Boost switch mode power supply from Lithium Batteries..

Discussion in 'The Projects Forum' started by roltex_rohit123, Jun 13, 2010.

  1. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
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    I have lithium cells, 4.1-4.2 volts 2 amperes each. i want to make a boost switch mode power supply by connecting 2 in parallel.

    the voltage in the lithium cells drops to 3.0 vlts in operation as they discharge. I want to make a threshold of 3 volts and gain an output of 5.5-6 volts at 500ma in operation.

    how canI proceed for it?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Look at the datasheet for Linear Technology's LT1172.
    Take the boost converter on the bottom left of page 1.
    Change R1 to 4.36k, leave R2 as is. Alternatively, change R1 to 3.2k and R1 to 910 Ohms.
    Add a 3.9uH inductor from your batteries' + terminal to the +5V in.
    Connect the batteries' - terminal to ground.

    If you want it to turn off at 3v, you will need to add a micropower comparator circuit to pull the Vc pin to ground when the battery voltage drops below 3v.
     
  3. Damo666

    Member

    Aug 25, 2009
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    Are you using Lipo cells by any chance? If so, you shouldn't run them this low as it will destroy them very quickly.
     
  4. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
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    No they are not LIPO batteries. I am using Li-Co batteries..
    the protection circuit has the values of 2.7 volts. i am giving it a threshold of 3 volts. and i am never going to discharge it below 3.5 volts because the purpose of making this charger is to chaarge cellphone only once or twice. so i need not worry. as soon as i get power i would charge the batteries.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Oops, you did not say that you wanted a charger; just something capable of taking your 4.2v in and stepping it up to 5.6v; I figured that 500mA was your minimum requirement.

    You will need to use a proper charging circuit to charge the cell phone battery. The cell phone is probably using a LiPo battery. If you overcharge a LiPo battery, it will rupture (even explode) and catch on fire.
     
  6. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
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    you are right, but cell phone has the control mechanism inside. the charger (cell phone charger) just provides a constant voltage. so we need not worry about anything beyond designing a switchmode boost charger. i take your advice of not connecting two cells in parallel. so i would just use a single cell in the charger. i have a couple of IC's sorted from various manufacturers. i need you help to choose the best out of them..

    TPS61220
    LTC3458L
    LTC3423
    LTC3499 high priority
    the LTC 3499 has a reverse battery protection also. you can obtain the datasheets directly from the company websites.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, it would have been nice if you had told us about this before.

    Why don't you post direct links to the datasheets, so that it shows you have some actual involvement in the process?

    If you expect others to do all of the work for you, you won't learn very much.
     
  8. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
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    yes it would have been, but by that time i had not made any searches. i was just wondering about how i could do it. direct links to the data sheets... from next time onwards i'l do that. i dont expect others to work for me, but help me in my work. since i am not of electronics, some of my basics are not clear, moreover i dont know many things.
     
  9. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
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    I have made a circuit using LM3478MM part. i am attachin a schematic of the circuit to this mail and wanted to know whether this is a smps module. i dont know much about smps. LM3478 Charger.jpg
    Thank you.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Why did you start with yet ANOTHER IC that is not on your above list? :rolleyes:

    I don't know why you ask for advice, receive it, and then go in different directions. It is very frustrating and discouraging for those who would like to help you.

    Now do you want to go with the LTC3499 which you have already, or try something completely different?

    I'm not going to play the "endless re-design" circuit game.
     
  11. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
    72
    0
    I am very sorry for this, but i didnt mean to discourage or frustrate anyone.. i was just playing with the software and it gave me the magical output i didnt expect the circuit to be designed by that software. actually should have posted it in another thread, i dont know what it is, it just gave me result of my input. please dont feel bad. i'm not trying different. i still want your advice.

    so if we could begin can you tell me the parts of an SMPS type i want? Regarding the IC i would leave it on you to decide the safest, the best and the simplest.. once again I'm very sorry..
    pls forgive me.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Rohit,
    I received your E-mail.

    Please understand that there are a LOT of people on these forums who need help, and comparatively few who actually provide it.

    I simply don't have the time to first come up with a suggested solution, and then experiment with lots of other possible solutions.

    If the solution you received from the other website is working, why not just use it?

    Do you think it would be fair to the other members who also need help, if I spent all of my time helping you with one project?

    Would that be fair to me?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Here, look at the attached.
    It's using one of the regulators that you already have. You will need to obtain the micropower comparator and the other components to complete the circuit.

    Open the simulation to see the exact output capacitor and inductor that I used in the simulation. You will need LTSpice, available from Linear Technology as a free download.

    It is better than 90% efficient.
     
  14. roltex_rohit123

    Thread Starter Member

    Jan 1, 2010
    72
    0
    Thank you for your circuit... i didnt intend a readymade help. actually i wanted to start with a part number understand it and then construct the circuit. anyways i'd like to reverse engineer the circuit, to know how each component works. would you help me in this? hope you have time for it, and i am not giving too much trouble.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    I did it the "quick and dirty" way.

    I used the existing models that Linear Technology supplied for the two ICs, and merged the schematics together.

    But, better than 90% efficiency from a "quick and dirty" gets the job done.

    If you would like to learn more about how the ICs work, please read the datasheets.

    I simply do not have the time to go over it piece by piece with you.
     
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