PCA9600D datasheet(21/30 Pages) NXP

21 / 30 page

PCA9600_4

Product data sheet

Rev. 04 — 11 November 2009

21 of 30

NXP Semiconductors

PCA9600

Dual bidirectional bus buffer

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

PCA9600 devices. When one TX drives LOW the other PCA9600 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 PCA9600s 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 24, the transient

undershoot will be limited, by the diodes inside PCA9600, to around

−0.8 V and that will

not cause problems for PCA9600. Those transients will not be passed inside the IC to

the SX/SY side of the IC.

Question: If we input 800 mV undershoot at TX, RX pins, what kind of problem is

expected?

Answer: When that undershoot is generated by another PCA9600 and is simply the

result of the system wiring, then there will be no problems.

Question: Will we have any functional problem or reliability problem?

Answer: No.

Fig 24. Wiring transients limited by the diodes in PCA9600

002aaf082

time (ns)

voltage

(V)

−2

horizontal scale = 62.5 ns/div

send

receive

5 V

PCA9600

send

300

Ω

5 V

2 meter

cable

5 V

300

Ω

GND

receive

5 V

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