LTC4234 seemingly random turn-offs

Hello,


We use an LTC4234 as hot-swap controller for a card powered at 5V DC nominal. The maximum current it can draw is in the order of 16A. The output load capacitance is 900uF in three groups of  300 uF (220uF polymer electrolytic + 8x10uF ceramic), serving the input of three identical voltage regulators. The first capacitors are about 5cm away from the LTC4234, the second group is 10cm away and the third group is 15cm away. The LTC4234's output is connected to  a solid wide power plane, distributing the curent to the regulators.

This is our schematic:

PG goes to a board supervision microcontroller, that among other things is responsible for board power-up and supply monitoring. The UV pin is also connected to a mosfet, controlled by the board supervision, to turn the circuit on an off. GATE and ISET are unconnected. The layout follows the datasheet recommendations, we mostly copied it from the evaluation board, compacting it to fit our available area.This is a 16 layer board with several ground planes.

According to our calculations, this cirucit should have an VOV = 5.45V, VUV = 4.66V, VPG=4.77V, Tcurrentlimit (TIMER) = 0.264ms

For now, we're bringing up the board and we have a current meter clamp attached to the supply cable and we're not drawing more than 5A total, with no significant peak loads. We've also checked the input supply, and it has low ripple (less than 50mVpp measured up to 20 MHz) measured at the board power entry point, right before the  LTC4234. The temperatures seem to also be fine. The LTC4234 remains cold to the touch, and the closest on-board temperature sensor reports normal temperatures.

We're experiencing apparently random poweroffs, seemingly uncorrelated to board function. They occur every several hours and don't seem to depend on board usage. We've got them while completely idle and also under load (at least as much as we can load the board for now). The board supervisor microcontroller turns off the power if anything fails, annotating the cause that led to poweroff. It tells us that it decided to power the board off because it detected LTC4234 PG suddenly going low. The microcontroller software has been tested enough to be quite sure that it's not reporting a false positive and bringing down the board because of a PG glitch or something. The PG input has schmitt-trigger and high hysteresis, and the PG trace is very short (2cm) and goes through an almost empty zone of the PCB. We're currently not monitoring the FLT pin (we can do it if required)

There could be several root causes for this issue, but before further investigation, could you check if there's anything fundamentally wrong with our schematic or setup that could explain the issue?



Current clamp meaning clarification
[edited by: dmarcos at 9:44 AM (GMT -4) on 13 Oct 2020]
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    •  Analog Employees 
    on Oct 22, 2020 11:28 PM 5 months ago

    Hi dmarcos,

    It seems that you've already looked at Vin and it doesn't seem to be the culprit. I would like to ask a few questions.  Is this occurring on 1 board or multiple boards?  Is the board supervisor activating the FET on the UV pin?  If it is random in nature, can you set up a scope in single shot and capture Vin, the UV pin, and the FLT pin?  You need to add a pullup to the FLT pin.  Also are you monitoring IMON for unexpected high load conditions?

    A schematic showing the mosfet on the UV pin might be useful to review.  Maybe you should probe the mosfet gate too.

    Regards

    Mike

  • Hi Mike,

    Thanks for your answer. I forgot to update this, but we found the root cause and the culprit isn't the LTC4234. We monitored the FLT pin and overcurrent wasn't the cause. The OV and UV thresholds were apparently fine and as-designed (we measured 4.7V UV threshold and 5.4V OV threshold). We then proceeded to probe the input voltage, the OV and UV pins and PG simultaneously and waited for the thing to poweroff. We captured several poweroff events and then it all got clear.

    For some reason there are rare regulation transients on the power supply, similar to what a PSU would do under a large load transient (i.e. the voltage shooting up a bit after a a load reduction before re-stabilizing) . The voltage was actually rising a bit over 5.4V during those transients, tripping the UV protection. The transients are several hundreds of microseconds long, less than a millisecond.

    We checked the PSU voltage for several hours before to rule out supply issues, with a current clamp simultaneously measuring the current, with the on-board FPGAs during their thing. The load isn't very dynamic but it does change over time, sometimes suddenly, and we found nothing suspicious. But it surely has some rare transients from time to time which go high enough to trip the overvoltage. The solution has been to change a resistor to increase the overvoltage threshold to 6V (it's still more than safe in this case).

    This all happened in the testbench setup, the final application for the board uses a much better power supply and the board is connected to a large backplane, so this shoudn't be an issue anymore. 

Reply
  • Hi Mike,

    Thanks for your answer. I forgot to update this, but we found the root cause and the culprit isn't the LTC4234. We monitored the FLT pin and overcurrent wasn't the cause. The OV and UV thresholds were apparently fine and as-designed (we measured 4.7V UV threshold and 5.4V OV threshold). We then proceeded to probe the input voltage, the OV and UV pins and PG simultaneously and waited for the thing to poweroff. We captured several poweroff events and then it all got clear.

    For some reason there are rare regulation transients on the power supply, similar to what a PSU would do under a large load transient (i.e. the voltage shooting up a bit after a a load reduction before re-stabilizing) . The voltage was actually rising a bit over 5.4V during those transients, tripping the UV protection. The transients are several hundreds of microseconds long, less than a millisecond.

    We checked the PSU voltage for several hours before to rule out supply issues, with a current clamp simultaneously measuring the current, with the on-board FPGAs during their thing. The load isn't very dynamic but it does change over time, sometimes suddenly, and we found nothing suspicious. But it surely has some rare transients from time to time which go high enough to trip the overvoltage. The solution has been to change a resistor to increase the overvoltage threshold to 6V (it's still more than safe in this case).

    This all happened in the testbench setup, the final application for the board uses a much better power supply and the board is connected to a large backplane, so this shoudn't be an issue anymore. 

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