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Original question:

LM5170EVM-BIDIR: LM5170 boost mode issue

LM5170EVM-BIDIR: Not able to operate the EVM in Buck mode with load connected

Gaureej Gauttam
Prodigy 55 points
Part Number: LM5170EVM-BIDIR
Other Parts Discussed in Thread: LM5170

Hi,

I am trying to use LM5170 EVM-BIDIR, in buck mode. I am able to produce the desired voltage when load is not connected (open circuit) but once I connect the load at the LV side, the voltage reduces to very low value of 0.13V. I am giving the PWM and the master enable using TI C2000 TMS F28379D. I have made all the connections as per the user manual, but I am not able to figure out what I am doing wrong. I have attached the board connection that I am using. Can you please help me out with this?

Thanks.

  • 0
    TI__Mastermind 23550 points

    Hi Gaureej,

    Why are you sending PWM to the EVM?

    The EVM can work in CV mode without external signals.

    You may follow these steps to operate the EVM in CV buck mode.

    1. You need to remove jumper cap for J3 if you want to use the circuit breaker on the board. Otherwise short the circuit breaker MOSFETs.
    2. Install 50k at R4 to pull UVLO high.
    3. use following jumper settings

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 55 points

    Hi Feng Ji,

    Thanks for you prompt reply. I am giving PWM at ISETD to control the duty of the Buck converter.
    I'll try doing the above connection and update you.

    Regards,

    Gaureej

  • 0 in reply to Gaureej Gauttam
    TI__Mastermind 19715 points

    Hello Gaureej,

    Looking forward to your update.

    Regards,

    Harry

  • 0 in reply to Harry Hofner
    Prodigy 55 points

    Hi Feng Ji,

    I tried doing as per what you suggested. Now, I am able to get some voltage at output for the buck CV mode.  But the duty ratio is varying with different input voltage, e.g.) Vin = 12V Vo = 9.33V D = 0.78, Vin = 14V Vo = 11.4V D = 0.81, Vin = 16V Vo = 13.55V D = 0.84, Vin = 20V Vo = 14.625V D = 0.73, Vin = 25V Vo = 14.613V D = 0.58, Vin = 30V Vo = 14.603V D = 0.48. This is when I did not use external ISETD and used the internal circuit which should set the ISETA.
    I also tried it with external ISETD from microcontroller with duty =40% at switching frequency 100kHz, but it showed the same results as above. 
    Can you please suggest what I might be doing wrong or what settings/connections I need to change?

    My final aim is to use this EVM to provide a constant 24V for any input grater than 24V. So, is there any way that I can set the voltage value for the control loop to 24V from 14V?

  • 0 in reply to Gaureej Gauttam
    TI__Mastermind 23550 points

    Hi Gaureej,

    You can set CV 24V by adjusting R78 to 2.49k.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 55 points

    Hi Feng Ji,

    Thanks for your help.


    I have 2.37k and 2.61k available with me can I use these or do I need to use exact 2.49k for setting CV 24V?

    Also, can you please provide me with the calculation on how did we decide the value of R78?

    Best regards,

    Gaureej

  • 0 in reply to Gaureej Gauttam
    TI__Mastermind 23550 points

    Hi Gaureej,

    V+ is 2.048V reference. Thus V- will also be 2.048V. R76 and R78 is a resistor divider. Vo=(R76+R78)/R78*2.048V. Please change R78 that the loop compensation will stay the same.

    Best Regards,

    Feng Ji

  • 0 in reply to Feng Ji
    Prodigy 55 points

    Thank you for the help.

    Regards,

    Gaureej

  • 0 in reply to Gaureej Gauttam
    TI__Mastermind 19715 points

    Hello Gaureej,

    I have not seen an update from you for the last 8 days, 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,
    Harry