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Need help with TI pT6656 to regulate 12v

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  • Need help with TI pT6656 to regulate 12v

    I am trying to regulate 12 volts with this setup and it doesn't seem to be working right. I have a PT6656 and 570uF of capacitance on the input and 320uF on the output. I couldn't land 560uF and 330uF capacitors but from my days of electronics school I seem to remember that capacitors in parallel add together and that there is generally a 20% tolerance where a circuit should still perform correctly.

    On the input I have one 470uF and one 100uF in parallel and on the output I have one 220uF and one 100uF in parallel. All cap's are 35v. I've triple checked all of my connections and everything and can't seem to find the problem. What the circuit is doing is mimicking a linear regulator. Any voltage above 12 get's brought down to 12v but anything below get's passed to Vo. I've tried tieing Vadj to Vo and Gnd. This adjusts the output to the high and low end but doesn't correct the problem. I've also tried tieing remote to the output this also fails to correct the problem. All of my test were performed with a small load on the circuit. Any ideas?

  • #2
    I looked up the PT6656 on ti's web site and that is how it should work. It is a step down only regulator and the input is between 15 and 24v so you will get 12 out as long as you have approx 15 or more in. It does not step up voltages lower then 12.

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    • #3
      You know. That's what I thought too (about it needing 15v in) but according to the pdf Vin can be 9v to 28v. It doesn't limit the pt6656 to a Vin of 15v or higher.

      About it being step down only, am I missing something? I don't see that in the pdf anywhere and I thought all ISR's were step up / step down. No?

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      • #4
        Read the pdf again. Notice the bit about Input Voltage Range minimum being Vo+3.


        Rob
        Old Systems retired due to new car
        New system at design/prototype stage on BeagleBoard.

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        • #5
          Oh... Yeah... I guess school is in, huh?

          It's been a while since I've messed with this type of project so I am re-learning some things.

          In light of this I think I will just plan to use 2 MC78T12CT's to step down the voltage and find something else to step up the voltage. If I do this I need to make sure the input to the 78t12's never falls below 12v so I need to step up from 8v or so (to account for engine cranking etc...) to 12v and it needs to output 5A... How about if I take 3 MC78T05's in series and then 2 MC78T12CT's in parallel?

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          • #6
            I don't understand what the 3 mc78t05 regulators in series is designed to do? How does that step up the voltage?

            As for putting regulators in parallel, be careful that one of the regulators doesn't end up doing all the work because of manufacturing tolerances on the regulated voltage outputs.


            Rob
            Old Systems retired due to new car
            New system at design/prototype stage on BeagleBoard.

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            • #7
              It's been a while so correct me if I am wrong but voltages in series add while voltages in parallel increase current capacity, right?

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              • #8
                Those TI ISR can be step down, step up and step up and step down so you do have to choose which to buy. 7005's in series won't do anything, the first would regulate the voltage to 5v which would be less that the vout+2 for the input of the next so it would give out approx 5v, then the third would do the same. Your best bet might be to either use a LDO reg such as the lm1084. You will be able to run anytime except starting with that. I suppose of you want to step down with a 7805 then use a step up isr to go from 5 back to 12 that could work. might me more trouble than its worth.

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                • #9
                  Okay, I am not sure I read your post right. Are you saying that three 7805's in series will NOT give me 15volts? Voltages in series add together right? Vt=V1+V2+V3 for voltages in series, correct? No?

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                  • #10
                    Originally posted by Pred8tr
                    Okay, I am not sure I read your post right. Are you saying that three 7805's in series will NOT give me 15volts? Voltages in series add together right? Vt=V1+V2+V3 for voltages in series, correct? No?
                    Voltage SOURCES in series add. not voltage regulators.

                    when you attach batteries in series, then you can add the voltages.

                    ~mike
                    Single Member of the "1000 Post and No MP3 Car" Club
                    PROJECT ON INDEFINATE HOLD... BOUGHT A HOUSE
                    2000 Cavalier Z24 [###-------] Only 30% Done ... Still

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                    • #11
                      Okay. Well.... Now I'm kinda at a loss I guess. All I need to do is regulate 8 - 16VDC in to 12V 5A out. I know I can do it with the PT5071 but they are just too darned expensive and it will take four of them to actually get to 5 amps. I am going to try building a unit with 3 of them for test purposes but I don't think this is really the answer. It shouldn't cost $60 to regulate 12volt. how do they do it in devices like CB radios etc the regularly sell from less thank $60... There's got to be an answer.

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                      • #12
                        Thats because in CB radios and other vehicle electronics they cheat. They take 12v in but ususally regulate that down to 9v or something. I took apart an old radio that had an 8v regulator inside. You have found what a lot of us did, its hard to regualte 8-16v to 12v. you can step down or step up easily but to do both is more complex. Is there a reason you need 12v at 5A

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                        • #13
                          I have one of the 2677 PSU's and I need to provide it a regulated 12volts. Also, I eventually plan to build my own atx DC-DC psu from scratch so I will have to overcome this hurdle eventually.

                          Ron

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                          • #14
                            I was using the 5071 for 12v trying to control its stby pin off the atx power on control, but the ISR's require a lot of current ton control that pin so it would probabally easier to use a relay on it. I haven't worked on it in a while as I plan on runing a second battery anyway.

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