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Using UART TX line to supply power when idle

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sebmadgwick:
I am designing a device with a connector for UART TX and RX lines.  The device will offer an alternative mode of operation where the UART TX line is not used for data and is instead used to supply power, e.g. 50-100 mA at 3.3 V.  A connecting peripheral would be powered by the TX line and send one-directional data on the RX line, similar to how old serial mice would operate.

My first idea was to buffer the TX signal through a line driver IC.  The TX line could then be held high to supply power.  Unfortunately line drivers seem to be limited to supplying no more than ~20 mA.

Load switches or similar ICs are not viable because they add output capacitance that would destroy the UART signal.

I tested the attached circuit but found this destroyed the UART signal.  I would like to support up to 3 Mbaud.

I'm now considering using a multi-channel tri-state line driver with all channels in parallel.  For example, the 74LVC125ABQ can supply up to 24 mA per channel.  The quality of the UART signal would not be compromised by the presence of the high-impedance outputs.  However, I'm not keen on this idea because any imbalance between the channels risks a runaway failure.

Any suggestions?


NiHaoMike:
Replace Q8 with a short from drain to source (less capacitance for the microcontroller to drive), then add a small N channel MOSFET to connect the output to ground if the input is high. You might have to add a resistor in series with that MOSFET to limit the cross conduction current. If you want the output to be noninverted, add an inverter gate in front of the input.

sebmadgwick:
I should clarify that the schematic in my previous post is intended to supply power or be high impedance.  It is connected in parallel with the UART TX signal, it does not drive the UART TX signal.

NiHaoMike:
Try a smaller MOSFET with less capacitance. Or a PNP transistor.

TomS_:
Check the maximum current that the VCC pin of the 74LVC125ABQ can handle. It may be less than the sum of all outputs.

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