SN74123: SN74123: Changing Output from Dual Retrigg. Monostable Multivibrator w/ Reset

Hi,

Yes you are right. If you want to scale your output voltage down, you will need to scale the supply voltage down. This is very uncommon. What is your application?

You can just tie your clear pin to Vcc if you are not using it. Then you would need to change the supply voltage. However you will not be able to use the SN74123 for this. As you see in the recommended operating conditions table, the device only operates from 4.5V to 5.5V Vcc.

Your best option is the SN74LVC1G123. But even that is only meant for operation down to 1.65V Vcc.

Thanks!

-Karan

Hi Karan, thanks again for your reply. I am trying to create a nanosecond pulses generator that is able to create nanosecond pulses (50ns) for a variety of output voltages varying from 1~5V. I got the right timing resistor and capacitor combination working to output nanosecond pulses - now I want to understand how to vary my output.

which has an input voltage of -0.5 to 7V, would this be another ideal monostable multivibrator for my use - I have to change the resistor-capacitor combination again to get the accurate 50ns pulse again - but by varying the input voltage through Vcc would result in varied outputs if I understand your answer right

Hi,

Varying the input voltage doesn't change the output voltage. Please look at VOH and VOL specs to know what the output of the device will be.

Varying Vcc will change the output voltage. Vcc has a recommended range of 2V to 6V for that device. None of our monostable multivibrators will go down to 1V. If power is not a concern, you can consider adding a resistor divider to the output to attain the voltage range you require.

Thanks!

-Karan

Hi Karan, I am a little confused from your reply - specifically this part - Varying the input voltage doesn't change the output voltage. Varying Vcc will change the output voltage.

Vcc is technically the input voltage, right? I am still pulsing 5V from the Ardunio and I am supplying 5V (Vcc) to my CD74HC221, I get 50ns pulses with the correct resistor-capacitor combination. 

By reducing the +5V Vcc I am able to get pulses of 4,3,2V however their periods become longer, from 5V to 4V the period of the pulse goes from 4V to 75ns, but 3V become 100ns and so on. To fix this issue should I have different resistor-capacitor combinations for each output pulse?

Thanks for the help, I really appreciate it! 

V_CC is the voltage at the V_CC pin. The input voltage is the voltage at an input pin, i.e., CLR/A/B.

The CD74HC221 Operating Conditions do not allow a supply voltage lower than 2 V. (The Absolute Maximum Ratings just guarantee that there is no permanent damage, but not that the device actually works.)

In theory, the output pulse duration depends only on the external resistor and capacitor. However, there is some internal resistance and capacitance, which has more influence on the result if the external RC is very small.

To make the pulse duration completely independent from the supply voltage (and to allow voltages lower than the minimum logic supply voltage), use the multivibrator's output to control an analog switch like the TS5A3160.

Clemens Ladisch said:

V_CC is the voltage at the V_CC pin. The input voltage is the voltage at an input pin, i.e., CLR/A/B.

The CD74HC221 Operating Conditions do not allow a supply voltage lower than 2 V. (The Absolute Maximum Ratings just guarantee that there is no permanent damage, but not that the device actually works.)

In theory, the output pulse duration depends only on the external resistor and capacitor. However, there is some internal resistance and capacitance, which has more influence on the result if the external RC is very small.

To make the pulse duration completely independent from the supply voltage (and to allow voltages lower than the minimum logic supply voltage), use the multivibrator's output to control an analog switch like the TS5A3160.

Hi Clemens, thanks for the reply. I really appreciate it! I understood what you said, but can you go into a little more detail about using the TS5A3160 analog switch to obtain nanosecond pulses of 3, 4V from the monostable multivibrator I am using.

The resistor-capacitor combination I have now works well to create 50ns 5V pulses. How can I use that output w/ a TS5A3160 to reduce that voltage to 4,3 and if possible 1 and 2V?

I looked up the Application Guide to designing with analog switches (http://www.ti.com/lit/sg/slyb125d/slyb125d.pdf), which schematic am I following?

Thanks again!

Hans

The multivibrator and the switch are powered with a constant voltage (whatever you want to use for logic signals). The switch connects the output to either ground or your desired voltage (which can have any value between 0 and V_CC).

There is a 32ns for start-up time and a 25ns turn off time. Since my pulses are in the 50ns range - I do not think I get proper output. I tried with a TS12A4514, which has a similar pin-out and function. I think the start-up and turn-off times are too large and cannot work with my pulses