CPC5903 Datasheet, PDF (11/15 Pages) IXYS Corporation – Bidirectionally Buffers I2C SDA Signal

INTEGRATED CIRCUITS DIVISION
4 Design Considerations
The minimum value of the pull-up resistor, R_PU, on the
I²C bus is chosen based upon the expected V_DD
supply voltage range and the weakest load current
sinking device on the bus. Note: Systems that do not
need maximum bandwidth and busses with lower
capacitive loading can use a higher value for the
pull-up resistor to reduce power consumption.
4.1 Side A Pull-Up Resistors: R_PUA
The weakest I²C compliant device on the Side A bus,
with R_PUA to V_DDA, must be able to pull the Side A
inputs below 0.4V for outputs rated at 3mA or 0.6V for
outputs rated at 6mA when V_DDA is at its maximum.
For example, if the weakest device is only guaranteed
to sink 3mA then the maximum allowed logic low
output voltage will be 0.4V. For designs with
V_{DDA_max} = 3.6V, the minimum voltage across the
pull-up resistor is:
Minimum R_PUA Voltage = 3.6 - 0.4 = 3.2V
For the I²C minimum current sink requirement of 3mA,
the minimum value of the pull-up resistor is easily
calculated as:
R_{PUA_min} = 3.2V / 3mA = 1066.7
Chose a standard value resistor that will not violate
this minimum value over tolerance and temperature,
such as a 1.1k, 1% tolerance, 100ppm/C
temperature coefficient resistor.
If all the non-CPC5903 devices on the Side A bus are
Fast-mode compliant (400pF capacitive loading
capable) with the required 6mA current sink capability,
then the bus can be configured for Fast-mode.
Resistor selection for Fast-mode is similar to the
example given above but because the logic low output
level is greater (0.6V) then the voltage across the
pull-up resistor will be less. Calculation of the
compliant Fast-mode bus minimum pull-up resistor
value is given by:
R_{PUA_min} = (3.6 - 0.6)V / 6mA = 500
The minimum E96 standard value 1% tolerance,
100ppm/C temperature coefficient resistor is 511.
CPC5903
4.2 Side B Pull-Up Resistors
Calculating the pull-up resistor for Side B is similar to
the process used for Side A but with some additional
considerations.
Before proceeding, it must be pointed out that Side B
of the CPC5903 is Fast-mode compliant with
V_DDB  4.5V. This means the CPC5903 Side B
outputs are 6mA capable, allowing bus operation of
400kb/s with up to 400pF of capacitive loading. For
V_DDB supply levels below 4.5V the CPC5903 outputs
are only rated for 3mA, but can be operated at
Fast-mode speeds of 400kb/s whenever the bus
capacitive loading C_LOAD  200pF. Greater capacitive
loading of the Side B bus limits the CPC5903 to data
rates of 100kb/s.
First, it must be determined if the Side B bus will be
configured for 3mA or 6mA operation. This is done by
evaluating the external (non-CPC5903) devices on the
Side B bus and the operational capabilities of the
CPC5903. There are three possibilities:
1) One or more of the external devices is limited to
3mA of output current sink.
2) All of the external devices are rated at 6mA of
output current sink and the Side B minimum supply
voltage V_DDB  4.5V.
3) All of the external devices are rated at 6mA of
output current sink and the Side B minimum supply
voltage V_DDB  4.5V.
For conditions 1 and 2 above the bus must be
configured for 3mA. Condition 3 is the only situation
where the bus can be configured for 6mA, a
Fast-mode requirement when capacitive bus loading is
an issue.
4.2.1 OB Pull-Up resistor: R_PU-OB
Selecting the pull-up resistor for the OB bus is based
upon the manner in which the bus is expected to
operate within the restrictions listed above. Although
the additional design considerations discussed below
for selecting the IOB pull-up resistor, R_PUB , are not
applicable to the OB pull-up resistor, the Side B
pull-up resistors should have the same value to
minimize skew between the clock and data. Therefore,
setting R_PU-OB = R_PUB is recommended.
R02
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