CPC5903 Datasheet, PDF (12/15 Pages) IXYS Corporation – Bidirectionally Buffers I2C SDA Signal

INTEGRATED CIRCUITS DIVISION
4.2.2 IOB Pull-Up Resistor: R_PUB
For the bidirectional buffer, it is necessary to configure
the IOB bus to be compatible with the CPC5903’s
lower logic low input threshold:
V_ILB = 0.2 • V_DDB - 60mV
As discussed earlier, this lower input threshold
requirement is to ensure the CPC5903 can drive a
logic low output that is recognized by the other I²C
devices on the bus, but will not accept it’s own logic
low output. This prevents latching of the CPC5903.
Additionally, this implies there can be no more than
one limited drive (Side B) CPC5903 interface on the
IOB bus, and that all other devices on the bus must
have V_IL = 0.3 • V_DDB logic low input thresholds.
Because the CPC5903 Side A inputs are compatible
with this requirement, any number of CPC5903 Side A
devices may be connected to the Side B bus.
For all modes, the minimum required voltage drop
across the IOB pull-up resistor at V_{DDB_max} by the
external non-CPC5903 I²C bus drivers is:
Minimum R_PUB Voltage = V_{DDB_max} - (0.2 • V_{DDB_max} - 60mV)
= 0.8 • V_{DDB_max} + 60mV
which gives the calculation for the minimum value of
the pull-up resistor as:
R_{PUB_min} = (0.8 • V_{DDB_max} + 60mV) / I_OL
where I_OL is the guaranteed logic low drive current of
the non-CPC5903 bus drivers.
For Standard-mode designs, with output drivers rated
at 3mA and a maximum supply voltage of 3.6V, the
minimum value of the pull-up resistor is:
R_{PUB_min} = (0.8 • 3.6 + 60mV) / 3mA = 980
The minimal standard value 1% resistor with a
100ppm/C temperature coefficient that will not go
below the calculated minimum due to tolerance and
temperature is 1k.
In Fast-mode designs with 6mA capable output drivers
and a supply voltage maximum of 5.5V, the minimum
Fast-mode pull-up resistor value is calculated to be:
R_{PUB_min} = (0.8 • 5.5 + 60mV) / 6mA = 743.3
For a Fast-mode design with high capacitive bus
loading, a 768, 1%, 100ppm/C resistor would
suffice. When the bus does not have a heavy
capacitive load, a larger value pull-up resistor can be
used thereby reducing overall power consumption.
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