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LM5170EVM-BIDIR: If I use changing REF voltage to adjust Vout, in the buck mode it is right, but in the boost mode, the Vout is not changed

Part Number: LM5170EVM-BIDIR
Other Parts Discussed in Thread: LM5170

Hello:

         Some time ago, I have asked a related issue about adjusting Vout using REF. I use the method is :to remove U13 (the Vref device), and add the flying wire across C95. In one pcb board, I have tested in the buck and boost mode, they are all working normally. But now that board is broken. I use another new pcb board to test, using the same way. But in the boost mode, the Vout can't be changed. When I change the REF voltage, but the Vout is still 50.2V. But in the buck mode, I can use changing REF voltage to adjust Vout from 6V to 20V. So could you help me analysis this phenomenon?  Is this board broken? Thank you very much! I will wait for your reply!

  • Hello,

    Thanks for reaching out with your questions and for using the LM5170.

    Can you let me know what you modified on the second board? Have you measured the voltage at pin3 of U2A? This should be the reference voltage that is changed to change the output voltage. In the boost mode is the output loaded?

    Thanks,

    Garrett

  • Hello:

            In my second board, I have modified two points: the first one,  remove U13 (the Vref device), and add the flying wire across C95; the second one, short out the circuit breaker FETs.Solder a copper foil to two exposed pads on the back side of the circuit  breaker FETs.

           Today I have measured the voltage at pin3 of U2A. When the VREF=1.82V(External provided),  the voltage at pin3 of U2A=1.818V. When the VREF=2.1V(External provided),  the voltage at pin3 of U2A=2.096V. But in the two conditions, the Vout=50.2V. It is not changed. So could you help me analysis it? I will wait for your reply!

    Thanks!

  • Hi,

    Can you please let me know the jumper settings for the voltage loop control? Please also provide the voltage that is on the ISETA pin. I think there might be a missed connection.

    Thanks,

    Garrett

  • Hello,

             Now I have used the "analog Outer Voltage Loop Control" on the board, so I closed the jumper J34,J35,J36,J37. And not give the voltage to ISETA pin. Use this way, in the buck mode, it is work normally. But in the boost mode, the outer is not changed.

    Thanks

  • Hi,

    It is not clear to me what is causing the problem on your board. I was able to change the output in boost mode doing the same method. Can you please chekc the U2A has the appropriate voltages? Pin 2 and pin 3 should be the same voltage and Pin1 should correspond to the ISETA voltage.

    Thanks,

    Garrett

  • Can we assume the issue is resolved?  Lets close the thread here, and you are welcome to reopen it by adding a new post.

    Thanks,

    Youhao Xi, Applications Engineering