CD74HC4538-Q1: Unused Pins Guidance and ESD Rating

David Shumate

Part Number: CD74HC4538-Q1

Hi Team,

For the CD74HC4538QPWRQ1 can you confirm what is the rated ESD capability of this component? (Human Body Model vs Machine Model, etc)

Pins 14, 9-12 are also unused for this design. Can you confirm what is the recommended connection for each of these unused pins? (tie to GND, tie to VCC5, or float)

Thanks,

David

6 months ago

0 Hannah Bludau 6 months ago

TI__Prodigy 270 points

Hi David,

The ESD rating is 4000V HBM.

All unused inputs should be tied to Vcc or GND. All unused outputs can be left floating.

[FAQ] How do I terminate any unused channels of a logic device? - Logic forum - Logic - TI E2E support forums

Thanks,

Hannah

0 David Shumate 6 months ago in reply to Hannah Bludau

TI__Expert 6865 points

Hi Hannah,

Thanks for the quick feedback, in the other E2E you referenced it mentioned tying unused output to Vcc through a resistor. Is there any concern about tying directly to Vcc instead of through a resistor?

Best,

David

0 Clemens Ladisch 6 months ago in reply to David Shumate

Guru 277650 points

Outputs should never be tied to anything.

But the I/O pins of transceivers can be both inputs and outputs. As inputs, they must be tied to some valid voltage, and this should be done with a resistor in case the pin is an output that drives a different logic level. But this applies only to transceivers (74xx245); the CD74HC4538-Q1 is not a transceiver.

0 David Shumate 6 months ago in reply to Clemens Ladisch

TI__Expert 6865 points

Hi Clemens,

I'm sorry I meant to say inputs not outputs. It's clear to me that the unused outputs should be left floating. I'm sorry I didn't quite understand your feedback in terms of the CD74HC4538-Q1, are you saying that unused inputs should be tied to Vcc through a resistor or that it is okay to not have the reisistor?

Thanks,

David

0 Clemens Ladisch 6 months ago in reply to David Shumate

Guru 277650 points

I was talking only about outputs.

Inputs must be tied to some valid voltage. You can use any resistor between 0 Ω and 500 kΩ. (You would want a resistor if you expected to need to change the input during debugging.)

0 David Shumate 5 months ago in reply to Clemens Ladisch

TI__Expert 6865 points

Hi Clemens,

Two additional questions from the customer

Do you have a voltage threshold for Machine Model (MM) ESD rating?
What are the limits of the Output Current Leakage on the device?

Thanks for all the help!

David

+1 Hannah Bludau 5 months ago

TI__Prodigy 270 points

Hi David,

David Shumate said:
Do you have a voltage threshold for Machine Model (MM) ESD rating?

We no longer provide MM ESD ratings.

David Shumate said:
What are the limits of the Output Current Leakage on the device?

This device does not have a high impedance output so there are no output current leakage ratings. The part has clamp diodes on all the inputs and outputs so when the power is off, the IOs are basically low impedance. [FAQ] Can the input voltage (Vi) to my logic device be higher than the supply voltage (Vcc)?

Thanks,

Hannah