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LM3668: Need DC/DC converter for hand crank mobile phone charger

Marvin Tolentino
Prodigy 100 points
Part Number: LM3668
Other Parts Discussed in Thread: TPS63070

Hello,

I'm trying to create a hand crank mobile phone charger. I will be using a 12v DC geared motor. In order to charge a mobile phone, I need a 5v output at 1A. Since this is a hand crank, I assume the voltage generated can be above or below 5v so I think I need a buck/boost converter. Can you please confirm if the LM3668 is the right one for my project? If not, please advise me. Thanks!

Regards,

Vino

  • 0
    Intellectual 550 points
    Absolutely NOT... Max Vin is 5.5 V !!

    This part seems to be a solution
    www.ti.com/.../tps63060-ep
    but has a Vin(max) of 12V, so you could be forced to limit the input voltage, by some means.
    Otherwise, a bit more complex, but...
    Something like www.ti.com/.../LMR33630
    and, an idea, a charge pump boost for the low-end voltage....

    with a hand-cranked generator, it probably has quite low-current output at low speed/rev's, so a charge pump could provide boost to 2x input voltage, maybe to a low-dropout linear regulator, then when the generator output goes a bit higher, the Power-good output of the DC/DC buck could be used to shut down the charge pump, maybe using a FET switch to completely isolate it from the potentially-higher voltage from generator;

    one needs to remember that, with the phone fully-charged, the generator will see almost no load, and its voltage, essentially open-circuit, could go quite high, unless you incorporate some kind of load-dump, or otherwise disconnect the generator output from the charging circuits.

    Only thinking :-)
  • 0
    TI__Expert 7925 points

    Hello Marvin,

    As Barry already suggested, LM3668 is not suitable unless you limit the generated voltage.

    For the start, I would measure the motor alone and determine the voltage-current characteristic for different spinning speeds and different loads (including no-load condition). Then you can determine the input voltage requirements for the converter and make appropriate selection. As for the buck-boost devices, the TPS63070 has the widest input voltage range (2 V to 16 V) and can provide up to 2 A. 

    Is it a brushed or brushless DC motor? 

    Best regards,
    Milos

  • 0 in reply to Milos
    Prodigy 100 points

    Hello Milos,

    It's a brushed DC geared motor. I think the TPS63070 is a good choice given the flexibility and the price. I will try to find a reference design here in TI for this converter. I will appreciate for any further suggestions regarding using this converter. Thanks a lot!

    Regards,

    Vino

  • 0 in reply to Barry Rowland
    Prodigy 100 points
    Hello Barry,

    I appreciate the explanations, insights including the suggestions. You're a great help in my project. I will appreciate any further thoughts.

    Regards,

    Vino
  • 0 in reply to Marvin Tolentino
    TI__Expert 7925 points

    Hello Vino,

    Once you determine the operating conditions for TPS63070, you can put them into Webench and simulate your design. You will get the component values and suggested bill of materials. 

    Additionally, make sure that if your motor generates more than 16 V you will need to add an input protection, for example a zener diode, to limit the voltage.

    Best regards,
    Milos

  • 0 in reply to Milos
    Prodigy 100 points

    Hello Milos,

    This resolves my specific concern. Bunch of thanks!