Important changes to forums and questions

All forums and questions are now archived. To start a new conversation or read the latest updates go to forums.mbed.com.

how high is the max current on DigitalOut?

Topic last updated 09 Apr 2013, by Geert Hospers. 1 reply
Sascha Krüger
08 Apr 2013

hey guys,

i will 4 Led´s connected on PIN17, 18 19 and 20 as DigitalOut.

one LED had 20mA. is that possible? or i need a mosfet?

thanks for your answers

Geert Hospers
09 Apr 2013

In case you use mbedLPC1768 read chapters 8, 9, 10.in this document http://www.nxp.com/documents/data_sheet/LPC1769_68_67_66_65_64_63.pdf
Especially paragraph 10.3 (Electrical pin characteristics) shows you the voltage drop by increasing the output current demand. In short: 3.3V with 0mA drops to 2.0V with 18mA. So your 20mA led actually will run on 1.8V. It depends of your led type if 1.8V still will produce light.
From the graph at 10.3 you can read that 18mA load gives a drop of approximately 1.3 Volt: it is like there is a resistor of 72 ohms. This means that you can safely connect almost any led without a separate resistor.
I think that if you really want at least 20mA going trough the led that you may need a mosfet.

But it depends on the led characteristics. Some examples: http://upload.wikimedia.org/wikipedia/commons/0/0c/Ledkarakteristieken.png