LT3081
Recommended for New Designs
The LT3081 is a 1.5A low dropout linear regulator designed for rugged industrial applications. Key features of the IC are the extended safe operating area...
Datasheet
LT3081 on Analog.com
MAX4007
Production
The MAX4007/MAX4008 precision, high-side, high-voltage
current monitors are specifically designed for monitoring
photodiode current in fiber applications...
Datasheet
MAX4007 on Analog.com
I’m building a digitally controlled lab supply using a pair of LT3081 linear regulators. With the help of circuit note CN0508, it was easy to figure out how to drive the set pin with an opamp. Unfortunately though in that application example, the current limit is still set by a resistor. So my question is how can I set the current limit with a voltage signal from 0-5V?
According to the LTspice simulation, it appears the LT3081 sources 6uA out of its ILIM pin which establishes a small voltage above VOUT and that sets the current limit. This is nowhere to be found in the datasheet. After tying the ILIMT pins of the two regulators, I’ve tried putting a small resistor like a 100 Ohm between ILIM and OUT and then inject some additional 0-250uA from another current source into the ILIM node, fooling the regulators into thinking the resistor is bigger than it is. I’ve been using a current mirror to source the extra current. I tried a pair of matched transistors as well as the MAX4007 10:1 current mirror.
In reality, this scheme more or less works, but not very well. It sets the current limit okay, but as soon as the regulators enter current limiting mode and heat up, there is significant drift. The output current does not stay still and drifts down tens of milliamperes in matter of seconds, especially when current limit is set to 2 - 3 amps. It seems somewhat correlated with temperature, but I’m not sure. It’s worth noting that this drift does not exist when using a potentiometer. The output current is rock solid during current limit when only a resistor is used to set the current limit.
Digital potentiometer was another option, but high voltage pots are hard to find and resolution is very low. Most I could find on the market is 256 taps. Ideally, I’d like to be able to digitally set the current limit between 0 to 3 A in 1mA steps.
So I’m reaching out here hoping some experts can help push me in the right direction. Please advise me on how to control the current limit digitally. Thanks for your help.
I came back to this project recently, and after some experiments, it turns out indeed the 6uA coming out of the ILIM pin of the regulator is not quite constant, but there is a temperature dependent tiny current added on top of the 6uA that boosts up the current limit voltage as the IC temperature rises. I verified this by feeding the 1uA/C out of the TEMP pin directly to the ILIM resistor (or to the middle of it using a pot) and observed that the output current now remained steady in current limiting mode as the IC temperature changed over time.
So this complicates the matter, and it mostly voids the attempt of feeding extra current to a small resistor at the ILIM pin. This method would be fairly unreliable for current limiting because the output current in current limiting mode would be all over the place, and it becomes difficult to calibrate.
I came up with a different solution that I want to share here, hoping to get feedback from the analog experts on this site, as well as to potentially help future readers who may be interested in controlling the current limit of this regulator digitally. I essentially added another opamp to monitor the IMON signal that simply brings down the set voltage as the current monitor value increases above a certain threshold. Here's what I came up with:
The compensation capacitor values can of course be tweaked as it depends on the opamp used, or a snubber network can be used at the opamp output instead. I built the above circuit with the OPA2189 precision opamp, and used a switch mode pre-regulator with a differential feedback as shown in CN0508 to keep the VIN 2 volts above VOUT. In reality, it seems to work very well. The current limiting is reasonably fast at a hundred microseconds or so, and it recovers without any overshoot. Best of all, finally I get a current limiting facility that doesn't drift around.
It's worth noting that with this circuit, one can easily detect when the supply is in current limiting by sensing the output of either opamp with a comparator. In contrast, the LT3081's own current limiting has no good way of having an indicator signal like this, to light up an LED for example when the device is in current limit.
Some readers may look at this and think, well at this point, why not just use a series pass transistor and just build the supply from scratch? 
The LT3081's here are used purely as an emitter follower any way. That would be an alternative, but then the frequency compensation of the voltage control loop would be annoying, and it would then be sensitive to pcb layout and may oscillate with different output capacitors and so on. The LT3081's regulate the voltage very nicely and they are quite stable. And they have all the requisite protections built in.
I'm open to comment and suggestions about how to improve the circuit above, and I hope that it can potentially help someone who may be interested doing a similar thing.
Yes, I did in fact build it, it works quite reliably. It's one of my better supplies.
Yes, I did in fact build it, it works quite reliably. It's one of my better supplies.
