LT8708: How does IC select (Constant current/ Constant Voltage) when used Li-ion Battery Charging Application

Hello,

  I am evaluating LT8708 for Li-ion Battery charging application. We require 2 modes of operation.

1. Constant Current Mode: For a 60V Battery lets say this mode starts when battery voltage is  37V. In this situation FBOUT and IMON_OP both Loops are active. FBOUT is set to have 63V and IMON_OP is set for 6A charging Current. How does it ensure that IMON_OP Error Amplifier dominates and IC works in CC Mode and not in CV Mode?

2. Constant Voltage Mode. After the battery is charged to 63V, only FBOUT error Amplifier must act to maintain battery voltage. This can be done as only FBOUT would be out of limit at this condition.

My confusion is how to ensure that IC operates in CC mode when Required? Do we need to disable the FBOUT Amplifier at that time? The examples in datasheet do not disable any of the Error Amplifiers, so what is the way to ensure its CC Mode operation when output voltage is below set limit.

Thanks

  • Hello Audison,

      I tried to run CC Mode on the development board DC2596A. 

    changes done were:

    1. Replaced RIMON_OP form 22.1k to 49.9k for 0.5A constant current mode.

    When i switched on the board with Electronic Load, set in CC Mode, the voltage went to 48V and it was working in CV Mode only. What other changes are required to run the board in CC Mode??

  • 0
    •  Analog Employees 
    on Jan 5, 2021 5:18 AM 3 months ago in reply to Urmi S

    Hi Urmi S,

    I suggest you run your Electronic load in CV Mode and set it below 48V to see constant current. The Electronic Load should pull the constant current you set in your circuit. The reason for this test is to emulate a battery in CC charging with terminal voltage below its CV mode.

    Regards,

  • Thank you. If RIMON_OP is 49.9k i.e set to 0.5A, initially when electronic load is off it will work in CV mode but when the load resistance is set to 30ohms, the board must enter in CC mode and restrict the current to 0.5A, which does not happen. It gives current= 48/30=Around 1.5A flows with output voltage maintained @ 48V. What could be the possible reason for this? 

  • 0
    •  Analog Employees 
    on Jan 5, 2021 8:32 AM 3 months ago in reply to Urmi S

    It could be that your Rsense2 is too low. Use the equation in page 43, RIMON_INP for computation of RIMON_OP. Replace Rsense1 with Rsense2, and I(in,fwd,limit) with I(out,fwd,limit).

    Regards, 

  • Yes i have used formulas on Page 43 for calculations.

    Rsense2 is 8mohms same as in evaluation board. I tried same by setting limit of  3.17A current with RIMON_OP 28kohms. This also did not work. It was allowing current around 4A  @ 48V.

    As the application is battery charging CC operation is required for two values of current.

    1. Precharge (0.6A)

    2. CC Charging (6A)

    The Rsense2 is chosen as per the max current 6A. This is same specification for output current in evaluation board.

    What else could go wrong, which is not allowing board to run in CC Mode?