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.
