LTC4125 - DC2770A-B-KIT - PTH1


I am using the DC2770A-B evaluation kit with the aim of recharging coin batteries with charging currents at 10 mA or 25 mA and a distance between the TX and RX coils of 10 mm.

The transmitter is set to guarantee optimum power search, but for the purpose of my work I would like to disable it. For this reason, I applied an input voltage of 1 V to IMON and I applied an adjustable voltage (0 -1) V with an r pull-up on pin PTH1, in order to manually manage the voltage on the coil.

Please let feel free to suggest and advise me, and let me know if you think that these changes are reasonable.

Also, from the datasheet I don't understand how the PTH1 pin works either with or without an applied voltage. In the tests carried out forcing this operation, the power is transmitted correctly. The problem is that the LED indicating the status of the transmitter is off as if it indicated a fault condition. I don't understand why and which fault condition is activated. NTC also does not return voltage (I think it is due to transistor Q1, activating when I apply a voltage of 1V on IMON and NTC goes to ground).

Since I don't need to use optimum power search, I wanted to ask you how I could modify the circuit further to take up as little space as possible. For example if there are components that can be removed. The features that interest me of the LTC4125 integrated are: the AutoResonant driver, Pulse Width Threshold, NTC and the Resonance Feedback Voltage.

I am attaching the circuit diagram of the transmitter with the proposed modification.



  • +1
    •  Analog Employees 
    on Nov 17, 2020 3:00 PM 5 months ago

    Hi Andrea,

    Setting IMON at 1V is a good way of setting the transmitter with fixed duty cycle. Instead of setting PTH1 voltage directly, I recommend you use PTHM pin to set up the duty cycle. Some protection features are only in T3 (please refer to Figure 6 of the datasheet). The protection feature triggers a pull-down on PTH1 and PTH2 to reduce the duty cycle to almost zero. This will not work if you drive PTH1 with external pull-up. As you set IMON at 1V, you fake the search logic into T3 directly as you meet the exit condition (IMON>0.8V).

    However, as IMON pin is fixed at 1V, it no longer indicates the amount of input current. The LED on the demo board is based on IMON pin voltage so it will not show the correct current. If you need LED current indication, you could use a current sense amplifier, for example LT6105, to feed into the LED indicator circuit on the demo board. 

    You are correct that Q1 needs to be removed if you apply a 1V on IMON. Actually Q2 is for that 1V reference voltage. You can refer to the -A in the version table for configuring Q2 to disable the optimum power search. 

    If you are only interested in the AutoResonant driver, Pulse Width Threshold, NTC and the Resonance Feedback Voltage, you can remove R5, R9, R6, R10, R7, R11. You probably still need C6 and C7 for NTC. 

    DC2770A-A is a good example of how to configure a fixed duty cycle AutoResonant transmitter. However, for LED indicator, you still might need to use LT6105 to convert current to voltage signal. 



  • Thank you for your reply, Wenwei!

    Very clear and helpful. I will certainly consider using PTHM pin to set up the duty cycle.



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