This application note discusses how the TCA9509 from Texas Instruments is implemented to resolve improper setup with static voltage offset (SVO) buffers like the TCA9517 and the TCA9617A/B.
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With the intention of being compliant with the I2C standard for maximum capacitance, I2C buffers or repeaters can be used to separate the parasitic capacitances associated at the inputs and outputs. Some I2C buffers such as the TCA9517, TCA9617A/B, and TCA9509 devices introduce a static voltage offset on one side of the device. A common design mistake is placing two I2C buffers with the SVO side facing each other. This application note takes a deeper look into this violation, the repercussions of not addressing this design mistake, and how to resolve this violation using the TCA9509.
As seen in Figure 2-1, the two sides containing the SVO are connected together, which must not be carried out. Note that the SVO output voltage output low (VOL) is considered to be “buffered low”, which is generally higher than the VOL levels of other peripheral devices. Similarly, the voltage output low of the external device (VOL,EXT) is required to be less than the voltage input low contention (VILC) at the SVO side to be able to be propagated as an input low into the buffer. These characteristics of the SVO feature result in a gap between the buffered VOL of the first device and the VILC requirement of the second device. In other words, the VOL of 0.52 V is higher than the VILC of 0.4 V, so an input low of the first buffer is unable to propagate a low to the second buffer correctly. For more information regarding the SVO feature, see also the Why, When, and How to use I2C Buffers application note.
To resolve the implementation issue, the TCA9509 can be used in place of the second TCA9517 as shown in Figure 2-3, allowing for both the B side of the devices to be connected together. This device has the SVO on the A side instead of the B side, which mitigates the concern of improper SVO sides being connected together. The improved performance is shown in Figure 2-4.