Texas Instruments TCA39306, User Manual, Page: 3 / 15

2.3 Load Capacitance Selector
Figure 2-2 shows the board with the load capacitance selectors labeled.
Figure 2-2. Load capacitor selector
The TCA39306 EVM provides a way for the designer to test the device under capacitive loading which is one
of the main constraints in an I
2
C system. Switches denoted as S1, S2, S3, and S4 provide the user with the
ability to change the I
2
C bus capacitance with up to about 525 pF of capacitance. The switched capacitor can
be selected in increments of approximately 8 pF, 1 6pF, 33 pF, 68 pF, 130 pF, and 270 pF. For example: if the
designer wants a total capacitance of 275 pF, the designer chooses to switch in 130 pF, 68 pF, 33 pF, 16 pF,
and 8 pF for a combined capacitance of approximately 255 pF, device capacitance and PCB capacitance can be
estimated to be about 20 pF resulting in approximately 275 pF.
Note that there is additional capacitance due to parasitics from the PCB line traces and ICs on the board. For
SDA and SCL pins of a device, assume 10 pF maximum per device on the I
2
C line. One important aspect to
remember is the TCA39306 device does not redrive an I
2
C signal. This means the capacitance on both sides
of the device are not separated but are technically connected (meaning they add up in parallel) when the signal
goes from LOW to HIGH from either SDA or SCL. This is highlighted because the I
2
C standard limits the bus
capacitance to 400 pF for standard and fast mode and 550 pF for fast mode plus. If the I
2
C system is expected
to exceed this amount, an I
2
C buffer and redriver will be necessary to separate the capacitance between the two
sides to be below the required capacitance limits.
Current I
3
C specifications state bus capacitance loading should be less than 50 pF, so all switches should be
turned to the off position when testing with I
3
C.
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SCPU045 – JUNE 2021
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TCA39306 EVM User's Guide 3
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