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LTC4125 - demo kit 2330A modified with smaller coil, high power consumption when receiver not in range

Category: Hardware
Product Number: LTC4125

Using the demo kit 2330A and 2445A-B (wireless charging transmitter LTC4125 and receiver LTC4120), I’ve been trying to adapt the application to a smaller coil (space constrained application only allows for 16 mm coil diameter).  In both options I tested, a 7.2 uH coil with 330n CTx, and a 26 uH coil (0.52 ohms resistance) with original 100n CTx, both coils with ferrite plate, there is power transferred to the receiver, but current draw by the transmitter increases when the receiver is not in reach, causing the Tx coil to get really hot. Can idle current consumption be limited, or is it the cause of improper parts choice? The problem (high power draw when idle) does not occur with the original coil mounted on the demo kit transmitter. It would not be necessary to use the full power this device is capable of, limiting it to 50% would be an option. However, increased power consumption without a receiver in range suggests there's a problem. Can you help with finfing the problem?  The coils used for the experiment are Qi charging coils from Würth; their power rating is significantly lower compared to the kit coil, but this doesn't seem to explain the superfluous current draw when idle.

 Excerpt from coil datasheet

  • Hi 

    With the original coil, the FB pin voltage can exceed VIN (5V) when no Rx is not in reach. However, with your new coil, the FB voltage no longer exceeds VIN voltage, so no protection is triggered. You need to readjust the FB resistors to make FB voltage over VIN and trigger this protection. 

    Another way to trigger protection when no Rx is in reach, is to use a transistor on IMON pin, and to trigger NTC when current is too high. In this way, when IMON pin voltage reaches around 0.7V, the NTC protection will be triggered and the LTC4125 will not trigger the exit condition of the optimum power search logic (see Figure 6 of the datasheet for details). 



  • Hi Wenwei,

    "You need to readjust the FB resistors to make FB voltage over VIN and trigger this protection."

    How do you go about calculating the precise value of the FB resistors? Is it through trial and error or is there some way to calculate the most suitable values?


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