The basics behind constant current LED driver circuits
Almost anyone who works in electronics is familiar with constant voltage LED drivers, although it may not be recognized as such. The classic digital output plus series resistor arrangement is essentially a constant voltage scheme; it may appear that the resistor is establishing a fixed current, let's say approximately (V DD – 0.7 V)/R series, but in reality the circuit consists of the diode's exponential current - Voltage relationship control.
The constant parameters here are V DD and R series; the current is then determined by the intersection of the load line and the diode characteristic curve. Changes in this curve (which, of course, are not the same for every part) can cause changes in current flow.
This approach is perfectly suitable for a variety of non-critical LED applications. But any circuit that doesn't actually control the current flowing through the LED has inherent weaknesses for the simple reason that forward current is more important than forward voltage when it comes to LED operation.
The brightness of the LED is determined by the forward current. When you get into the details, the question becomes a little fuzzy because forward voltage is related to forward current according to the exponential relationship mentioned above. Therefore, it is difficult to separate current from voltage when considering the effect on brightness. But it makes sense to think of current as a quantity that determines brightness, since a more or less linear relationship between forward current and brightness is more direct and useful than the relationship between forward voltage and brightness. So if you want to adjust the brightness, you need to control the current.
If the forward current is exceeded, the LED may be damaged. Applying too much voltage is not a big problem since the voltage drop across the diode does not increase significantly after entering the more vertical part of the exponential current-voltage relationship. What is increased is not the voltage drop, but the forward current, which is an amount that needs to be limited according to the specifications in the datasheet. It's easy to adhere to the forward current specification when all you need is a simple indicator - you can use a suitably sized series resistor to keep the current well below the limit. But what if you want to maximize the luminous intensity of an LED (that is, get as much light as possible from a single LED)? In this case you need to push the forward current to the value and do it safely,
Perhaps a simple way to achieve constant current LED control is to use an integrated circuit designed specifically for this purpose - there are many such devices available. These LED drivers include a variety of useful features; they can simplify your design and, with their power-saving features, help extend battery life in portable applications.
For those who prefer to design their own circuits, op amps can provide adjustable constant current LED drive:
The action of negative feedback causes the op amp to increase or decrease its output current until the voltage across the resistor matches the control voltage applied to the non-inverting input terminal.
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