We all know that you can drive an LED with nothing more than a voltage source and a series resistor. But this is not the optimal approach. I won’t go into all the details because we already have an article that covers this topic. Here’s the abbreviated version:
Current vs. Voltage
The critical parameter when driving an LED is forward current, not forward voltage or applied voltage. The relationship between forward current and brightness is approximately linear, whereas the relationship between forward voltage and brightness is exponential. This means that small changes in forward voltage correspond to large changes in brightness; it also means that precise control of LED brightness is more easily achieved by adjusting the current.
If you drive your LED with a voltage source and a resistor, the actual current (and therefore the actual brightness) depends on the exact current–voltage characteristics of each LED. As shown in the following figure, part-to-part variations will cause some LEDs to be brighter than others.
Maximum brightness requires maximum forward current, but too much forward current can damage the LED. Thus, if you want your LED to provide as much illumination as possible, you need to carefully control the current.
Circuit diagrams regularly contain two mythical creatures: fixed voltage sources and fixed current sources. These idealized constructs are helpful for analysis but are not representative of reality.
However, at some point in your journey through the world of electronics, you probably noticed that there is a mysterious lack of equilibrium with regard to real-world sources: (nonideal) fixed voltage sources are everywhere, whereas we might be well into third- or fourth-year engineering courses before we have a clear idea of how to create a (nonideal) fixed current source. Thus, we should address the issue of how exactly to go about implementing constant-current LED drive.
If you like the more homemade approach, you can use an op-amp.
This technique is discussed in a previous article. Basically, you use an op-amp’s output voltage to drive an LED; the voltage across the LED determines the forward current (as usual), and you use negative feedback to fix the current according to a reference value.
In my experience this strategy is quite effective, and I imagine that you could achieve very high precision if you were careful about component selection. But as usual, it’s often more convenient to use an IC that is specifically designed for your application (or at least something similar to your application). In this article we’ll take a look at the BCR401U, which is a new constant-current regulator from Diodes Inc.
No External Components
As you might already know, I enjoy ICs that require few external components. It allows me to reserve my intellectual energy for other things. Thus, the BCR401U receives my initial approbation:
As you can see, the internal circuitry is fairly straightforward (at least when the details are omitted):
Diagram taken from the BCR401U datasheet.
However, it’s important to keep in mind that the precision provided by a circuit like this is much lower than what could be achieved with an op-amp. The BCR401U gives you output current with ±10% accuracy; however, the idea here is that the ±10% is adequate in many applications, and of course the BCR401U offers advantages over an op-amp implementation.
One such advantage is that the BCR401U accepts supply voltage anywhere from 1.4 V to 40 V. (If I understand correctly, you need at least 1.4 V across the part itself, meaning that you have to account for the voltage drop of the LEDs that are in series with the BCR401U.)
Also, as mentioned above, no external components are required if you’re happy with the default 10 mA output current. If you want to adjust the current (up to 100 mA), you need an external resistor. You’ll also need some additional components if you want to include PWM dimming functionality (the BCR401U can handle PWM frequencies up to 25 kHz). Page 7 of the datasheet provides information on PWM control.
Another interesting feature is what the datasheet describes as “self-protection.” The BCR401U has a negative temperature coefficient, i.e., higher temperatures lead to reduced output current.
Plot taken from the BCR401U datasheet.
This avoids the less-than-desirable situation in which increasing temperature causes increased output current, which causes more power dissipation and thus higher temperature, which causes increased output current . . . you can see where this is going. The problem is called thermal runaway, and it comes into play when you are driving LEDs in parallel.
Do you have any experience with high-performance LED applications? Let us know in the comments.