CD74HC221: Pulse width versus supply voltage

ArTzy

Part Number: CD74HC221

Hi Experts,

Seeking your assistance on this query from customer named Sam:

In the datasheet for the CD74HC221, it says that "The calculation for the pulse width is tW = 0.7 RXCX at VCC = 4.5V ." However, when we measured the pulse width for this part in our circuit, it was approximately 15% higher than the value expected based on the supplied R and C values. Our setpoint is VCC=3.3V.

Could you provide a graph of the pulse width vs VCC setpoint? Is this pulse width discrepancy expected for this lower setpoint?

Regards,
Archie A.

over 1 year ago

0 Albert Xu1 over 1 year ago

TI__Mastermind 22355 points

Hey Archie,

I would take a look through our FAQs regarding MMVs seen here: https://e2e.ti.com/support/logic-group/logic/f/logic-forum/913667/faq-h-monostable-multivibrators---top-questions-answered

There is the potential of a lot of variation due to the resistor/capacitor duo directly influencing the pulse width.

0 ArTzy over 1 year ago in reply to Albert Xu1

TI__Genius 14719 points

Thank you, Albert.

We haven't found the graph to be particularly helpful. In our system, despite having a lower VCC (3.3 vs the standard 4.5 V figure), we saw a pulse width substantially longer than the 0.7C_xR_x number we would expect. Any insight as to why this is?

Regards,
Archie A.

0 Albert Xu1 over 1 year ago in reply to ArTzy

TI__Mastermind 22355 points

Hi Archie,

I don't have much insight into this currently. Can you send over a schematic and maybe some scope shots?

In addition, do you see this behavior in multiple devices?

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CD74HC221: Pulse width versus supply voltage