SN74AHC123A: maximum value of timing capacitor

Pavel Vencalek

Part Number: SN74AHC123A

Hello,

is there any limit on value of timing capacitor of SN74AHC123A? Datasheet does not mention this and graph on fig. 5 shows C values up to 5uF only. My concern is temperature stress of discharging transistor inside the chip....

thank you,

Pavel

over 8 years ago

0 Clemens Ladisch over 8 years ago

Guru 317550 points

The characteristics of the discharge MOSFET are not specified. I would assume that it is designed to be safe, but when in doubt, use a smaller capacitor with a larger resistor.

0 Emrys Maier over 8 years ago

TI__Guru* 97066 points

I have personally used this device with up to 100uF electrolytic capacitors without any problems, however the device was designed for lower values (as you pointed out).

As Clemens said, it's better to use a smaller cap when you can. I typically will avoid using anything larger than 1 Mohm for resistance, and 10uF for capacitance, which limits my usage of this part to ~10s delays. I have seen customers use it beyond 30s effectively (even one that was ~210s). I have not yet seen one fail due to 'too large of capacitance.'

0 Pavel Vencalek over 8 years ago in reply to Clemens Ladisch

Prodigy 90 points

...yes, but larger resistor would be too large (leakage current vs voltage drop)

0 Emrys Maier over 8 years ago in reply to Pavel Vencalek

TI__Guru* 97066 points

Pavel,

How large of a delay are you trying to get?

0 Pavel Vencalek over 8 years ago in reply to Emrys Maier

Prodigy 90 points

...what about to connect say 500R between the capacitor and RX/CX pin? I am going to use at least 100k as the timing resistor so the created voltage divider would be negligible. Discharge time compared to overall pulse width would be small as well...

0 Pavel Vencalek over 8 years ago in reply to Emrys Maier

Prodigy 90 points

2 seconds would be ideal. Accuracy is not critical (+/-30% is ok)

0 Emrys Maier over 8 years ago in reply to Pavel Vencalek

TI__Guru* 97066 points

For this you should be able to just use a 10uF cap and ~220kohm resistor (you might have to tweak the resistance value a bit at prototyping). With large capacitances, you should expect to see a lower K value than that shown on the datasheet, ie your pulse width will be t_w = K * R * C, with K being (probably) between 0.7 and 0.9 for this

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SN74AHC123A: maximum value of timing capacitor