Can I direct drive an LED from a KW41Z low power I/O at 1.8V

[stevenriedl]

I'm trying to drive both discrete LEDs and an OptoTriac directly from the KW41Z with the DC-DC in Buck mode and V I/O at 1.8V. After some digging in the data sheet I found the low power current to be 2.5 mA. The forward voltage of the  optotriacs seems to be 1-1.6V. They have a max current of 60 mA, but it looks like I won't come close to that.

Any comments on driving them directly from the pins with no dropping resistors?

http://www.vishay.com/docs/84797/vo4154.pdf 

https://www.fairchildsemi.com/datasheets/FO/FOD410.pdf 

Yes, I could set the dc-dc to 3 or 3.3 V, and the main reason I might have to do that is if I want an RGB or blue LED.

[stevenriedl]

We must be reading different manuals for different parts. Section 7.2.3 Table 12 of the MKW41Z512 data sheet.

You have high drive and low drive pins, when at 1.8 V VIO can source or sink 2.5 mA, at 3.3 V VIO it can do 10 mA with a chip total of 100  mA.

I guess I will experiment. I was planning on running VIO at 1.8 for lowest current consumption and feeding the DC-DC with 4.03 V, also for lowest current consumption.

Was wondering if anyone had experience driving low forward voltage LED's direct off the port. I guess my options are,

1) Drive LED off a low power port, should current limit to 2.5 mA

2) Drive LED off high power port, should current limit at 10 mA

3) Sink LED driven off VIO with a resistor, may have to bump up VIO in DC-DC

4) Sink LED driven off VBAT (4V), can the chip handle that if the VIO of the pin is 1.8 or 3.3V

[jorge_a_vazquez]

Hi Steven Riedl

Please first consider that if you going to use your DC-DC in Buck mode, to start up properly, the battery voltage needs to be higher than ~2.1V. After DCDC startup, the DCDC can continue to operate if the battery voltage drops as low as
1.8V.

Now, for this devices, the max current that a pin can supply is 25 mA, where do you find the 2.5 mA specification? As long as it doesn't get close to max current of 60 mA, it seams that there should not be problem by the MCU, it is related with the current that this pin can supply.

Best Regards

Jorge Alcala