LTC4125 resonant freq and foreign object detection scheme

Hi ADI Expert

I have few questions about LTC4125 and hope if you can provide me some suggestion

Application: 

 I use a LTC4125 as the Tx to transmit power to a  LTC4126 for a 50mA battery charger, the air gap is around 3~4mm and should be fixed, Ith is set at 0.6A

1. I picked the TX resonant freq  around 157.5Khz (L:6.8uH , C: 0.15uF), RX 170Khz (16.5uF C:0.056uF)

I got the resonant freq around 165Khz  as the below pic1 when there is no RX closed to TX, and the Optimum power search is stopped because of VFB>Vin (pic2)

however if a RX is presented, the resonant freq is up to 200Khz when the exit condition is found and power starts delivering from TX to RX  (pic3)

I am not sure if the behavior is correct? (will Resonant freq change quite a lot when RX is closed to Tx ?)

pic1                                                                                                          

  

Pic2

  

Pic3

2. According to the fig11 of the datasheet ,when a foreign object is presented, the resonant freq is increased with lower Vtank amplitude 

however it's not what I found when I put a Aluminum foil on the top of my TX coil(please see pic4), the freq is even lower than when TX/RX is matched and the Vtank is higher

so I couldn't use the FTH pin to setup a fault for foreign objects. (I am now using IMON with a BJT to cause NTC fault to stop power delivering)

but still, I would like to know why the freq did not rise as I expected. I am not sure if the material would be a problem (Copper is used in the test of the datasheet)

                                                                                                     

    

Pic4

 

any suggestion is appreciated

thank you

Ken

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  • +1
    •  Analog Employees 
    on Jul 8, 2021 2:51 PM

    Hi Ken,

    Yes resonant frequency will change when you have a receiver on top of the transmitter coil. That behavior is normal.

    It is also possible that with foreign object, the frequency might not go up. FTH is not the only method to prevent charging the foreign object. I would recommend the BJT on IMON solution if foreign object detection is a key factor to consider, and that is why the newer wireless charging demo kits (DC2770A, DC2554A) all include this method. If a valid pattern on FB is not found, the LTC4125 will keep increasing its duty cycle until the IMON pin turns on the transistor and pull down the NTC pin. 

    Thanks,

    Wenwei. 

  • Hi Wenwei

    thanks for the prompt response, it's helpful

     appreciated~

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