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Voltage Controlled Current
Voltage controlled current is an indispensable tool in any engineer’s toolbox. Today I want to talk about one of my favorite Op Amp circuits. Using an Op Amp to control a current through a load has many applications. The one that seems to come up again and again is the need to easily control a current with a microcontroller. Yes, in the days of Internet of Things (IoT) the sensing and controlling of our environment has become paramount.
Figure 1Voltage Controlled Current Source
In Figure 1 we have an op amp providing control of a N-Channel MOSFET. The goal is to control the current in a load at around 1A. If the load current is 1A then the sense voltage is 100mV. This is what we are feeding back to the op amp. When Vin is set to 100mV the op amp will put out the necessary 1.6V so that 1A flows through the sense resistor and Vin+ and Vin- will be the same.
In this case I am using a FDN337 N-Channel MOSFET. We have talked about this transistor before in Issue # 9. Last time we used it as a switch. Today we will use it in its active region. Looking at the datasheet we see that a 1 amp current corresponds to VGS of 1.25. Add the 100mV and the gate voltage must be 1.6V. Also from the datasheet VDS = 0.15V; so the resistance of the transistor is RQ1 = 0.15/2 = 0.075. At 1 amp this has the transistor dissipating P = I2 x R = 1 x 0.75 = 75 mill-watts. This is safely within the 0.5-watt max power dissipation for the device.
The op amp is an ISL28194. This is a great op amp for battery operated circuits. It has a gain-bandwidth product of 3.5kHz and a slew rate of 1.2V/ms which means it is stable, does not couple with high frequency noise, but it is very slow reacting. The tradeoff is the current draw is only 330nA when in use and the enable pin allows it to drop to 2nA when off. This is a very economical device from a power standpoint. There are many applications where a slow reaction is appropriate and the power savings is critical.
Low input voltage
This works fine for 1A but what is you need to control a 10mA load. The input voltage is getting pretty low and even though this is a rail to rail device, things can get a bit funky when you get to close to the edges. There are a couple of ways to deal with this issue. One way would be to power the op amp with +/- 2.5. Then the input would be in the middle of the power supply range and life is good.
On the other hand, if you don’t have those voltages available or if you need the output to be 4.5 volts and your power supply is 5.5V you are stuck. In that case you could add another gain stage to the sense voltage and bring it up to 1 or 2 volts. If you do that, you might see some ringing in response to step changes of the input voltage because you have two control loops with the same response time and some lag in between them. You can slow down the transition of Vin either in code or with a low pass filter, or you can use an op amp with a higher slew rate on the sense line.
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