Electronics > Projects, Designs, and Technical Stuff
Adjustable Voltage Supply: Schematic Concerns
LoveLaika:
I'm developing an adjustable voltage supply to give out positive and negative voltages as well as current from each supply, up to 2.5 amps. The schematic is shown below. I'm using the LM2673, working off of a 15 V input going to both of the regulators, and I have a 1-ohm sense resistor with an op-amp for each supply in order to adjust the current. I ran this in SPICE, and it seems to work well in practice, but I wanted to get a review of my schematic. The values for the components match up with TI's guidelines, but I was wondering if there were any improvements that I should look into or consider. One thing that concerns me as well is the output. The positive output is straightforward, but for the negative output, I labeled the connector accordingly based on how I had it hooked up in SPICE, so if I take the voltage difference between NEG_RETURN and NEG_OUT, NEG_OUT should be at a lower voltage. Overall, does this circuit look okay?
xavier60:
The CC loop might be unstable. What does pin 7 of the op-amp look like?
LoveLaika:
Well, the first op-amp acts as a differential amplifier (G=1) while the second one acts as a comparator, with pin 7 being its output. The diff. amp. converts the current measurement into a 1-to-1 voltage, going to the comparator. Pin 7 goes to around 3.5 V when the 'current' exceeds the reference voltage set by the potentiometer going into pin 6. Otherwise, it stays below 0.5 volts. If it goes above 1 volt, it turns on the diodes at pin 7 and controls the regulator feedback voltage that way, decreasing the output voltage in order to maintain the current limit. I put those LEDs there to act as an indicator when you're reaching the current limit, but I can change that around to swap it around with a regular diode or something.
I got the idea from this link below. The LM2673 feedback voltage pin can go as high as 14 volts, so this seems to be within the acceptable limits. Though, I do see your point. The comparator output behaves differently for both the negative and positive voltage regulators, but they both behave the same way. I've attached a waveform of how they behave along with my test circuits. The negative output voltage circuit is set up the way it is because of issues with the model, so that's why the output voltage for that one is positive.
By the way, you think the 7805 regulator's okay? It's only powering the op-amp and the current reference, but those are only just a few milliamps.
xavier60:
Although there is a lot of activity at pin 7, it might not matter. It could be clean up with some capacities feedback on the compatitor op-amp but this will slow the response.
7805 regulators don't make the best references but might be ok for your application. Check to see how much the output varies with change in input voltage with a real part.
LoveLaika:
Thanks for your reply. Considering that my timescale is in milliseconds, if a small capacitor can clean that signal up fast enough, then I'm all for it. Is it as simple as adding a capacitor at the output in order to reduce that noise? Would it be a good idea to add a small resistor like 1k to limit the op-amp current? Might be best to clean up that signal so as to not affect the regulator feedback.
The 7805 has a dropout of roughly 2 V, so since it's being powered at 15 V, as long as it's above 7V, it should be fine, right? What other ICs would you recommend here, or should I just splurge on a pre-made buck module fixed for 5V?
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