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LM5123-Q1: Handling backword currents with LM5123 boost controller

Part Number: LM5123-Q1
Other Parts Discussed in Thread: LM5177, LM5170, LM5122

Hey guys,

Quick question for you regarding the LM5123 . I want to use that chip as a boost controller to power up a 24V motor from a 12V battery. Regarding this part of the logic, so far so good

The thing is the following :my motor can sometimes gain energy. I want to send back this energy to my battery. I don't need to change the voltage or anything of that energy, I can send it directly to the battery it can handle it.

The behavior of the circuit would be the following : the motor gains energy, the voltage at the ouput of the boost  thus increase. The LM5123 will then see that the voltage at the output is greater than the 24V. I then expect it to reduce the duty cycle of the high-side PWM. By extension I also expect the duty cycle of the low-side PWM to rise as it behaves the oppsite way of the PWM of the high-side. 

The low-side mosfet would then be fully closed(full conduction mode) because it's PWM is greater. This mosfet would allow currents to flow from the output to the input. My battery will then recharge a little bit and when the energy at the motor is dissipated the output votlage will fall, the LM5123 will detect that and the compensator will then start to increase the PMW of the high-side and I would go back to normal behavior.

My question is the following : will the LM5123 behave as I think it will ? Does this chip have any ''problems'' with seeing reverse current ? 

Thanks a lot guys,

  • Hi Amine,

    Thank you for using E2E forums, regarding your query when the low side FET is fully closed for the current to go in reverse wouldn't that mean that the current in the inductor has to change instantaneously, and that is physically impossible. if I am misunderstanding anything please correct me.



  • Hi Haroon,

    Thanks for your reply. I'm not really sure to undersdant the issue here. When the motor will gain energy, the boost controller will reduce its PWM, reducing then the inductor current. Energy would then start flowing the other way around and the inductor would indeed see '' negative '' currents. 

    I don't believe this is an issue but maybe I understood wrongly your suggestion.

    Thanks a lot for the answer Slight smile

    Best regards

  • Hi Amine,

    Well the LM5122/21/23 I know do not allow or function with current flowing from output to input, in other words they are not bidirectional. Maybe you could check LM5170 or LM5177 devices and see if they fit your application.



  • I had a similar question for this part. According to the datasheet, it sounds like the LM5123 can reverse current in Forced PWM mode 

    "The maximum reverse current is limited to 145 mV/RDS(ON) in FPWM mode."

    That, to me, makes it sound like it can direct current from output to input, no?

  • Yeah, I'm really not sure to understand why it would not. In fact , the only thing it has to do is correctly drive the mosfets and the rest would do itself..

    I'm thinking about maybe ordering an eval board and trying it out in lab..

  • Great! I will be doing the same. I will make sure to update with any findings.

  • Sweet will do the same.

    Talk to you soon

  • Gentlemen,

    Please do not confuse average current with peak current.

    The section in the datasheet only refers to a light load condition where the average current is close to zero.

    In this case, and when the LM5123 is configured to FPWM mode, some amount of reverse current will be allowed (see the upper graph in the image below). This current depends on the chosen HS FET and is limited to 145 mV/RDS(ON). Nevertheless, the average current is usually still positive. 

    The LM5123 can NOT be used for a controlled bi-directional operation, where you might want to discharge e.g. a battery on the output side.

    It is not possible to program / configure a certain reverse current.

    Best regards,


  • Thanks Harry! 

    I guess my application is still a little different. I don't want to configure a configure a particular reverse current. That would be limited by the output resistance of the boost side. I just want to regulate the output to 42V and source or sink current as needed to maintain that voltage.

    All bidirectional regulator ICs seem to assume I want to regulate going both directions, but I only want to regulate the boost output voltage.

  • Hello Tyler,

    If on the output side there is nothing but the output capacitors and your load, then this is exactly what FPWM mode is meant for. 

    When you look at the diagram from the previous post:

    In DEM or Skip mode, the controller will skip pulses when the output voltage is too high, and the load current is so small that the output capacitors will be discharged very slowly.

    In FPWM mode this is different.
    There are no skipped pulses because the output capacitors can be discharged via the reverse current.

    The drawback is that this mode is less efficient because - even with no load - there will always be some current going back and forth which will cause losses.

    Best regards,


  • I got the exact same application. Still really not sure if available or not. Our sales rep might ship us some sample pretty soon so I can try it out..

  • Gentlemen,

    We have not seen an update from you for three weeks, so I assume the questions are answered and the issue is solved.
    I close this thread now. If there is still something open, please reply and the thread will get opened again.
    If you have any other question or of the thread has been locked, please open a new one.

    Clicking the Resolved Button also helps us to maintain this forum.

    Best regards,