CD74HC123: tW = 0.45 Rx Cx in case of Vcc = 3.3V and Ψjt

Hi Matsumoto-san,

<Question 1>

The calculation of the pulse width is done by the equation tW = k*Rx*Cx. For the 5.0V case the value of k is .45.

To find the value for the other voltages, the following table provided in the datasheet can be used.

Also please make sure that the values of Cx and Rx are chosen correctly. Here is a table also found in the datasheet that helps with finding these values.

<Question 2>

To find Ψjt for you, I will need the full part number. If you are looking for CD74HC123M96 for example the Ψjt = 10.1 C/W however this value will differ for each package.

Thank You!

- Karan

Hello Matsumoto-san,
The datasheet table for K factor is only listed for 10nF. This value will typically decrease with larger capacitors, and increase with smaller capacitors.

This is one reason why we always recommend prototyping the circuit prior to production.

From the information you have provided, your capacitor produces a K value of:
0.173 = K * 0.953uF * 270 kΩ
K = 0.672

You can expect that the K value will remain consistent with similar size/style capacitors.

We expect up to see up to 10% variation across temperature/process for pulse width from a monostable multivibrator (typically under 1%, not including manufacturing and temperature variation in the timing components), so I would recommend getting as close to 150ms as possible to guarantee that the pulse will always fall in the desired range:

0.150 = 0.672 * 0.953uF * R
R ~= 234 kΩ