Datasheet

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R1 =
1 µs
50 pF
= 20 kΩ
LocalI CPullup
2
R2 =
1 µs
3000 pF
= 330 Ω
BufferedBusPullup
LocalBus
ProposedBusExpansion
5V
0V
V
CC
GND
I C
2
3nF=CableWiringCapacitance
R3
R2R1
SDA
SDA
SDA
LDA
Sx
Lx
Lx
Sx
I C
2
I C
2
R3 =
30 pF
= 33 kΩ
RemoteI CPullup
2
1 µs
2 I CDevices×
2
Strays
P82B715
Total
20pF
20pF
10pF
50pF
WiringCapacitance
Total
3000pF
3000pF
1 I CDevices×
2
Strays
P82B715
Total
10pF
10pF
10pF
30pF
EffectiveCapacitance
RemoteI CDevices
2
EffectiveCapacitance
BufferedLine
EffectiveCapacitance
LocalBusI CDevices
2
P82B715
I
2
C BUS EXTENDER
SCPS145A – DECEMBER 2007 – REVISED FEBRUARY 2008
To calculate the current sunk by the Lx pin of the buffer at I
2
C 1, note that the current in R1 is sunk directly by
the device at I
2
C 1. The buffer, therefore, sinks only the currents flowing in R2, R3, and R4, so the effective
pullup is R2 in parallel with R3 and R4.
In this example that is 1 k Ω ||10 k Ω ||10 k Ω = 833 Ω . For a 5.5-V supply and 0.4-V low, the buffer is sinking
16.3 mA.
The P82B715 has a static sink rating of 30 mA at Lx. The requirement is that the pullup on the buffered bus, in
parallel with all other pullups that it is indirectly pulling low on Sx pins of other P82B715 devices, does not cause
this 30-mA limit to be exceeded.
The minimum pullup resistance in a 5-V ± 10% system is 170 Ω .
The general requirement is:
(V
CC
(max) – 0.4)/R
P
< 30 mA
Where:
R
p
= Parallel combination of all pullup resistors driven by the Lx pin of the P82B715
Figure 5 shows calculations for an expanded I
2
C bus with 3 nF of cable capacitance.
Figure 5. Typical Loading Calculations
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