SN54SC3T97-SEP: Typical output impedance

Sean Martin

Part Number: SN54SC3T97-SEP

Tool/software:

What is the typical output impedance for Vcc=5V?

I would like to drive a 900pF capacitive load directly with this device and need to size a current limiting resistor and not stress the output of the logic gate. I'd like to limit max current below 25mA where Imax = Vcc/(Rs+Ro) where Rs is the external resistor and Ro is the output impedance of the logic gate.

8 months ago

0 Clemens Ladisch 8 months ago

Guru 317000 points

The typical output impedance can be computed as V_OL / I_OL or (V_CC − V_OH) / I_OH, e.g., 0.3 V / 8 mA = 37.5 Ω. (See [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?)

0 Sean Martin 8 months ago in reply to Clemens Ladisch

Prodigy 80 points

Got it, this makes sense. For a worst case does it make sense to use V_OL = 0.75V, I_OL = 0.024A ?

Also, assuming 37.5Ω and no external series resistor, when driving a capacitive load the gate will source (5V/37.5Ω) = 133mA. This violates the abs max spec, but only for a brief period of time. How long is acceptable to violate the continuous current rating? (I assume this is where the 50pF recommendation comes from?)

Thanks for the help.

0 Clemens Ladisch 8 months ago in reply to Sean Martin

Guru 317000 points

Yes; you can compute the worst-cast output impedance from the worst-case output voltage.

The absolute maximum ratings must not be exceeded, not even for a short time. (Actually, loads up to 70 pF are allowed to be connected directly.)

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SN54SC3T97-SEP: Typical output impedance