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ID: 840
Case Type: faq
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My device has a 3.3v VCCIO, can the device IO drive an LED directly?

That will depend on the LED voltage required for the LED to light up, and that depends on the color of the LED. The following are some typical and max voltages required for various colors of randomly selected LEDs:

Red, Vf typ = 1.8v, V max = 2.6v, If typ = 30mA
White, Vf typ = 3.2v, V max = 4.0v, If typ = 20mA
Blue, Vf typ = 3.0v, V max = 4.0v, If typ = 30mA

Of interest is the V max, that is the voltage required max to get the If current flowing through the LED. You can see that even at a typical white LED voltage of 3.2v and with a VCCIO of 3.3v with 5% tolerance (3.135v to 3.465v), that the voltage available from the VCCIO to drive the LED is potentially not enough to drive the white and blue LEDs, especially given that a series R would be added to limit the current used by the LEDs (LED current changes rapidly with applied voltage, series resistors are added to limit the current through the LED). The 3.3v VCCIO voltage is enough to drive the red LED with an added series R.

Whether or not the IO can drive the LED directly also depends on the current required to sufficiently light the LED in the intended application. Often times, 20mA of current is overkill with today's LED optical efficiencies, with 5mA or less being adequate. The 20mA current setting is also the maximum provided by many of the Lattice device IOs with VCCIO at 3.3v. In applications requiring more than 20mA of LED current, you will probably want to drive the LED with an external FET device, and possibly with the LED connected to a higher voltage such as a 5v supply. If you don't want to add the FET, you can also parallel device IOs with series Rs to each IO. The limitation here is that the total IO current can not exceed n*8mA where n is the number of IOs between successive GND pins on the device, or a GND pin and the end of a bank
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