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.

[MartinBL]

Start 2/2:

Regarding the PWM option; Unfortunately we havn't tested, and hence have no values, for maximum peak currents on the GPIO. So while you might be within limits of the LED, you are in the blind on the nRF52. Proceed with caution and at your own discretion.