link to page 9 link to page 10 link to page 10 link to page 11 LTC4125 operaTion AUTORESONANT DRIVE coupling conditions, such a strong magnetic field will be Consider the series resonant structure in Figure 2. If a inefficient and may damage the receiver. Given dissipative square wave voltage source is used instead of a sinusoi- elements in the transmit circuitry, transmitting any more dal voltage source, the analysis for the rest of the circuit power than necessary will result in reduced efficiency. does not change significantly assuming the values of Therefore it is desirable to adjust the strength of the R, L and C result in a high quality factor (Q greater than magnetic field generated by the transmit coil such that 10). The frequency selectivity of a high Q circuit ensures just enough power is available to support the load at the that primarily the fundamental component of the square receive coil—the optimum transmit power point. wave affects the voltage and current waveforms across Aside from efficiency, there is also a matter of safety. the inductor and the capacitor (Figure 3). When a conductive object is placed in the magnetic field At start up, the LTC4125 will drive the LC tank with a 50% generated by the transmit coil, eddy current will be gener- duty cycle square wave at 2.5kHz. When current is devel- ated in the object. These eddy currents generate heat due oped in the LC tank, the LTC4125 detects this condition, to the object resistance. This heating is undesirable for and adjusts the frequency of the drive voltage accordingly. safety reasons, especially in higher power applications. LTC4125 has features that address these two issues: I V L IN VL improved efficiency across all coupling conditions and foreign object detection/protection that enhances safe operations. OPTIMUM POWER SEARCH OPERATION 4125 F03 The Optimum Power Search takes advantage of the fact that Figure 3. LC Tank Voltage and Current Waveforms withSquare Wave Input at the Resonant Frequency transmit power can be adjusted by varying the pulse width of the full bridge driver. AutoResonant Drive continues AutoResonant Drive ensures that the voltage at each SW to operate as pulse width is varied to control the amount pin is always in phase with the current into the pin (refer of transmit coil current. Figure 4 shows tank current and to the Block Diagram: when current is flowing from SW1 voltage waveforms using a drive pulse width resulting in to SW2, switch A and C are on while D and B are off; and a duty cycle less than 50%. vice versa in reverse). Locking the driving frequency cycle by cycle with this method ensures that LTC4125 always VIN V drives the external LC network at its resonant frequency. IL L This is true even with continuously changing variables that affect the resonant frequency of the LC tank such as tem- perature and the reflected impedance of a nearby receiver. 4125 F04 OPTIMUM POWER SEARCH BACKGROUNDFigure 4. LC Tank Voltage and Current Waveforms with SquareWave Input at Less Than 50% Duty Cycle for a Series RLC Circuit In a wireless power system, the magnetic field at the transmit coil needs to be strong enough to ensure that The drive duty cycle is proportional to pulse width. Figure 5 sufficient power can be delivered to the receiver load at shows how tank current increases as duty cycle is varied the worst coupling condition. However, under best case from 0% to 50%. Note that controlling the amplitude of transmit coil current is equivalent to controlling the volt- age amplitude across the coil at a particular frequency. 4125f 10 For more information www.linear.com/LTC4125 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
