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LM2576: Used as a constant current source

Part Number: LM2576
Other Parts Discussed in Thread: BQ21040

I am developing a lithium battery charger and I've been thinking about using a constant current source for a faster charging, since with constant voltage is a bit slow. I will be charging at about 700mA, being quite high for a liner regulator, so I have found the LM2576. However, as a current source varies the voltage to maintain the current, I don't know if I need a step-up or step-down converter, or one having both. I have an input voltage of 4.2V and the battery will between 3.5V and 4.2V. I suppose that I need a higher voltage than 4.2V to have a high charging current.

  • Hi Dani,

    Most batteries, especially lithium-ion, are charged with a constant-current/constant-voltage charging profile. You have to have the constant voltage at the end to prevent over charging, which will damage a battery.

    Yes, it will be easier if you start with a voltage that is a little higher than your maximum expected output voltage.

    I have quite a bit of experience charging and managing lithium-ion batteries (I'm working on it right now) and I always like to start with a charger IC that already implements the CC/CV charging profile. You can find all of TI's offerings here: www.ti.com/.../products.html

    The BQ21040 might be an excellent choice for you. I found it with the TI search, choosing 1 cell, lithium-ion/li-pi/li-polymer, with a charge current of at least 700mA. The BQ21040 linearly regulates the input voltage to a maximum of 4.2 volts and a maximum current that is set with the ISET resistor. This will allow you to feed the charger with your 4.2V, set the max current at 700mA, and it will safely charge to 4.2V. The charging will stop when the current is below the termination current (10% of ISET in this case) . The BQ21040 might not be your perfect solution, so go ahead and browse the selection at the first link. Other charger ICs have more features and more flexibility (adjustable charge voltage, termination current, etc.). I hope this helps.

    Best,
    Aaron