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LM5122: Cannot Get Chip to Regulate

Michael Stagg
Prodigy 120 points
Part Number: LM5122

I used this chip in a past design in a boost configuration using the recommended values and schematic from WEBENCH. It creates an 11V output from a 7.4V battery. This worked fine using an IRFR3707ZTRPBF and STD35NF3LLT4 as the MOSFET devices. So now we are making a variation of the part and the output goes to 12V and the output current is much less. So we changed the mosfets to AO3400A.

The new design will only occasionally (like every few seconds) momentarily jump to 12V but then immediately discharges back to battery voltage (following the output capacitor discharge) and the output basically follows battery voltage.

Can this MOSFET change actually be causing this behavior or is there something else we are missing? We cannot find any other changes.

  • 0
    TI__Mastermind 21720 points
    Hi Michael,

    Can you please provide the schematic? There might be something incorrectly connected. I would not expect that changing the MOSFETs would cause this issue.

    Thanks,

    Garrett
  • 0 in reply to Michael Stagg
    TI__Mastermind 21720 points
    Hi Michael,

    The connections on the schematic look correct.Please ensure the that device is not running in to current limit with the SS pin being pulled low.

    Can you please provide waveform of the switch node, COMP pin SS and VOUT? What are the application parameters? {VIN range, VOUT, IOUTmax}. Just to confirm, when the other MOSFETs are used this design works fine?

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 120 points
    Garrett,

    I will work on getting this information. We have now got the circuit to work using a different shottky diode. Diode B340-13-F seems to make the circuit work okay but diode SS24T3G does not work. Maybe this can give you a clue? I do not notice anything significantly different on the diode specifications?
  • 0 in reply to Michael Stagg
    TI__Mastermind 21720 points
    Hi Michael,

    On the schematic which diode are you talking about? I imagine it is parallel with the high side FET. The only difference I see between the two diodes is that B340-13-F has a larger surge current rating. Maybe the inrush current from the initial turn on is damaging the other diode.

    Are you seeing this on multiple boards? My first though is that this sounds like a soldering issue. Normally these the diode selection doesn't have this large of impact on operation.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 120 points

    It is Diode D2 in the above schematic. I am sure it is not a soldering issue as we have changed the part multiple times. We have not verified it on other boards. I have checked the critical diode parameters and there seems to be no real difference between this diode and others. All of the diodes seem to fall within the chip recommendations. Are there any specific characteristics this diode must have? Is it possible there is a capacitor problem on the caps surrounding it?

  • 0 in reply to Michael Stagg
    TI__Mastermind 21720 points
    Hi Michael,

    The boot strap diode should be a low leakage schottky diode and the capacitor needs be a high quality low ESR/ low ESL selection.
    The boot strap circuitry is also affected by layout. The loop with the capacitor and diode should be as tight as possible. This really means that the diode and the capacitor should be close to the IC and high-side gate driver.

    When you remove the original diode that was not working, is it damaged? Looking over the datasheet, the diodes seems pretty similar, nothing sticks out to me that would cause a difference. Take a look at a some other boards, if it is possible, to see if the issue occurs.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 120 points
    The issue is repeatable on other boards of the same layout and build. However when the components are interchanged with an older design using the same power supply the older design continues to function correctly (so componenets are not damaged). Looking at the SS pin it pulses high (does not stay high) and then the output rises to the regulated level and immediately begins discharging. Once the output level reaches the input level it pulses again repeating the cycle. There is obviously something we are missing.
  • 0 in reply to Michael Stagg
    Prodigy 120 points
    Garrett,

    Is there anyone at TI I can speak to that can assist me to figure out this issue?
  • 0 in reply to Michael Stagg
    TI__Mastermind 21720 points

    Michael,

    It sounds like you are running into hiccup current limit. To test this please just put the RES to GND. Section 7.4.3 of the datasheet describes the hiccup mode operation. 

    Also can you please provide waveforms of VOUT, SS, RES and COMP? This will help to trouble shoot the issue.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 120 points

    Yes..I was going to post a followup to this today. It is definately the hiccup mode as grounded the res pin corrects the issue. However, I am not understanding what about the design is causing this issue. I will read up further and see if I can figure out why it goes into this mode in one layout and not the other. I am out of office but essentially, the res pin waveform was looking like a sawtooth (until it was grounded). The other pins (I do not have details offhand) but were staying low until the res pin was grounded.