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LTC4079 random termination

Hey everyone,

I'm using an LTC4079 to charge a set of 4 series connected 18650's. The charge voltage is set to 200 mA and the input voltage is 30V. I don't see any heating on the device nor have I had any issues charging these batteries in the past.

The voltage on the prog pin is around 1V and randomly drops to 0V for a few seconds while the current drops from 0.2A to 0A. The device charges the batteries for about 2 seconds and then it stops again for about 10 seconds and then repeats the 2 second charge cycle. This constantly repeats. I've got no idea what could cause this as the only thing from the datasheet that resembles this is charging high resistance batteries and that doesn't seem to be my case. I'm also not using a timer to terminate the charging, rather I've connected the timer pin to ground. Furthermore the enable pin remains high throughout this constant charge/stop cycle as well as the input remains stable and is currently just a DC bench power supply.

Here's a rough schematic of the configuration:

R91 is actually a  thermistor.

Here's a link to the batteries I'm using where you can find the datasheet:

http://www.mantech.co.za/datasheets/products/LGABB41865-190226A.pdf

http://www.mantech.co.za/ProductInfo.aspx?Item=15M0457

Thanks in advance for any help!

  • Hello,

    I notice your thermistor is labeled as 1K. Is that the nominal value? Might you be operating near the edge of the hot threshold? The nominal thermistor value should generally be the same as the NTC bias resistor value.

    Also, when you say the voltage on the prog pin and the charge current "drop", is this an immediate drop to 0 or a slow decrease?

    Is your PROG resistor value really 1.5ohms? If so, that's probably the issue.

    Regards,

    Zack

  • Hi Zack!

    Thank you so much for the assistance!

    1) Is your PROG resistor value really 1.5ohms? If so, that's probably the issue.

    So I made a typo there, the PROG resistor is 1.5 kilo ohm.

    2) Also, when you say the voltage on the prog pin and the charge current "drop", is this an immediate drop to 0 or a slow decrease?

    It's an immediate drop, The chip isn't warm at all prior to this drop.

    3) I notice your thermistor is labeled as 1K. Is that the nominal value? Might you be operating near the edge of the hot threshold? The nominal thermistor value should generally be the same as the NTC bias resistor value.

    That was just an incorrect value as well. The thermistor that's used in this project is the following:

    https://za.rs-online.com/web/p/thermistor-ics/7546645/

    The bias resistor connected to the thermistor is also 10 kilo ohm

    The device is operating in a room with a temp of about 25 Degrees Celsius

    Thanks again for the help!

  • Hello,

    So does this happen on multiple boards? If you've only tested on one board so far, try building up another so we can rule out an assembly issue.

    Otherwise, perhaps it is just terminating and recharging. Can you capture a scope plot of this happening over a duration of maybe 10 sec? I'd like to see the battery voltage & current.

    Also, if you add a timer cap, does this go away?

    Regards,

    Zack

  • Hi Zack,

    I'm swapped it out and it's working now. Although the device does seem to be overheating, which doesn't quite make sense.. The input voltage is about 22V, battery voltage is currently 15.38 V the charge current is set to 0.2A. It seems to work well and then after a while the temp rises and it starts shutting the current down to about 0.07A or 70mA.

    According to my calculation, the device should be seeing a drop of 6.62V across it with a current draw of 0.2A which would result in 1.324 W.

    From page 15 on the datasheet

    Ta = 118 - (1.324)*43

    Ta = 61.068

    So the charge current should start being reduced when the ambient temperature is 61 degrees?

    I'm not sure why it is heating up as much as it is. I can place my finger on the IC and feel that it's warm but no where near the point where I burn my finger. Placing my finger on the IC also seems to draw some heat away and it starts momentarily charging at 0.2A again

    Any help with this will really be appreciated

  • Hello,

    That's right, and I'm guessing your ambient temperature is not 61 degC.

    I tested this on the bench. In your setup, I do see a rise to about 60degC, but that's nowhere near what should be engaging the thermal regulation. For that to be the case, you should have some burnt fingers by now.

    Are you sure this is due to the temperature rising? Not just approaching CV mode?

    You might also check that the IC is adequately grounded. Are there vias on the GND paddle going to a GND plane?

    Regards,

    Zack

  • Hi Zack,

    > Are you sure this is due to the temperature rising? Not just approaching CV mode?

    CV mode should start around 16V, the batteries are about 14.5V now, it also looks like if I decrease the voltage by 2V the current rises back to 0.2A and stays there, furthermore if it has decreased I can place my finger on it and it starts to somewhat use my finger as a heatsink and the current shoots back to 0.2A. It does seems to get quite hot.

    > You might also check that the IC is adequately grounded. Are there vias on the GND paddle going to a GND plane?

    The ground plane has a solid piece of copper connected to 6 vias routing it to a solid ground plane layer.

  • Zack here is the connected ground plane connection, the vias connect to a ground plane which fills the whole board. I've got a feeling this might not be adequate ?

  • Hello,

    Good tests, I agree with you that it is power-dissipation-related.

    Thanks for sharing the PCB footprint. Right, that's not adequate. Those vias should be directly below the IC for heat sinking and grounding purposes. I can't be sure that this is the root of the problem, but it does seem likely and I can't think of anything else that would impact the thermals so heavily.

    It is still surprising that this could be nearly enough to approach thermal regulation, but perhaps the bottom of the IC is getting significantly hotter? It would be good to see what the other routing is like on the board.

    I'm sending you a friend request so you can share your whole layout with me if you'd like. I'll take a look at it for thermal performance if you do.

    Regards,

    Zack