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# How to set the current limit of an LT3081 using a DAC?

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.

Parents
• Regarding a digital potentiometer, the AD5272-20 is a 20kohm digipot (I2C) with 10-bit resolution (1024 steps). You could put it in parallel with a fixed resistor to adjust the range and resolution of the net resistance on the I_LIM pin.

Regarding using a resistor, I wonder if you can make an op-amp based negative resistance circuit to skew the effective resistance on the I_LIM pin : https://electronics.stackexchange.com/questions/52860/negative-resistance-impact-on-voltage-source

I'm working on a somewhat-similar personal project and I'm using the LT3086, which has a more explicit current-limit circuit. I'm tentatively using the AD5272 and an analog switch (ADG5401) as a hi/lo range toggle.

• Regarding a digital potentiometer, the AD5272-20 is a 20kohm digipot (I2C) with 10-bit resolution (1024 steps). You could put it in parallel with a fixed resistor to adjust the range and resolution of the net resistance on the I_LIM pin.

Regarding using a resistor, I wonder if you can make an op-amp based negative resistance circuit to skew the effective resistance on the I_LIM pin : https://electronics.stackexchange.com/questions/52860/negative-resistance-impact-on-voltage-source

I'm working on a somewhat-similar personal project and I'm using the LT3086, which has a more explicit current-limit circuit. I'm tentatively using the AD5272 and an analog switch (ADG5401) as a hi/lo range toggle.

Children
• Thank you for your suggestions. I'm afraid AD5272 will not be suitable in my application because it's only rated for 5V whereas I need something that can withstand at least 30V.

But I'm curious to learn more about your idea with negative resistance. Would you please draw up a quick schematic to show what you mean by skewing the effective resistance?

The link you provided shows an example of this concept being the Howland current pump which is something I was also simulating and considering to prototype as a current source. Something like the following:

Is this what you mean? Or were you thinking of a different configuration?

I haven't prototyped this circuit yet, and the reason is that it is very susceptible to mismatches in resistors. Perhaps it requires a precision difference amplifier, something I don't have around atm. Also I'm not sure how safe this is since the datasheet for LT3081 suggests an absolute maximum difference between ILIM and OUT pins to be 200mV.