SN74LVC244A: dis/step at rising edge

Zach Xia

Part Number: SN74LVC244A

Hi，

Will this dip/step at the edge be concern? I think the time is short and should not cause an additional pulse and influence the following device.

Could you help confirm?

over 2 years ago

+1 Clemens Ladisch over 2 years ago

Guru 295220 points

This is a violation of the Δt/Δv limit (which is missing from the SN74LVC244A datasheet, but it still exists), and it is near V_CC/2, where it can actually lead to oscillations at the output.

The trace to the input should be properly terminated; see p. 47 of the LVC Designer's Guide.

0 Zach Xia over 2 years ago in reply to Clemens Ladisch

TI__Expert 7205 points

Thanks, Clemens.

But I need to ask if any TI colleague could provide the missing part for me, since my customer is asking me to explain the reason inside the chip.

This is a violation of the Δt/Δv limit (which is missing from the SN74LVC244A datasheet, but it still exists), and it is near V_CC/2, where it can actually lead to oscillations at the output.

0 Albert Xu1 over 1 year ago in reply to Zach Xia

TI__Mastermind 29295 points

Hi Zach,

I would refer to the app note linked below:

https://www.ti.com/lit/wp/slla364a/slla364a.pdf

The typical rise time minimum will be 10ns/V for LVC family.

0 Zach Xia over 1 year ago in reply to Albert Xu1

TI__Expert 7205 points

Hi Albert.

To confirm, should the Δt/Δv limit be followed at everywhere of the edge or just the total Δt/Δv needs to be complied?

From customer plot, the total Δt/Δv is OK. And the dip/step lasts for very short time. Should that be an issue?

Thanks,

Zach

+1 Clemens Ladisch over 1 year ago in reply to Zach Xia

Guru 295220 points

In theory, the Δt/Δv limit applies to all points of the edge. In practice, you need to care only near the actual switching threshold, which is near VCC/2, but that is where the problem actually happens.

You do need to properly terminate the traces (simplest and cheapest is source termination), or use a device with Schmitt-trigger inputs.

0 Zach Xia over 1 year ago in reply to Clemens Ladisch

TI__Expert 7205 points

Thank you so much!

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SN74LVC244A: dis/step at rising edge