LTC4020 not detecting battery / Charging current lower than expected

Dear community,

I previously designed a 10S charger using LTC4020 which is working great.

Here are my settings:

・Vin =from 30V to 55V
・ Float voltage = 42,0V
・ Max Charging current = 4.16A (Rsense = 12mΩ)
・ Max Inductor current = 5A (Rsense = 10mΩ)
・ Switching power supply frequency = 200KHz
・ Charging method = CC / CV charging
・ Battery specifications = 10S3P (36V, 9600mAh)

I also join the schematic to this message.

For a newer version I tried to implement a precise charge current setting using a DAC and an OP Amp coupled to a mosfet as current sink which is controlling the RNG/SS pin. It works well sometimes, however:

- I noticed that when I'm powering the LTC4020 and the DAC & OP amp are not powered (3V3_CHARGER is OFF), the LTC4020 is set to what I understand as a "no battery" mode with a voltage to the battery equal to VFLOAT + 10% (46V approx). It is problematic as it could damage the battery. Do you have any clue on what is happening here and how this pin can make the LTC4020 to think there is no battery plugged?

- If the DAC & OP amp are powered, it seems to work as intended and I'm able to control the voltage of the RNG/SS pin from 0V to 1.2V through the current sink and set the charging current accordingly, but at 1.2V on RNG/SS pin the charge current is only 3.6A instead of the 4.16A set with Rsense = 12mΩ . 


Best regards,

Victor




  • Ok, I found the issue by myself, the PMOS drain and source are reversed... so the LTC4020 cannot turn off the path towards the battery.

    I modified the board and it fixes the 110% Vfloat sent to Vbat and allow me to set the current from 0 to 4.16A when 3V3_CHARGER is ON, so it works as expected.

    However, when the 3V3_CHARGER is off and VIN is powered, the maximum charge current is 5A instead of 4.16A. 5A match the max inductor current set by the 10mOhms resistor, any clue on what could cause the charge current to be above the limit set by the 12mOhms sense resistor?

    Best regards,

    Victor

  • 0
    •  Analog Employees 
    on Jul 20, 2020 5:14 AM 15 days ago in reply to VictorG

    Hi Victor,

    I'm not very familiar about this product but I can say something about the R-sense. Noise (from the DC source) can be a big factor of inaccurate current sensing. Make sure that the Kelvin sense layout is good and following the guideline while putting in place the necessary filters.

  • 0
    •  Analog Employees 
    on Jul 22, 2020 3:30 PM 12 days ago in reply to VictorG

    Hi VictorG,

    I apologize for the delay. Fil is correct in the aspect of making sure that the kelvin sense layout should be up to the guidelines to minimize noise from the DC source. I would try and apply power to U801 and U803 separately and test whether having a certain one on/off causes the current regulation issue. For instance, does turning U801 off cause the system to no longer regulate or does turning off U803 cause problems? Maybe the problem only appears when both are off and this could well be an issue with the amplifier. Let me know if you have any additional questions ! 

    Best regards,

    Zaid