P82B715
NXP Semiconductors
I2C-bus extender
The P82B715 has a static sink rating of 30 mA at Lx. The requirement is that the pull-up
on the buffered bus, in parallel with all other pull-ups that it is indirectly pulling LOW on Sx
pins of other P82B715 ICs, will not cause this 30 mA limit to be exceeded.
The minimum pull-up resistance in a 5 V ± 10 % system is 170 Ω.
The general requirement is given in Equation 2:
VCC(max) – 0.4 V
< 30 mA
(2)
------------------------------------------
RPU
Where: RPU = parallel combination of all pull-up resistors driven by the Lx pin of the
P82B715.
Figure 7 shows calculations for an expanded Standard-mode I2C-bus with 3 nF of cable
capacitance.
local bus
proposed bus expansion
R2
V
CC
5 V
R1
R3
SDA
SDA
2
I C-BUS
Sx
Lx
LDA
Lx
Sx
SDA
2
I C-BUS
2
I C-BUS
3 nF = cable wiring capacitance
GND
0 V
effective capacitance
local bus I C-bus devices
effective capacitance
at common Lx node
effective capacitance
remote I C-bus devices
2
2
2
2
2 × I C-bus devices
strays
20 pF
20 pF
10 pF
1 × I C-bus devices
strays
10 pF
10 pF
10 pF
wiring capacitance 3000 pF
total capacitance 3000 pF
P82B715
P82B715
total capacitance
50 pF
total capacitance
30 pF
2
2
local I C-bus pull-up
buffered bus pull-up
remote I C-bus pull-up
1 µs
1 µs
1 µs
R1 =
= 20 kΩ
R2 =
= 330 Ω
R3 =
= 33 kΩ
50 pF
3000 pF
30 pF
002aad692
Fig 7. Typical loading calculation: adding an extension bus with P82B715
P82B715_8
© NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 08 — 9 November 2009
8 of 23
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