PCA9600DP datasheet(20/32 Pages) NXP

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NXP Semiconductors

PCA9600

Dual bidirectional bus buffer

should be observed when using reasonable care with the ground connection of the

‘scope. It is more likely that undershoot observed at a driving PCA9600 is caused

by local stray inductance and capacitance in the circuit and by the oscilloscope

connections. As shown, undershoot will be generated by PCB traces, wiring, or

cables driven by a PCA9600 because the allowed value of the I2C-bus pull-up resistor

generally is larger than that required to correctly terminate the wiring. In this example,

with no IC connected at the end of the wiring, the undershoot is about 2 V.

002aaf081

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

Figure 23. Transients generated by the bus wiring

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.

PCA9600

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Product data sheet

Rev. 6.1 — 20 December 2021

20 / 32

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