TS556 not working in monostable operation with 3.3V power supply

Paul-François DOUBLIEZ · ‎2018-05-31

Posted on May 31, 2018 at 22:32

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,