Doubling the LTC4120 Wireless Power Receiver for Higher Battery Charge Current

The LTC4120 is a constant-current/constant-voltage wireless receiver and battery charger. The charge current is programmed up to 400mA with an external resistor. If more than 400mA is required, then use two LTC4120 devices operating in parallel as shown in this schematic. In this case the two LTC4120 operate from one transmitter with one receiver coil and one rectifier. Simply connect the two LTC4120 inputs and battery outputs together, and short the two DHC pins. Optionally the /CHRG and /FAULT flags may also be shorted together to obtain wired-OR status flags. The two LTC4120s can be operated at the same or different frequencies and whichever has the lowest V_FLOAT (matching in feeback divider) turns on first during recharge.

The basic wireless transmitter requires some improvement to support the higher power level. Simply increase the current capability of the two 68uH input bias inductors to support the additional power. The demoboard, DC1968A, uses two small TDK VLCF5028T-680MR40-2 inductors with saturation current of 410mA and ESR of 400mOhm. Replace these inductors with two TDK SLF10145T-680M1R2 inductors with saturation current of 1.2A and ESR of 140mOhm, and voila the transmitter power level is doubled.

The basic transmitters input supply is 5V +/- 5% supplying up to 1A. While the battery is charged to 4.2V with a charge current of up to 800mA. This circuit charges a battery with up to 3W with an overall efficiency of about 60%.

Further information can be found in the LTC4120 datasheet's Applications Information section, AN138: Wireless Power Users Guide and DC1969A's Demo Manual and Schematic.

 

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