• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 3 answers
  • Subscribers 64 subscribers
  • Views 1856 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5176: Use as battery charger

Horst Schlemmer
Prodigy 160 points
Part Number: LM5176
Other Parts Discussed in Thread: LM5175

Hi,

I'm wondering if it's possible to use the LM5176 as CCCV battery charger.

A general synchronous 4-switch buck-boost topology can work bidirectional and I'm not sure what will happen for example during soft start with a battery or a power source connected to the output as well? Will it be possible that power is "converted" from the output back to the input? Will the high side boost FET for example stay off during soft start or will it be permanently on? Will the device switch of the synchronous FETs at low current?

What pin do you recommend to control the output current with an additional external circuit? Should we use the SS-Pin as you do internally or the Comp-Pin. Or do you recommend to use the internal current measurement circuit and add a controlled offset?

Thanks for your support.

Horst

  • 0
    TI__Mastermind 21720 points

    Hi Horst,

    Thanks for reaching out with your questions.

    The LM5176 can be used as battery charger. However it is only functional in one direction (unidirectional) and doesn't have bidirectional functionality. In the unidirectional configuration there is no problem starting with a pre-biased output voltage like a battery.

    Since the LM5176 operates in FPWM the high side boost MOSFET will be active during the start-up procedure.  

    To control the output current the SS pin could be used. The solution below is another approach. This will provide a more accurate current limit.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 160 points
    Hi Garret,

    thank you very much for your quick response. Your schematic is exactly what I'm looking for, great.

    You mentioned that during start-up, the high-side boost MOSFET is active (constantly on). I'm wondering if exactly this may be a problem, because if this MOSFET is on and the charger is working during soft start in "Buck-Mode" (I think this is what happens), the system may work in reverse direction like a boost converter and transfer the battery voltage back to the input of the circuit?

    I don't need the bidirection function. My concern is, that there is a potential issue with an undesired reverse mode where the input voltage rise to unacceptable values, even for a short time.

    I read lot's of application notes and there is for example an IC from a competitor that mentioned that the high-side boost MOSFET will stay off during small currents and soft start. Will you deactivate any synchronous FET if there is a small current?

    Regards,

    Horst
  • 0 in reply to Horst Schlemmer
    Prodigy 160 points
    Hi Garrett,

    thank's for the circuit. Is there any thought or concern related to my upper post?

    Regards,

    Horst
  • 0 in reply to Horst Schlemmer
    TI__Mastermind 21720 points
    Hi Horst,

    The LM5176 works in FPWM mode. If you need diode emulation functionality please use the LM5175.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 160 points
    Hi Garrett,

    thank you for your reply. To be honest, I'm a little bit confused now. FPWM or Forced PWM means, that the system is always working in CCM mode. Doesn't that mean that the current direction in the coil doesn't matter and charging of a battery will NOT work without issues?

    I mean during soft start in buck mode, both input MOSFETs are switching with a small duty cycle, (low side MOSFET is most of the time on) will act as a boost with high duty cycle in other direction, if there is a battery at the output.
  • 0 in reply to Horst Schlemmer
    Intellectual 310 points
    Dear Horst,

    that's a really interesting question. We use the LM5176 as a battery charger and it looks like we sometimes have the same issue. For a quick test, we interchange the boost high-side MOSFET with a Diode and one of the issues doesn't happen anymore.

    We are currently investigating this point (without much success) and it would be great to get more support. The posts attached let me suppose that there is no issue with charging of a battery but your thoughts may explain the issue we sometimes see without the diode at the output. Unfortunately we havn't triggered this situation with a scope and when it happens, the system is destroyed. This makes it not easy to debug.
  • 0 in reply to Horst Schlemmer
    TI__Mastermind 21720 points
    Horst,

    Yes FPWM means that the LM5176 will operate in CCM for all loading conditions. If this doesn't work for your application please take a look at the LM5175.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 160 points
    Hi,

    thank you Garret. I'm a little bit confused about charging. For me your last post explained, that charging is NOT possible without getting trouble with current flowing in the reverse direction during startup. On the other hand, you said that charging is possible.... What should I do? As DH mentioned, he saw more or less the same behavior.

    Is there a synchronous buck-boost you recommend for high voltage charge application. Unfortunately the LM5175 will not fit because of the lower voltage rating.


    Regards,

    Horst