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LTC4121-4.2 charging current issue

Category: Hardware
Product Number: LTC4121-4.2

Good morning,
I made a circuit using LTC4121IUD # PBF as shown in the schematic of the datasheet in figure 12.
As a test bench I've connected at the input Vin a DC power supply and a series resistance (Rin = 100R) and a Lithium battery of 3.7V/2000mAh at the output BAT.

My measurements do not agree with the datasheet values, in particular despite the maximum charge current has been set to 400mA (Rprog = 3.01k) the battery current was different; it increases with the supply voltage in the initial section but at 14V it is limited at a maximum of 100mA. Further increasing of the input voltage lead to a drop of the current to 90mA.

The graph in figure 13 of the datasheet shows that the charge current is initially increasing until it becomes constant at 400mA when the input voltage is 28V.

My question is the following: why does the battery charge current on my circuit does not increase more than 100mA despite the selected Rprog is 3.01k (Ichrg = 400mA)?
What should I do to bring the charging current to its maximum value (400mA)?

  • Hi Nic84,

    To debug, would you please first disable the MPPT  feature (connect MPPT to either INTVCC or IN with a minimum 10k resistor) and check whether the LTC4121 can deliver 400mA ? Also would you please also check PROG pin voltage when it is charging, and see if it is at 1.227V? 

    The LTC4121 is very sensitive to input loop (CIN to VIN pin to GND and back to CIN_GND) parasitic inductance. With larger parasitic inductance, the part could be mal-functioning. So it is good to debug it with a simpler setup to check the basic feature first. 



  • Hi Wenwei, thank you for your reply,
    I've made the suggested modification: with the connection of the 10k resistor on the IN pin I have disabled the MPPT feature but the battery charging current (Ibat) is always about 100-120mA as the supply voltage varies (Voc) (no change from the previous test). The PROG pin remains stable at around 0.3-0.4V.
    Apparently, the IC fails to provide the expected maximum current of 400mA. I've tried to use another IC but the results are the same. Would you please give me another debugging procedure that could be used to test the application circuit? Otherwise, I guess that means I have to change my design for the customer.

  • I feel like this could be layout related. I know that the LTC4121 is sensitive to VIN cap location. Could you share your board layout screenshots, especially the portion that shows the loop of CIN_TOP-IC-GND, and also the loop of C_INTVCC-IC-GND? 


  • Please, find attached the screeshots of the first version of the designed prototype.

    As you can see, I've used a matrix board to have a first evaluation of the IC behavior in my application. Since the experimental measurements does not fullfill the expected results I have not yet realized a pcb, but I guarantee you that the routing is correct.
    The requested loop have been highlighted, please let me know if you need any further clarification.