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LM317L / Constant-Current Battery-Charger Circuit

K.Yaita
Guru 29690 points
Other Parts Discussed in Thread: LM317L, LP2951

Hi Team,

My customer considers to use LM317L for NiMH battery charger as Figure 8.




The datasheet shows "VI should be greater than VBAT + 3.75 V".
What is concern if VI is lower than VBAT+3.75V?

I would like to know the function under the following condition.
"VI=9V, VBAT=5.25～8.4V, Rout=62Ω (20mA)"

Best Regards,
Yaita / Japan disty

  • 0
    TI__Genius 12410 points

    There is a dropout voltage from the input to the output as well as from the output to the adjust so it will not be able to charge the battery if you do not allow the 3.75 headroom. 

    Thank you,

    Cameron

  • 0 in reply to Cameron Phillips
    Guru 29690 points

    Hi Cameron-san,

    Thank you for your support.
    Is there another device that has low dropout for this application (Constant-Current Battery-Charger Circuit)?

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points
    Hello Yaita-san,

    It is possible to implement this application with the LP2951 device. www.ti.com/.../LP2951
    Input and output capacitors will likely be needed for stability.

    Also, Icc of the device will need to be considered when setting the output current because the Icc will ultimately flow to the output.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 29690 points

    Hi Michael-san,

    Thank you for your support.
    Is the folllowing LP2951 schematic correct for this application?


    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    Guru 29690 points

    I would also like to know if it could damage the device under Vin=0V (disconnected) and OUTPUT=8.4V (Battery voltage).
    Is there absolute maximum rating for OUTPUT pin?

    Your support would be appreciated.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    Guru 29690 points

    May I have your advice?

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points
    Hello Yaita-san,

    Yes, the way that you connected it in your drawing is correct. With Feedback connected to the output.

    As for the Vin question.
    If Vin is disconnected altogether then you should be fine as there is no path for current to flow, except to maybe charge the capacitor on the input. If, however, you actually have the input grounded, then this might cause some current backflow. It is not recommended to connect it in this manner. A recommendation would be to put a diode from the source to the input before the input capacitor to prevent any large backflow.

    Does this make sense?

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 29690 points

    Hi Michael-san,

    Thank you for your answer.

    Is it no ploblem for LP2951 under the following condition in terms of absolute maximum rating?
    I concern if it is possible to apply a voltage to GND pin and OUTPUT pin more than INPUT pin. 

    - INPUT=0V
    - Battery voltage=8.4V.




    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points

    Hello Yaita-san,

    If the input is actually connected to a true ground then this would not be recommended. 

    If you think there is a scenario in which this might happen then the recommendation would be to put a forward facing diode into the input to prevent current backflow. This may however affect your battery charging voltage, so it would be recommended to choose a diode with a very small forward voltage (small enough so that the drop from the diode + drop from the ldo does not exceed the 0.6V headroom needed for the battery voltage.)

    If the input is going to be floating or disconnected then there will only be some current backflow into the input capacitor which should not cause damage to the device.

    Best,
    MIchael

  • 0 in reply to Michael J Schultis
    Guru 29690 points

    Hi Michael-san,

    I apologize for asking many time but I would like to confirm one more..

    I concern if the following Absolute Maximum Ratings are referenced with respect to GND pin.
    Is it possible to apply voltage to GND pin more than INPUT pin(Vin)?



    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points
    Yaits-san,

    I apologize, i believe I misunderstood your question from the beginning.
    As you mentioned, if the input is grounded and the GND pin is a higher potential (higher than 0.3V) it is possible and likely that there would be damage to the device.

    I would not recommend using the device in this kind of scenario. I would try to make sure that the input disconnects and that it is not connected to ground. A forward diode from the input to help prevent current backflow would be recommended.

    In short, yes, the device should not have a higher potential on the GND than the input.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 29690 points

    Hi Michael-san,

    Thank you for your reply.
    I understood LP2951 should not have a higher potential on the GND than the input.

    Is the same equally true of LM317L? (LM317L should not have a higher potential on the OUTPUT than INPUT.)



    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points
    Yaita-san,

    Please allow a day for me to verify.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Guru 29690 points

    Hi Michael-san,

    Thank you for your support.
    I will wait for your feedback.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 24485 points
    Yaita-san,

    Yes, by having the adjust pin higher than the input, the out pin would also try to be above the input. This may cause backflow of current into the output of the regulator which could damage the device.

    Best,
    Michael