Timeline for How to control a buck-boost converter circuit from a microcontroller?
Current License: CC BY-SA 3.0
12 events
| when toggle format | what | by | license | comment | |
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| Jul 5 at 5:02 | comment | added | Unknown123 | What should I do if I want to control the current such that it change into constant current mode? | |
| May 5, 2015 at 8:42 | comment | added | Piotr Sarnacki | Sorry to bother you again, but I have no idea how to make it work in LTSpice, so again, I'm probably missing something. I basically took LTC3780 example circuit from Linear and added R3 and CFF. No matter which values I choose the output voltage doesn't work as expected. The circuit is here if you (or anyone else reading this) wants to play with it: dropbox.com/s/ua4pxbw47x8s730/3780.asc?dl=0. I tried a few different values of R1, R2 and R3. Here is a screenshot of a relevant portion of the schematic: cl.ly/image/0b2Z193e2j2T/… | |
| May 1, 2015 at 0:28 | vote | accept | Piotr Sarnacki | ||
| Apr 30, 2015 at 23:06 | comment | added | Zulu | @PiotrSarnacki, even though \$V_{OSENSE}\$ is an input, it is "servoed" to 0.8V. You can think of the buck-boost converter as a giant power op-amp, with an [internal] 0.8V reference connected to its [internal] non-inverting input. The output of the converter (\$V_{OUT}\$) will be driven to whatever is needed to make its inverting input (\$V_{OSENSE}\$) to equal its non-inverting input, which is 0.8V. This is the basic idea of all control systems. So when doing calculations, you can simply assume that, once the converter has reached steady-state, \$V_{OSENSE}\$ will be 0.8V. | |
| Apr 30, 2015 at 21:27 | comment | added | Piotr Sarnacki | So, I'm probably seeing it totally wrong, sorry if that's the case. Anyways, since Vosense is an input pin, it should get the Vout divided with voltage divider. If you look at the example in the datasheet, they want to set Vout to 12V and they choose 20k for R1 and 280k for R2. In this case: Vosense = R1/(R1+R2) * Vout. So for 12V voltage at Vosense will equal 0.8V. If Vout is higher than voltage at Vosense will be higher etc. In your response you calculate Vout based on Vctrl, while Vfb should be the "output" here. | |
| Apr 30, 2015 at 16:53 | comment | added | Zulu | @PiotrSarnacki, you are correct: \$V_{OUT}\$ connects to \$V_{OUT}\$, \$V_{FB}\$ connects to \$V_{OSENSE}\$, and \$V_{CTRL}\$ connects to a D/A converter from the μC. And as described, the output voltage will be \$V_{OUT}=\left(1+\frac{R_2}{R_1}\right)0.8-\frac{R_2}{R_3}(V_{CTRL}-0.8)\$. "If that's true I fail to see how would that work with setting proper voltage on Vosense." What do you mean? | |
| Apr 30, 2015 at 12:15 | comment | added | Piotr Sarnacki | I was playing with the circuit from the last option in LTSpice and either I really don't understand it or I do something wrong. The problem is that as far as I understand the datasheet, Vosense is an input pin. Vout voltage is steered by the buck/boost circuit on the right (MOSFETs etc) and then Vosense reads the output voltage divided by the voltage divider and adjusts MOSFETs duty cycles accordingly. It seems that on your circuit Vout would be connected to Vout, Vfb to Vosense and Vctrl to the uC. If that's true I fail to see how would that work with setting proper voltage on Vosense. | |
| Apr 29, 2015 at 16:39 | comment | added | Zulu | @PiotrSarnacki, \$C_{FF}\$ is a feed-forward capacitor. You might pick a value of, say, 1nF. The presence of \$R_3\$ reduces the loop gain, so you may want \$C_{FF}\$ to keep the loop gain invariant to the feedback divider ratio. That said, you also might not want it. As mentioned, you'll want to carefully evaluate the regulator stability... and component selection could be a lengthy question (and answer) of its own. So... you could leave \$C_{FF}\$ non-populated, and add it only if you determine it's needed. | |
| Apr 29, 2015 at 9:28 | comment | added | Piotr Sarnacki | this is a very informative answer, thanks! in last option, what's the role of CFF? is it for stabilizing output? And if yes, what value should I choose, I suppose something small like 470n with a fairly small R3? | |
| Apr 28, 2015 at 22:17 | history | edited | Zulu | CC BY-SA 3.0 |
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| Apr 28, 2015 at 21:33 | history | edited | Zulu | CC BY-SA 3.0 |
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| Apr 28, 2015 at 21:28 | history | answered | Zulu | CC BY-SA 3.0 |
