The specification you refer to is the port pin sink current (via the N-channel MOSFET). A higher voltage across the LED means a lower voltage at the pin, and therefore less current through the LED.
The output characteristic of the MOSFET, when conducting, may be represented by a resistance to the negative rail. For 3 volt supply, the calculated resistance value may vary between approximately 130 ohms and 670 ohms (unit-to-unit). A red LED would have a voltage drop in the vicinity of 1.8 volts, so the current through the LED may vary between about 2mA and 9mA, with no external series resistance. If this current is insufficient, or the variation of current is too wide, an external buffer transistor or MOSFET would be necessary (including a resistor in series with the LED).
For a 5 volt supply, the internal N-channel variation would be approximately 120 to 330 ohms.
Message Edited by bigmac on 2006-06-17 01:03 AM
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