Datasheet LTC4125 (Analog Devices) - 9

link to page 9 LTC4125
operaTion INTRODUCTION
There are various ways of producing a large AC current in A wireless power system is composed of two parts sepa- an inductor from a DC voltage. The LTC4125 is designed rated by an air gap: transmit circuitry with a transmit coil, to employ one of the simplest and most efficient methods and receive circuitry with a receive coil. The LTC4125 is the using a series LC resonant circuit. power controller for a simple but versatile wireless power transmitter. The LTC4125 enhances a basic wireless power
SERIES RLC
transmitter by providing three key features: an AutoReso- Iasin(ωt + θ) R nant function that maximizes available receiver power, an + Optimum Power Search algorithm that maximizes overall Vasinωt VL L IL C wireless power system efficiency and foreign object detec- – tion to ensure safe and reliable operation when working 4125 F2 in the presence of conductive foreign objects. In order to understand these features, an overview of wireless power
Figure 2. Simple Series Resonant RLC Circuit
systems is required. Figure 2 shows a simple series resonant circuit. When In a typical wireless power system, an AC magnetic field driven with a sinusoid voltage at the resonant frequency is generated by a transmit coil which then induces an the impedance of the inductor and the capacitor cancels AC current in the receive coil—like a typical transformer leaving a pure resistance R. The resonant frequency can system. The main difference between a transformer sys- be calculated as: tem and a wireless power system is that an air gap (or 1 other non-magnetic material gap) separates the primary fn = (transmitter) and secondary (receiver). Furthermore, the 2π LC coupling between the transmit and the receive coils is typi- Therefore at resonance the amplitude of current developed cally very low. Whereas a coupling of 0.95 to 1 is common in the inductor is simply: in a transformer system, the coupling coefficient in the wireless power system varies from 0.8 to as low as 0.05. V I a a = R L TX LRX COIL COIL Notice that at resonance, with a low enough R value, a LTC4125 RECEIVER TRANSMITTER significant amount of inductor current can be generated. CIRCUIT CIRCUIT V + IN – RL Furthermore, the inductor voltage is proportional to the driving voltage: AIR GAP 4125 F01 LOW COUPLING BETWEEN COILS V = QV
Figure 1. Typical Wireless Power System Setup
L = Ia • ωnL = Va • ωnL a R In order to induce enough AC current in the receive coil where Q is the familiar quality factor of the series tank. with such low coupling, a strong magnetic field is needed. The LTC4125 enables a series LC to be driven at exactly Since the magnetic field generated by the transmit coil is its resonant frequency with ease. It uses a patent pending proportional to the current flowing in the coil, a large AC AutoResonant method to automatically detect the resonant current needs to be generated in the transmit coil. frequency of the series LC connected to its switch pins and drive it at that frequency. 4125f For more information www.linear.com/LTC4125 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts