.
In the past, i implemented this monostable operation with a NE556 and 5V TTL signals, and it worked perfectly. Now, i would like to replace the NE556 by some IC working with 3.3V signals.

I spent many hours on issue, but now i do not know how to solve it. Could you help me please?

Do you have some ideas of root causes why it is not working?
Do i maybe miss something?
For example, is it correct to use the TS556 with a 3.3V power supply, and with a trigger input signal of 3.3V?
What do you suggest to solve the problem?

i would be very interested and gratefull to you to get your support on this issue.


Best regards,

Attachments

Outcomes