SN74AXC2T245: External pull-up resistor value

ken kodaira

Part Number: SN74AXC2T245
Other Parts Discussed in Thread: SN74AVC2T245

Tool/software:

I will be using it with VCC=1.8V. The data sheet states that the external pull-up should be 7kΩ or less, but as long as the VIH specification = VCCIx0.65 is satisfied, is it okay to use an external pull-up of 7kΩ or more? Also, what is the variation of the internal pull-down of 71kΩ?

over 1 year ago

+1 Clemens Ladisch over 1 year ago

Guru 316760 points

The datasheet says:

If an external pull-up is required, it should be no larger than 7kΩ to avoid contention with the 71kΩ internal pull-down.

Your external pull-up forms a voltage divider together with the internal pull-down; if the pull-up is too weak, the voltage will be too low (below V_IH).

0 ken kodaira over 1 year ago in reply to Clemens Ladisch

Prodigy 30 points

Hi Clemens-san,
Thanks for your comment.

If I check the voltage division, is it okay to use more than 7kΩ?

For example, if I use 10kΩ for an external pull-up at 1.8V,
the voltage divided with 71kΩ becomes 1.58V.
This satisfies VIH＝1.8x0.65=1.17V for Vcc=1.8V.

0 Clemens Ladisch over 1 year ago in reply to ken kodaira

Guru 316760 points

71 kΩ is a typical value. The datasheet recommends 7 kΩ as the highest value that works in the worst case. (So in practice, use 6.8 kΩ.)

0 ken kodaira over 1 year ago in reply to Clemens Ladisch

Prodigy 30 points

Hi Clemens-san,
Thanks for your comment.

I will use this as a reference.

0 Michael Ikwuyum over 1 year ago in reply to ken kodaira

TI__Guru 50736 points

Hi Ken,

From another referenced thread, i noticed customer insists on the external pull-up. No concerns if customer is okay with the impact from the divider network.
If concerned, customer may also see SN74AVC2T245 without internal pull-up, thanks.

Best Regards,

Michael.

Logic

Logic forum

SN74AXC2T245: External pull-up resistor value