SN74HC32: Do I need current limiting resistor on input?

Bojan Jovanovic

Part Number: SN74HC32
Other Parts Discussed in Thread: SN74HCS32, SN74AHC32

Hello, guys.

We have SN74HC32PWR part in our design. We noticed that the clamp current is +/-20mA (page 4 of datasheet).

Do we need to provide some kind of current limiting resistor at the inputs or it is safe to connect 3.3V of logic HIGH directly from power supply source?

Is there any internal pull-down resistor inside SN74HC32PWR ?

Thanks in advance for your time and efforts.

Bojan.

over 6 years ago

0 Clemens Ladisch over 6 years ago

Guru 327330 points

Why would the input voltage be clamped? Is the OR gate's power supply lower than 3.3 V?

0 Bojan Jovanovic over 6 years ago in reply to Clemens Ladisch

Prodigy 145 points

I would mean to put some current limiting resistor between 3.3V and input. Is that necessary or we can connect 3.3V directly to the input pin of OR gate?

Power supply voltage is 3.3V.

Regards,

Bojan.

0 Clemens Ladisch over 6 years ago in reply to Bojan Jovanovic

Guru 327330 points

CMOS inputs have a very high impedance; a series resistor would have no noticeable effect. You can connect the inputs directly to V_CC or GND or a logic output.

Please note that the clamp current limit is specified for voltages outside the recommended operating conditions.

Consider using the more modern SN74AHC32 or SN74HCS32 instead (they are drop-in replacements).

0 Bojan Jovanovic over 6 years ago in reply to Clemens Ladisch

Prodigy 145 points

That is what I wanted to double check! Thank you very much for your clarification!

Cheers,

Bojan.

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SN74HC32: Do I need current limiting resistor on input?