• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 211 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.

LM7310: LM73100RPWR Priority power mux - not working as expected

Martin Vit
Prodigy 145 points
Part Number: LM7310
Other Parts Discussed in Thread: LM70880-Q1

Tool/software:

We configured LM73100 as priority mux (see schematic) but observed here different behavior than expected.

We tried this test scenario:

1) Turn on VIN2, VIN1 is off, at the output is 12.1V as expected

2) Turn on VIN1, VIN2 is keept ON, at the output is 12.3V as expected

3) Turn off VIN1, VIN2 is keept ON, at the output is 12.1V as expected

4) Turn on VIN1, VIN2 is keept ON, at the output is 12.3V as expected

5) Turn off VIN1, VIN2 is keept ON, at the output is 0V, but was expected VIN2 = 12.1V

Looks like secondary path went to Fault condition during step 5. Why that happened?. After step 5, we must do power cycle on both power supplies VIN1 and VIN2 to make output again working.

Could You please explain that. Thank You.

Martin / Kontron

  • 0
    TI__Mastermind 39045 points

    Hi Martin,

    I see you have used 5 test conditions (mentioned above). Did you follow this in sequence? Test condition #3 and #5 are identical but with different Vout values. Do you have any waveforms which might help in understanding what might be causing this behavior of FLT tripping the device. Also, this shouldn't be FLT tripping, this is more likely to be startup failed. What was the load when you did the testing and any difference between condition #3 and #5.

    Best Regards,
    Arush

  • 0 in reply to Arush Gupta
    Prodigy 145 points

    Hello Arush Gupta,

    Thanks a lot for You reply. Finally I was able solve reason for this malfunction. There was a noise around ideal diode which caused this malfunction. I added 100nF decoupling capacitors close to pins 5 and 8. But there was still malfunction caused during startup. As soon as the left ideal diode in the photo started lead current, it increased current thru nearby coil from nearby DC/DC. From bootstrap capacitor which was exactly close to enable pin of ideal diode was coupled some additional noise to the ideal diode circuit ant it stopped working and close itself until power cycle. It was really crazy to discover this phenomenal, because all parts are very small. Finally I was able move bootstrap cap with resistor next to pins 5 and 4 of the U14. Rest of switching node polygon under coil is cuted so coupling noise to the enable pin of ideal diode doesn't heppen with this modification. I also added snubber close to DC/DC to avoid ringing on switching point.

    Anyway I have another question. As we have this DC/DC running from 55V downto 12V. My colleague selected for bootstrap capacitor 47nF/100V/0805.

    As I understood from EVM for LM70880-Q1 is enough to have here cap 47nF/0402/X7R/ 25V. Is that correct? I understood that bootstrap cap is charged to VDD (which is internal LDO inside LM70880-Q1 running on 8V) - in case that bottom switch is on. Somewhere I found that bootstrap capacitor should be rated to 2 x VDD = 2 x 8V = min 16V. Is that correct?. Can we redesign this bootstrap cap 47nF/0805/100V to 47nF/0402/50V/X7R as I did. Thanks for answering this. Also is welcome some application note regarding bootstrap circuit design.