Uising LCT4125 with LTC4124 configured at 10mA

Hello,


I am involved in designing a wearable device that is to be charged wireless, the system includes a docking wireless transmitter.

I have chosen LTC4125 Power Transmitter to work with LTC4124 Power Receiver configured at 10mA.

I know you have Eval kits configured for 100mA (DC2770A-B-KIT) and 50mA (DC2770A-A-KIT).

But, the eval kits for 25mA (DC2769A-B) and 10mA (DC2769A-A) are not based on LTC4125 power transmitter, they use a quite simple oscillator based on LTC6990 instead.

I find the power transmitter based on LTC6990 very useful for testing but, for my final product, I miss all extra protections and less power consumption of LTC4125.

I would like to know if there is any drawback in using LTC4125 for a just 10mA wireless receiver. I am planning to use proposed design for 50mA at DC2770A-A-KIT Eval Kit as it is.

Thank you,
Alberto Farre

Parents
  • +1
    •  Analog Employees 
    on Apr 6, 2020 1:15 PM

    Hi Alberto, 

    The DC2770A-A-KIT may not be good for 10mA wireless receiver, without any changes. The design should be re-configured for such low output power. 

    The DC2770A-A-KIT is configured to run fixed duty cycle for the Auto-Resonant driver. For 10mA receiver, it will send too much power to the receiver and will make the receiver coil overheat. The DC2770A-A-KIT is configured quite differently from the normal operation as described in LTC4125 datasheet. The LTC4125 optimum power search feature is disabled by Q2 (please refer to DC2770A-A-KIT schematic in the demo manual). As the search period begins, it immediately stops by the 1V (Q2 regulated voltage) on IMON pin. The duty-cycle is locked at the start of the search, which depends on the voltage on PTHM pin. So the duty cycle (in proportion to output power)  is programmed by R8 and R4. As IMON pin is used to lock the duty cycle, it might not be used as input current protection. 

    As the DC2770A-A-KIT outputs too much power for the 10mA, you can lower R8 to reduce the output power of LTC4125. You may also change the transmitter coil LTx1 to a higher impedance coil (thinner winding). If you would like to implement input current protection, you could use a current sense amplifier at the input rail. (For example, LT6108). 

    For 10mA application, I do not think that LTC6990 will consume more power than LTC4125 based solution. From my experience, it is difficult to implement optimum power search feature for 10mA as the voltage step would be too fine to be detected by LTC4125, and LTC4125 will miss the trigger locate the optimum power condition. 

    Thanks,

    Wenwei. 

Reply
  • +1
    •  Analog Employees 
    on Apr 6, 2020 1:15 PM

    Hi Alberto, 

    The DC2770A-A-KIT may not be good for 10mA wireless receiver, without any changes. The design should be re-configured for such low output power. 

    The DC2770A-A-KIT is configured to run fixed duty cycle for the Auto-Resonant driver. For 10mA receiver, it will send too much power to the receiver and will make the receiver coil overheat. The DC2770A-A-KIT is configured quite differently from the normal operation as described in LTC4125 datasheet. The LTC4125 optimum power search feature is disabled by Q2 (please refer to DC2770A-A-KIT schematic in the demo manual). As the search period begins, it immediately stops by the 1V (Q2 regulated voltage) on IMON pin. The duty-cycle is locked at the start of the search, which depends on the voltage on PTHM pin. So the duty cycle (in proportion to output power)  is programmed by R8 and R4. As IMON pin is used to lock the duty cycle, it might not be used as input current protection. 

    As the DC2770A-A-KIT outputs too much power for the 10mA, you can lower R8 to reduce the output power of LTC4125. You may also change the transmitter coil LTx1 to a higher impedance coil (thinner winding). If you would like to implement input current protection, you could use a current sense amplifier at the input rail. (For example, LT6108). 

    For 10mA application, I do not think that LTC6990 will consume more power than LTC4125 based solution. From my experience, it is difficult to implement optimum power search feature for 10mA as the voltage step would be too fine to be detected by LTC4125, and LTC4125 will miss the trigger locate the optimum power condition. 

    Thanks,

    Wenwei. 

Children
  • Thanks for your answer.

    If I got it right, you recommend using the easy power transmitter based on LTC6990 for so low power (10mA). I also understand the LTC2415 is quite difficult to tune for so low transmit current and hard to take advantage of all advanced functions it provides.

    I’ll will use LTC6990 as you say.

    I have already prototyped both power transmitters, LTC6990 and LTC4125, I succeed with first one but unable to tune second one at using both with LTC4124 power receiver prototype (10mA). The same with LTC4125 Eval Board DC2330A from kit DC2386A-B I already had.

    About power consumption, I can say Eval Board DC2330A (LTC1425) takes about 4mA with no load, where most of them are taken by power LED. My LTC6990 prototype takes 20mA in same condition and no power LED, going to 50mA when coupling coil with LTC4124 power receiver prototype (10mA).

    Because my power transmitter is also powered with a rechargeable battery, I am afraid I have to manage to control power consumption by adding some sort of load detection.

    Thanks again.

    Alberto