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LM5170-Q1: Not going into buck mode

Michael Gorski
Prodigy 165 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Hello,

I have a prototype board that has 4 LM5170 controller ships on it to regulate 4 independent sources onto one common bus that can source and sink current. I have 7 boards total and I am having trouble with one converter on 3 of the 7 boards. The one converter doesn't seem to want to go into buck mode when the DIR pin is set high. I have watched the SS pin during the transition and I see it cycle low and recharge. When I scope the FET's they are still cycling in boost mode for some reason. The other converters seem to respond correctly and as far as I can tell they are all build the same. When I check the ISET pin it is being commanded to the same voltage as a functioning converter. I have verified the DIR pin is set high, 3.3v. The converter seems to work just fine in the boost direction.

Another thing I noticed that seems off and may point to what I am doing wrong. The IOUT current relative to the LV current shifts going from boost mode to buck mode. Boost mode seems to track with what I would expect the LV current to be but buck mode, on the functioning converters, shifts to read significantly higher. I see the shift in the IOUT signal but the ISET signal still controls the same LV current that I would expect it to. 

I am imagining there may be something in the layout or parts that is on the edge of a threshold but I don't know what to look at to figure out what it could be. Unfortunately the design is under NDA with my customer so I am not able to share the schematic or layout. If I could get any reason(s) why the converter would ignore the DIR command that I can look into it would be greatly appreciated.

Other notes for the setup:

VHigh: 24v-32v

VLow: 12v-16.5v

Switching freq: 150Khz - set and phase controlled from micro

Thanks!

  • 0
    TI__Guru* 78300 points
    Hi Michael,

    Thank you for designing with the Lm5170, and sorry to hear about the issues. Since your 4x Lm5170 in the design, the circuit is a complex one and what you described does not give me enough clue. I understand that you cannot post the schematic here in E2E website. Is it possible to share the schematic through the Private Message System (PMS)? Or, are you able to find an local TI FAE in your region, and let me work with him on debugging your circuit?

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 165 points

    Hi,

    I attempted to send you a private message but the system didn't resolve your name to a contact. I sent you an email to subscribe??? whatever that means. Whenever I am able to directly contact you I can send you the schematic for the LM5170. All 4 converters use the exact same schematic. The layouts have minor differences due to their orientations relative to the main power node, but are very similar. The biggest difference is the current sense routing. The converter with the issue has the most direct and shortest routing of the four converters. 

    Is there anything that could cause the chip to stay in boost mode control and ignore the direction pin? Like I mentioned, I can see the SS cap cycle when the direction pin is changed, both directions, but watching the FETs the converter stays in boost mode. When the DIR pin is set to buck mode the switching cycle gets minimized even though the ISET command has increased to the point that it should be attempting to move current. 

    Other things I should have mentioned earlier:

    circuit breaker is bypassed per the datasheet

    Supplies are setup per the datasheet, filters are in place

    ISETA is used with the suggested RC filter configuration and ISETD is grounded

    Thanks!

  • 0 in reply to Michael Gorski
    TI__Guru* 78300 points
    Hi Michael,

    I am contacting through the Private Message System (PMS). Please accept and send your schematic to me.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    TI__Guru* 78300 points

    Hi Michael,

    I heard from my colleague in your region that the issue was resolved, so let me close this thread. If you have further questions, please open a new thread.

    Again, thank you for using our products.

    Best Regards,

    Youhao