Datasheet

Constant Current Driver

In order to provide a consistent level of illumination each of the R, G, B LEDs needs a

constant current supply of about 350mA. We opted for linear regulation for its simplicity and

low-cost, despite it being less efficient (and as such, dissipating more heat) than switching

regulation.

The constant current circuit is pretty cool. Let’s explain it by first considering the path of the

current through the LED. The current comes from the 5V supply, through the LED, then

through a nFET (Q1/3/5) then through a 1.74[Ohm] shunt resistor. The more resistive the

FET becomes between its drain and source, the smaller the current flowing through this

path. Now let’s see how we can use this to our advantage.

The NPN transistors Q2/4/6 have a specified 0.6V drop between base and emitter when on.

This means the voltage across their respective shunt resistors R1/3/5 will always be 0.6V.

According to Ohm’s law, this means that the current through them will be 0.6[V]/1.74[Ohm],

or about 344mA. Close enough. If the current were to decrease, the base voltage would

decrease proportionally, resulting in the NPN having more resistance between its collector

and emitter, thus causing a higher voltage on the collector (recognize the voltage divider

formed between the NPNs and their R2/4/6 pull-ups?). But this would result in a higher

voltage on the FET gate, causing it to become less resistive between source and drain and

as a result, higher current through the LED. The same logic can be applied in the opposite

direction. The conclusion is that this circuit is self-regulating the LED current.