Imagine ou like to connect a white LED to a microcontroller operating from a 3 V supply voltage. Unfortunately, this doesn’t work and your nice white LED only lights up feebly or not at all.
Why does it work perfectly with red and green LEDs, but not with white?
A bit of data sheet research reveals the reason: white LEDs have a forward voltage of 3.2 V, so a 3 V supply is simply not enough to let them light up properly. The advice you often see in online forums is to use a boost converter to generate a higher voltage, along with a transistor switch to control the LED. For just a single LED, this seems like a lot of overhead.
The good news is that th ere’s an easier way. And it only needs one inexpensive component: an inductor, which costs next to nothing. If you wire it up right and drive it the
right way, your white LED will light up nicely — and this even works with a microcontroller supply voltage as low as 2.5 V. Magic? Not at all.
The circuit shown in Figure 1 is a boost converter. It also goes by other names, such as step-up converter. But how does this minimalistic boost converter work?
Operating principle of a boost converter
Take an inductor L (a coil) and connect one end to the input voltage UIN and the other end to a switch S tied to ground. When the switch S is closed, a gradually rising current flows through the inductor L, creating a magnetic field. When the switch S is opened a bit later the magnetic field collapses, generating an inductive voltage over the coil (Figure 2), the same as an ignition coil in a car.
This voltage adds to the supply voltage, so the voltage UOUT (UA in the drawing) )at the diode D (which charges the capacitor C) is higher than the supply voltage. When the switch S is closed again, this process repeats. A LED is basically just a diode, so the diode D can be replaced by a LED (Figure 3). And the LED can be connected directly to ground.