Many times in an embedded system, we need to supply a constant or variable current to power a transducer, excite a sensor, or charge a laser. In this article, I briefly show how a current source works and review different types of sources I have seen or used.
IV curve and compliance voltage
A constant current source IV curve is usually depicted as a line parallel to the voltage axis; that is, the source is a constant current that is independent of the load voltage. In the real world, a constant current source is independent of voltage only over a limited range. Figure 1 shows a typical IV curve for a real-world constant current source.
Figure 1. IV curve of a real-world current source
This curve shows the current is constant (independent of voltage) only over a limited voltage range. Once the load voltage increases beyond the source's ability to regulate, the source falls out of regulation. The voltage limit is known as the compliance voltage. In general, it is up to the designer of the source to make certain that there is enough compliance voltage so that when the limited load changes, the current will remain constant.
The simplest current source is the series resistor connected to a load. Usually, the series resistor value is much larger than the expected load resistance. The idea here is to overwhelm the small load resistance so that the current to the load appears constant over a very limited voltage range. This source is shown in Figure 2.
Figure 2. A large value series resistor as a current source
In general, the compliance voltage for a series resistor current source is typically on the order of a few millivolts. There are times when a simple and cheap series resistor can do the job. In general, this type of source is very good when the expected voltage change in the load is very small. The equation for this type of source is trivial and is shown in Equation 1.
Equation 1. The current source equation for a simple series resistor current source
Active Device Current Sources
Things get a bit more interesting when we add transistors or ICs. Figure 3 shows a typical current Zener diode transistor current source. We get much better regulation here.
Figure 3. Zener transistor current source
The primary disadvantage with this source is the low compliance voltage and the temperature dependence of the current with respect to the transistor junction temperature. Using a matched transistor in place of the forward biased diode may mitigate some of the temperature effects.
Equation 2 shows the relationship between the current (is), programming resistor (Rs) and the diode breakdown voltage (Vz). The regulation is much better here than with the simple series resistor current source.
Equation 2. Zener transistor current source relationship
Equation 3 shows the compliance voltage (Vcompliance) for this current source. Here, the rail voltage is shown as (Vcc) and the biasing diode forward voltage is shown as (VF).
Equation 3. Zener transistor current source compliance voltage