GPIO drive high Vf LED w/o current limiting resistor?

[butch]

Specifically, sinking a Cree C503B green LED (Vf 3.2V at 20ma) from a GPIO configured for high drive, without a current limiting resistor, from VDD 3.6V?

Figure 23 “GPIO drive strength vs Voltage, high drive VDD=3.0V” of the nrf52832 product spec, seems to show that the voltage drop across the GPIO rises with current. My reasoning is that the circuit would stabilize at about 0.5V drop across the GPIO, 3.1V across the LED, and 13mA current. I know an LED should be driven from a constant current source, is a GPIO in some sense regulating the current, at least in this circuit? I am not a electrical engineer.

[RK]

it's not a very good idea. High drive will still limit current, although much less-so than low drive. However it will let you pull about 15mA through the pin. That's ok as long as nothing else is pulling current through pins. If it is you're exceeding the total GPIO current draw of 15mA and bad things are likely to happen.

If you really want to drive a 20mA LED you should really just put a driver circuit on it, an sot-23 mosfet is plenty good enough, and a current limiter (unless your power supply is sufficiently close to 3.2v you're on Vf anyway). The nRF series aren't like the chips they put into arduinos which are designed to drive things directly, it's low power, doesn't have a big fat power bus in there.

[butch]

I was actually considering using PWM to the LED to limit the current, without a resistor. Like a switching power supply, slightly more efficient and less heat. Isn't the real limit not current but a thermal limit, the chip can't dissipate the heat? My internal, electrical model of a GPIO is a MOSFET, but a poorly characterized one. Thats why some suggest using an external MOSFET, because it is more fully characterized and you know what you can safely do with it. I'm not sure Fig. 23 corresponds to a simple MOSFET model.