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TPS2384 PoE PSE Power Manager - Optical isolation for longer lifetime

tripple_power
Prodigy 140 points
Other Parts Discussed in Thread: TPS2384

In the application notes for the TPS2384 opto isolator devices are used to get access to the I2C bus. For SDA_I and SCL the driver is connected to the LED cathode and the LED only flashes, if activity is on the I2C bus. Also, no polarity inversion is necessary, since the transistor on the output of the optical isolator is connected to GND.

For SDA_O, a more complicated approach is used. The LED is always on, except, when the SDA_O (here it is called SDA_OUT) pin goes low. The output of the opto isolator is now inverted and another inverter with open drain output must be added.

Wouldn't it be possible to use the solution for SDA_O below? If the SDA_O is inactive, no current flows through the LED and the output of the optical isolator (transistor) has a high impedance, thus, the pull-up of the I2C bus on the right side will cause a high level. If the SDA_O is active (low level), the LED is on and the output transistor of the opto isolator connects SDA with GND.

The benefit of this solution are not only the lower component costs but also a much longer lifetime of the opto isolator devices, since they are only active a small amount of time.

  • 0
    TI__Genius 9415 points

    Depending on the SDA_OUT pull up voltage, the opto may be on all the time. If you trim things to get the opto to switch, then SDA_OUT pin carries the total opto input current.

  • 0 in reply to Bob Kando
    Prodigy 140 points

    For my understanding, the SDA_O is an open drain output.

    In the inactive state, the current only flows through the resistor and the zener diode and only 5.6 volts (from the zener)  are on the output SDA_O. The data sheet of TPS2384 shows, that the leakage current at 5V is 100nA typ. I assume, it would always be low enough to keep the LED dark.

    In the active state, the full current flows through the output transistor of SDA_O and the current should be set somewhere around 5mA. There is no limit for this current stated in the data sheet, but it is noted, that the voltage is below 200mV at 10mA, so this should not be a problem.