Datasheet

PCA9600 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 5 — 5 May 2011 20 of 31
NXP Semiconductors
PCA9600
Dual bidirectional bus buffer
10.2.1 Example with questions and answers
Question: On a falling edge of TX we measure undershoot at −800 mV at the linked
TX, RX pins of the PCA9600 that is generating the LOW, but the PCA9600 data sheet
specifies minimum −0.3 V. Does this mean that we violate the data sheet absolute
value?
Answer: For PCA9600 the −0.3 V Absolute Maximum rating is not intended to apply to
transients, it is a DC rating. As shown in Figure 23
, there is no theoretical reason for any
undershoot at the IC that is driving the bus LOW and no significant undershoot 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 I
2
C-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.
Fig 23. Transients generated by the bus wiring
002aaf081
time (ns)
2
0
4
6
voltage
(V)
−2
horizontal scale = 62.5 ns/div
send
receive
5 V
RX
TX
SX
PCA9600
send
300 Ω
5 V
2 meter
cable
5 V
300 Ω
GND
receive