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LTC4124 charge complete behavior


LTC4124 datasheet page 10 says “After the 3-hour charge termination time expires, charging stops completely”, but does not clarify what it means:

Does it mean the ACIN input (resonant tank) is being grounded?

Does it mean the CHRG led is to go off?

I have seen it continues taking power from transmitter even after several hours, so it seems the ACIN is never grounded. Why not? if charging is off.

I have also seen the led remains ON (no blinking) forever.

If it is the normal behavior, it does not make much sense for me:

in my particular implementation the transmitter is also battery powered and takes 5mA at idle (no RX coil coupled) and 55mA when RX coil is coupled EVEN AT CHARGING COMPLETE STAGE. It is too much power just for nothing.

From my point of view, a “charging stops completely” should also include ACIN grounded and led to go off (CHRG high impedance). My doubt is if it is so and I have something wrong at my setup.

Thank you,

Alberto Farre

  • Hi Alberto, 

    When the 3-hour charge termination timer expires, LTC4124 charge current reduces to zero, and no longer follows the CV charging curve, until the Vbatt falls below recharge threshold, or remove the power source is cut off and then reapplied. "Charging stops completely" means the charging controller, that works in CC/CV mode, completely stops working. 

    The wireless power manager (please refer to the top left of the block diagram) portion of the IC is still working after the termination timer expires. This is for keeping the VCC voltage 0.85V-1.05V higher than the battery voltage. In this mode, anything connected to the VCC is still powered by ACIN, wireless power input. If VCC is loaded by a LED or anything, the VCC pin will be discharged to 0.85V above Vbatt, and the ACIN will not be grounded. If the VCC load is small enough to keep VCC higher than 0.85V for a long time, then the ACIN will be grounded for longer time. 

    CHAR_ pin will be pulled down when battery enters C/10 end of charge, or when the 3-hour charge termination expires. So there are two possible causes for the pull-down event. If you "CHAR led" you mentioned is connected between VCC and CHAR_ pin, as indicated on the front page typical app, then the LED will stay on after the 3-hour charge termination expires.

    What type of transmitter are you using for transmitter? The 55mA might be reduced. If the VCC load is small enough while the charging is terminated, ACIN will be grounded most of the time. For some type of transmitter, the input current should be reduced when ACIN in grounded. 

    You are right that in your application, it is good to terminate the ACIN and make CHAR_ pin high impedance. However, when this part is designed, most of the customer is using a power supply or a larger battery for the transmitter. It is good to have the LED on to indicate that the charger is connected to a supply, otherwise it is difficult to differentiate whether the charger is powered or not. For the ACIN logic, most of the customers expect to supply the load from input while input power is available, instead of using the battery to power the load once the charge is done. So that is why the input power manager circuit is still running. 



  • Hi Wenwei,

    Thank you very much for your extended explanation. Everything works for me as you describe but not sure when you say “ACIN grounded” if you mean permanently or some sort of switching behavior.

    What I can say for sure, in my setup ACIN never gets grounded “permanently”, even after leaving it ON whole night once led remains on (no blinking) and removing it (no led consumption).

    I believe this is the normal behavior as you describe at your last paragraph.

    My problem is that I need some sort “full charge” detection (entering C/10 stage would be OK) at transmitter side to send it to sleep. I was in the hope that measuring current consumption would be enough but, as far as I can see at my setup, power consumption from "led blinking" to "led on" is almost the same.

    I am afraid I have to add some sort of external comms between TX and RX to let TX known when RX is in C/10 stage. It is no difficult to implement but I am afraid it is going to be easier going to some system that includes it.

    Regard to my setup, it is exactly the same like EVM DC2769A-A, but using my own resonant tanks tuned to same frequencies: LTC6990 at 1MHz, TX tank tuned at 1.3MHz and RX tank tuned at 1MHz. Signals at Oscope are exactly the same like those at datasheets.

    About TX power consumption, once adjusted coils separation to 2mm, it is 12.8mA with no RX load and 25mA with RX load at C/10 & >3h stages, with no led (all currents referred to 5V).

    I know 25mA it is not too much but it would take a batt of 1000mAh@4.2V in just 33 hours… not suitable for me.

    Thank you again for your support,