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I'm modifying a step-up circuit based on UC3843 SMPS Controller to add a current limiting feedback to it. I'm using a low-side sense resistor (Rsense) amplified by an LM358:

schematic

simulate this circuit – Schematic created using CircuitLab

The feedback seems to work but there is a problem with the op amp: its output is slowly rising from the time i turn it on by about 1mV/s making the output current of the step-up module slowly going down. The LM358 behaves like this even when disconnecting its output. I also tried replacing it to no avail.

Can you help me solve solve this issue?

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  • \$\begingroup\$ Maybe the sense resistor is heating up, resulting in its resistance changing? Does the LM358 output keep rising, or does it reach a stable state after a while? \$\endgroup\$ – marcelm Jun 26 '16 at 15:04
  • \$\begingroup\$ Thanks, i checked the voltage drop on Rsense but it is actually stable while the output keep raising... The rise is quite stable and doesnt stop! At least not after 5 min :( \$\endgroup\$ – Michele M. Jun 26 '16 at 15:19
  • \$\begingroup\$ What gain have you got i.e. what is the 10k pot resistance wise? \$\endgroup\$ – Andy aka Jun 26 '16 at 16:25
  • \$\begingroup\$ I'm currently testing with a gain of 16, i'll check for temperature drift now according to your answer \$\endgroup\$ – Michele M. Jun 26 '16 at 17:02
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It could be input offset voltage drift with temperature.

The data sheet says it is typically 7 uV/degC and with a gain about 680 (10k resistor set to zero), this will cause a shift in output voltage of 4.7 mV for every degC that the LM358 warms up. It should eventually settle down of course.

Try touching the LM358 with the tip of a soldering iron to see if there are significant rapid shifts in output voltage.

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  • \$\begingroup\$ Thanks, i just tested that. I leaved it on for few minutes, and the LM358 was not even warm to the touch (unfortunately i dont have anything to measure temp, but to me it seemed to be at room temp). Touching it with a soldering iron made the output shoot down like 20mV for few seconds, but when removed it rised back up and kept rising by that 1mV/s. I also double checked the Rsense drop and it was stable all along, kinda frustrating :( \$\endgroup\$ – Michele M. Jun 26 '16 at 17:30
  • \$\begingroup\$ I think you should choose a better op-amp i.e. one with much smaller tempco on offset voltage and bias currents. It's gonna cost more though. You might also consider replacing the pot with a fixed value resistor. \$\endgroup\$ – Andy aka Jun 26 '16 at 18:26
  • \$\begingroup\$ Since i cant find a way to stop this behaviour that's what ill do... which IC would you suggest, possibly with the same pinout? \$\endgroup\$ – Michele M. Jun 26 '16 at 21:01
  • \$\begingroup\$ There are plenty - try using Linear technology's search engines or TI or ADI. Try also using Farnell (UK distributor' search engine). Try initially looking at TI's OPA333 - it might be suitable. The AD8605 from ADI also might be useful. \$\endgroup\$ – Andy aka Jun 27 '16 at 7:16
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Since i solved the issue, i may as well post the answer.

The problem was related to the actual wiring on the stripboard: floating inputs (LM358, one pin of the trimmer, etc) and long wires was causing relevant parasitic effects generating the problem.

A re-design and re-wiring of the circuit fixed the problem!

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