The LTC4120 from Linear Technology is an all-in-one receiver chip for wirelessly charging battery-powered devices. It measures 3 x 3 mm and requires a pick-up coil at its input and a rechargeable battery at its output. A voltage is induced in the coil when it is in close proximity to the transmitter coil of a separate charging unit.
As well as the convenience of just placing your cell phone on a charging pad, this method is also ideal for hand-held devices that can’t use a conventional plug-in charger for reasons of hygiene or harsh/volatile atmospheres.
The battery charging functions allow for both constant current and constant voltage modes and a programmable float voltage level between 3.5 and 11 V accommodates a wide range of cell chemistries. An external resistor sets the charge current up to a maximum of 400 mA. It senses cell voltage and can initiate a low-voltage preconditioning phase if necessary.
The chip employs a technique called Dynamic Harmonization Control to de-tune the pick-up coil when the battery achieves full charge. This has the effect of both reducing energy induced in the receiver loop and power drawn by the transmitter circuit. A FAULT output indicates a bad battery or over-temperature condition.