My LTC4121 circuit is not charging the Li-Ion battery

I laid out the circuit as close as possible to the evaluation board. The circuit would charge intermittently well below the programmed current and eventually stopped charging completely well below the FB resistor programmed float voltage. I bought the evaluation module DC1977A-B and found that it had a mod BAT capacitor (4.7uF) was moved to share its GND with the PROG resistor, but the module appeared to work to specification. I implemented this mod on my circuit and found that initially the circuit charged at the programmed current for many hours but eventually stopped charging well below the programmed float voltage.

  • Hi LAR1922,

    Would you please share your layout ? As you have same components and settings as the DC1977A-B, and DC1977-B works as expected, the difference could still be the layout. 

    Thanks,

    Wenwei.

  • Overlay

    Top Layer

    Layer 3 Routing

    Bottom Layer

  • This layout looks okay to me. The thermal relief pads can be changed to solid copper. What is the oz of copper did you use for all your layers ? The key components are the IN pin capacitor and the INTVcc capacitor. These two capacitors need to form as close a current loop as possible to make sure the LTC4121 works properly. The current loop is the from the positive terminal of the cap, to the IC pin, and from IC ground (IC bottom pad) to the other terminal of the cap. 

    For the battery connection, did you use short and thick cables? and did you have remote voltage sense for the FBG pin resistors ? It might be good to try with remote sense to see if it can be improved. 

  • 1 oz copper for top and bottom layers

    0.5 oz copper for inner layers

    What do you mean by remote voltage sense for the FBG pin resistor? What do you improve with remote sense?

    My FBG pin resistor is 845 kohm and is as close as possible to the FBG pin.

  • The demo board uses 2oz for the output layers and 1 oz for the two inner layer. The two inner layers can provide good ground return path from the 10uF input cap to the bottom pad of the IC. This might be the difference between your design and the demo board design. 

    When battery is connected with long wires, the BATSNS/FB top resistor (R12 in the demo board schematic) can be connected to battery positive terminal, similar to a kevin sense connection. In this way, the LTC4121 can regulate the real battery voltage on the battery terminal. This is to prevent high voltage drop between the BAT pin and the battery terminal. 

    Thanks,

    Wenwei.