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
Figure 4 shows a very popular current source configuration using athree-terminal voltage regulator. Some of the advantages of thisconfiguration are the relatively high compliance voltage, goodtemperature coefficient, and the ability to generate appreciable amountsof current. The three-terminal current source can be used to generateseveral hundred milliamps at constant current with very good regulationif proper thermal strategies (heatsinking) are used.
Figure 4 . A current source using a three-terminal voltage regulator
The current transfer characteristic for this source is shown asEquation 4. The current is set via the programming resistor value (Rs ).
Equation 4 . Three terminal voltage regulator current source programming
The compliance voltage of this current source is quite good. Given the drop-out voltage, (VDO ) of this source, we can show the compliance voltage as Equation 5.
Equation 5 . Compliance voltage of the three terminal voltage regulator-based constant current source
Closely related tothe constant current source is the transconductance amplifier, which isany circuit that converts a voltage to a current. This amplifier can bethought of as a variable current output device with a voltage input.Typically these amplifiers are connected to the digital-to-analogconverter (DAC) output of a microcontroller to program a specificcurrent for excitation.
The classical example of a transconductance amplifier is the Howland Current Pump, which converts a differential voltage, (V1 and V2 ) to a load current using some feedback resistors (Ri and Rf ) and a series measuring resistor (Rs ). The Howland current pump schematic is shown in Figure 5.
Figure 5 . The Howland current pump transconductance amplifier
The transfer function of the Howland current pump is shown inEquation 6. Great care must be observed with the resistor selection. Ifany positive gain is placed (ratio of Rf / Ri ),then this gain will reduce the input voltage programming of the pump.For example if we desire a 100 milliampere current source passingthrough the sense resistor Rs , and we have a gain (Rf / Ri ) of ten (10), then the voltage difference between V2 and V1 will only be 10 millivolts!
Equation 6 . Transconductance transfer function of the Howland current pump
The advantages of the Howland current pump are its programmability,two quadrant operation, reasonably high compliance voltage (variable),and relative ease of programming. Its disadvantages are its low currentoutput (unless a power amplifier is used inside the feedback loop) and arelatively small current programming range. Howland current pumps aretypically used when a two-quadrant low current programmable currentsource is desired.
Another transconductance amplifier is the single quadrant highcurrent sink. This configuration is shown in Figure 6. The regulation ofthis configuration is very good. However, care must be taken to selectthe proper loop time constant for stability. This means the timeconstant of the product of (RT x CT ) must beproperly selected. This amplifier is quite commonly used for circuitssuch as laser diodes that require high current levels. Since the timeconstant (RT x CT ) is in the low audio range, thiscurrent source can be easily modulated with an RF signal, which allowsthe laser to be used as an RF transmitter.
Figure 6 . A single quadrant high current programmable current source
Figure 6 shows the gain resistors, (Rf , Ri ) and the sense resistor (Rs ). The control voltage is shown as (VICTL ) and we get a monitor output (VIMON ) for free! It is also recommended that the sense connections to the sense resistor (Rs ) be connected as a Kelvin connection.
The transfer function of the single quadrant current sink is shown in Equation 7. The circuit layout also shows that the VIMON should track the VICTL values to within the tolerance of the resistors.
Equation 7 . Transfer function for the single quadrant current sink
The compliance voltage for the single quadrant current sink isusually very good. In general, the compliance voltage is limited by thesaturation resistance of the FET (RDON value) plus theresistance of the sense resistor. Of course, the maximum available railvoltage of the amplifier will have some effect for very high currentapplications.
Since this type of current source is linear (class AB) in operation,it is very important to follow very good thermal management practicesfor this type of circuit.
It is also important to note that the amplifier connected to the gateof the FET must be able to source voltages much higher than the gatethreshold voltage of the FET. The gate threshold voltages of most powerMOSFETS are in the 3.00 to 3.5 volt range.
The advantages of the single quadrant current source are itsprogrammability, high current capability and ease of programming. Itsdisadvantages are its single quadrant operation and relatively lowerbandwidth of operation compared to the Howland current pump.
Variations on a Theme
One of the problems withvery low current excitation in the DC bandwidth range is that we areplaying around with very high impedances. In general, this means we mustdeal with a lot of noise. This can be a considerable problem if we wishto provide a very low and precise current while taking care of noise.
One of the ways we can solve this problem is to provide a veryprecise, low current, programmable AC current source. Figure 7 shows amethod of solving this problem by taking the series resistor currentsource (Figure 2) and dressing it up using a microcontroller and somecircuitry.
Figure 7 . A microcontroller programmable AC current source with feedback
Here, we have a precision AC voltage reference along with a2-quadrant multiplying DAC to set the peak-to-peak voltage level intothe series resistor. We then AC couple the load through a high-passfilter and bias the load voltage for proper analog-to-digital converter(ADC) operational input. We can further band limit the input signalusing a band-pass filter to further reject the noise and make a morerefined signal acquisition.
Of course, it would be good to have some PLL and synchronizingcircuitry. This will allow us to properly sample the incoming signalfrom the load synchronous with the precision AC reference. With thisdigital synchronous detection, we can go quite far with suppressingnoise while using a very low current source.
The other thing we can do is to vary the amplitude gradually byprogramming the DAC. Sometimes this is necessary if the load in questionis a biologically active sample that would be destroyed with theapplication of too much voltage or current. The programming andread-back of this type of source can make this effectively a constant ACcurrent source via the programming algorithm embedded in themicrocontroller.
We can also get rid of the AC voltage signal generator if we use afast enough microcontroller (e.g. DSP). In this case, we can generate areal time pattern (usually a sine wave) as the input to an on-chip DAC.In this case, we would need to add an anti-aliasing filter to the DACoutput to reduce the noise generated at the sampling frequency of thepattern generator.
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