PCA9601DPZ datasheet(19/31 Pages) NXP

19 / 31 page

NXP Semiconductors

PCA9601

Dual bidirectional bus buffer

002aaf078

time (ns)

voltage

(V)

- 2

horizontal scale = 62.5 ns/div

send

receive

5 V

PCA9601

send

300 Ω

5 V

2 meter

cable

5 V

300 Ω

GND

receive

Figure 24. Transients generated by the bus wiring

Question: We have 2 meters of cable in a bus that joins the TX/RX sides of two

PCA9601 devices. When one TX drives LOW the other PCA9601 TX/RX is driven to

-0.8 V for over 50 ns. What is the expected value and the theoretically allowed value

of undershoot?

Answer: Because the cable joining the two PCA9601s is a ‘transmission line’ that

will have a characteristic impedance around 100 Ω and it will be terminated by pull-

up resistors that are larger than that characteristic impedance there will always be

negative undershoot generated. The duration of the undershoot is a function of the

cable length and the input impedance of the connected IC. As shown in Figure 25, the

transient undershoot will be limited, by the diodes inside PCA9601, to around -0.8 V

and that will not cause problems for PCA9601. Those transients will

not be passed

inside the IC to the SX/SY side of the IC.

PCA9601

All information provided in this document is subject to legal disclaimers.

Product data sheet

Rev. 3.1 — 4 January 2022

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PCA9601DPZ Datasheet(PDF) 19 Page - NXP Semiconductors