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How to use a DC/DC Switcher to get a current regulated output?

Richard Deutsch33
Prodigy 70 points
Other Parts Discussed in Thread: LM25085, LM22679

Hello,I want to make a battery charger with a DC/DC Buck converter. I need the output current regulated. I remember TI has had some example schematics, but I can´t find them anymore...

  • 0
    TI__Guru 73235 points

    I have attached a document that may help.  You can find much more info on the TI website: go to power management products and serach the technical literature.snva557.pdf

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hello,

    thank you very much, that´s what I´m looking for. Maybe you can help me in answering the next step: The DC/DC converter gets 29V, the max. output voltage should be 28.5V. Is there any type of DC/DC converter in the TI range which can do that? I have to charge with 5A.

    Another question: what´s the diverence between "Voltage Mode" and "Curent Mode" ?

    Easier would be to use a charger IC- but they are all limited to 4 cells LiPo. I need 8s LiFePo (28.8V). Any ideas?

  • 0 in reply to Richard Deutsch33
    TI__Guru 73235 points

    Such a close input/output ratio will require a large duty cycle.  I would try to increase my input voltage or use two separate regulators for the charging.  Voltage mode and current mode refers to the control method used to regulate the output voltage.  Voltage mode is simpler to implement, but is harder to compensate. Current mode is more difficult to implement, but is easier to compensate and may give better transient response.

     

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hello,

    thank you- searching for another part I found the LM25085. I out can be set with R-sense and R adj. The question is, how can I change the circuit to make 3 different current limits? For example switching an additional R adj with a little Mosfet?

    Can that be done? Would solve all problems...

  • 0 in reply to Richard Deutsch33
    TI__Guru 73235 points

    That device has an adjustable peak current limit on the inductor.  The average output current will change with Vin and Vout.  The LM22679 is a 5A device with adjustable current limit also.  A MOSFET in series with the Radj can be used to adjust the current limit.  You will need to experiment and be sure that this will work for your application.

  • 0 in reply to Frank De Stasi
    Prodigy 70 points

    Hello,

    Thank you. Yes I have seen this part first,  would be perfect, but the Duty Cycle is only 90% (I need 100%) and has no "Enable Pin". I need less than 100µA current draw in standby.