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Cell voltages are ~60% of true values, and CRC passes

Category: Hardware
Product Number: LTC6813-1

We have a daisy-chain configuration with two LTC6813 chips, each managing 14 cells. The measured cell voltages from the first chip in the daisy chain look like a normal distribution centered around 2.0V, even though the true values are around 3.5V:

Meanwhile, the 14 cells from the next LTC chip read correctly, as a normal distribution centered at 3.5V:

Sometimes, all the cells voltage reads will be correct (~3.5V), while other times, all of them will be wrong, but still normally distributed around some seemingly arbitrary mean like 2.0V or 2.3V.

I know this isn't some strange bit shift issue because the CRC is passing, so these numbers are truly what the LTC is reporting. I've replicated this issue on multiple boards as well.

I checked the STAT registers, too, and the chip with the strange measurements reports a low analog supply (2.9V) while the good one sees 5.0V. Similar low voltage on the digital supply. We checked REF1 with a multimeter and it appeared to be 5.0V though.

Any idea what could be causing the ADC reads to be incorrect in such a way?

Thanks,

Matthew

  • If I remember correctly, we watched VREF1 over time, and it would occasionally drop below 5V for a very short time. Every time it would dip, the ADCs would then read the cell voltages correctly for a couple cycles, then return to reading the scaled voltages upon VREF1 recovering to 5V. The chip also sometimes reports correct cell voltages for the first few cycles upon powering on before switching to the scaled ones. This behavior aligns with the voltages being 60% of expected, because the reference is 1/(60%) of what it should be.

    I should note that this issue only began to appear after the board was working perfectly for a few days. Some other copies of this board seemed to start off with this behavior. I wonder if there just isn't enough current for VREF1's zener diode to work? Are there other places you recommend probing?

  • Added a 100 nF capacitor where you specified, and the issue persists. Measured VREF1 again and it's ~5.3V, while VREF2 is ~5.6V. This issue has been replicated on multiple boards. Is it possible the voltage reference fails high instead of low?

  • I repeatedly set REFON to 1 during the test. Later tonight I can verify using the RDCFGA command that it is actually being set. Assuming REFON is in fact 1, what do you think it could be? I'll also double check VREG with a multimeter but I believe it was similar to VREF1 and VREF2. Also, it's no problem if I send WRCFGA repeatedly as long as it's always REFON=1 right?

  • Sounds good. I have tested on an eval board and it worked fine. Our schematic only has the bypass caps on VREF1 and VREF2, though is it possible that too much load on VREG (say, from thermistors) could cause this issue? I tried disconnecting all the thermistors and still saw the same problem.

  • Our thermistors are on a separate board and they're powered through a p-type FET whose gate is pulled down by the LTC6813's WDT pin. I doubt they're the problem since their power draw is in microamps and I tested with the FET disconnected.

    Would it be possible for you to review the schematic, layout, and/or gerber files? If so, they're accessible at this Google Drive folder.

    Later tonight I plan on doing more testing, so are there any additional things besides VREG/VREF1/VREF2 that you recommend I probe or check?

    Also, I've been able to get a decent estimate of cell and thermistor voltages for most boards by just multiplying them by (3.15 / VREGD), where VREGD is what the LTC perceives its digital supply is as seen in Status Register Group B. This temporary fix may work but I'd rather not rely on it.

  • We ordered this version and fixed the unconnected cell 5 trace manually (verified continuity). This issue sometimes occurs on both ICs, sometimes neither, sometimes just 1. The ratios will vary from around 55% to 65%, and this ratio seems to be an intrinsic property of each LTC chip as it's very consistent for each one. We've been able to communicate with not only both ICs within this board, but 6 ICs across 3 boards through daisy-chaining. A couple LTCs still work fine, but the majority of them experience this scaling issue. Some that are currently experiencing the issue had been working perfectly fine for a few days. Also, the ones experiencing the issue will occasionally read correct cell voltages, but rarely, and usually only within a few conversion cycles of startup.