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Why is the logic LOW level output voltage, VOL1, up to 0.8V on Side1 of the ISO1540 and ISO1541 bidirectional I2C isolators?
To achieve the bidirectional functionality of the isolated I2C device, the device needs to be designed with two unidirectional channels connected back-to-back to achieve a single bidirectional channel. Connecting the two unidirectional channels back-to-back directly would lead to a lockout situation, where both channels are LOW. To avoid this, a diode has been introduced at the output of Side1. This is to make the LOW output of the output channel on Side1 look like a HIGH for the input channel of Side1. The diode placement is shown in the diagram below.
VOL1 will have a voltage of up to 0.8V because of this diode. When a LOW is seen on Side2, Side1 will turn on the FET, allowing the diode to conduct, generating a nonzero forward voltage. The thresholds in the ISO154x device are carefully designed to make sure the bidirectional channel operates smoothly, as long as the VOL specifications are acceptable and VIL specifications are met, and falls within the thresholds of the ISO1540 device shown in the Electrical Characteristics section of the ISO154x datasheet. This has been the common practice in the industry for achieving bidirectional I2C function. A non-zero voltage of logic level LOW will still be compatible with I2C specifications.
Please note that this only applies to VOL1. Since no diode is required on side2 of the device the VOL2 will be 0.4V max, which is common in most digital isolators.
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