Increasing digital I/O pin current with MOSFET

Hi Digi-Key,

I’m currently working with a microcontroller that operates at a steady 3.3 V when plugged into a constant voltage source (or can alternatively be operated by a Li-Po battery which operates from 4.2 V [when fully charged] to 3.0 V [when fully discharged]). This microcontroller produces 15 mA through its digital I/O pin; however, I wish to use this microcontroller to control the pulsing of a diode which has a threshold current of 33 mA and an operating current of 63 mA. The operating voltage for this diode is 0.9 V +/- 0.05 V.

Based on my own research, I believe I will first need a MOSFET transistor to increase amperage from the digital I/O pin and then a resistor to drop the voltage. It would be extremely helpful if you could point me in the direction of the proper components to prevent the diode from frying and allowing it to operate optimally.

Thanks for your help! And awesome YouTube channel btw.

Cutler Whitely

Hello Cutlerwhitely,

Looking over your circumstances I would wonder first what the actual failure mode of your diode would be. Just because it operates at 63mA doesn’t mean it couldn’t handle more, so be sure to check its documentation. If you need to have a finite level of control a operational amplifier circuit with a specific gain tuned to your microcontroller, or an infinite gain with a 1V source could work for you. (Op amps with a ~60-150mA out can be found here.)

I am sure a great deal of solutions exist in this case, a linear voltage regulator as a solution also comes to mind.

Hi Cutler,

To help further, we could use some clarification on your diode. Your description is a bit unusual. What type of diode is it, and what is it’s part number? Is this a standard diode, a Schottky diode, a zener diode, an infrared light emitting diode (IR LED), other?

An arrangement such as the below would be a typical approach. R1 selected as per this post, Q1 selected to have adequate voltage and current withstand ratings, while having sufficiently low gate voltage requirements as to be controllable from the available drive source. TN0702N3-G being a possibility.

Such approaches depend on regulation of the supply voltage to maintain consistent current flow. Where such regulation is unavailable, a current regulation IC such as a BCR421 can be used instead to improve regulation.

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FETs behave more or less as voltage-controlled resistors; the resistance between drain and source of the suggested device will be many megaohms when voltage between gate and source is zero, and approximately 2 ohms with 3v applied between gate and source. This behavior can be used to implement an on/off switching function.

The BCR421 device suggested incorporates an enable function; this could be driven from an I/O pin to achieve switching function without addition of a separate FET. Note that the device requires supply voltage at least ~1.4V greater than the forward voltage of the driven diode for proper operation. While it would appear that this requirement would be met in this case, the 0.9v Vf figure mentioned for the referenced diode seems low; please verify that this figure is correct.