Datasheet

P82B715_8 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 08 — 9 November 2009 8 of 23
NXP Semiconductors
P82B715
I
2
C-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:
(2)
Where: R
PU
= parallel combination of all pull-up resistors driven by the Lx pin of the
P82B715.
Figure 7 shows calculations for an expanded Standard-mode I
2
C-bus with 3 nF of cable
capacitance.
Fig 7. Typical loading calculation: adding an extension bus with P82B715
V
CC max()
0.4 V–
R
PU
------------------------------------------
30 mA<
002aad692
Lx Sx SDA
R3R2
LxSx
R1
SDA
SDA
V
CC
local bus
I
2
C-BUS
I
2
C-BUS
proposed bus expansion
3 nF = cable wiring capacitance
LDA
I
2
C-BUS
5 V
0 VGND
effective capacitance
local bus I
2
C-bus devices
local I
2
C-bus pull-up
2 × I
2
C-bus devices
strays
P82B715
total capacitance
20 pF
20 pF
10 pF
50 pF
R1 = = 20 kΩ
1 µs
50 pF
effective capacitance
at common Lx node
buffered bus pull-up
wiring capacitance
total capacitance
3000 pF
3000 pF
R2 = = 330 Ω
1 µs
3000 pF
effective capacitance
remote I
2
C-bus devices
remote I
2
C-bus pull-up
1 × I
2
C-bus devices
strays
P82B715
total capacitance
10 pF
10 pF
10 pF
30 pF
R3 = = 33 kΩ
1 µs
30 pF