Battery-free applications with wireless power
The Wireless Power Transfer (WPT) system consists of two parts separated by an air gap: a transmit (Tx) circuit (including a transmit coil) and a receive (Rx) circuit (including a receive coil) (see Figure 1). Much like a typical transformer system, the alternating current generated in the transmitting coil generates alternating current in the receiving coil by magnetic field induction. However, unlike a typical transformer system, the degree of coupling between the primary side (transmitting end) and the secondary side (receiving end) is usually low. This is due to the presence of non-magnetic material (air) gaps.
Figure 1. Wireless power transfer system.
Most wireless power transfer applications today use a wireless battery charger configuration. The rechargeable battery is located at the receiving end and can be wirelessly charged as long as it has a transmitting end. After charging is complete, separate the battery from the charger, and the rechargeable battery can supply power to the terminal application. The back-end load can be connected directly to the battery, indirectly to the battery via PowerPathTM ideal diodes, or to the output of an integrated battery-powered regulator in the charger IC. In all three cases (see Figure 2), the end application can be run either on the charger or off the charger.
However, if a particular application does not have a battery at all, instead, when a wireless power supply is available, just provide a regulated voltage rail, what happens? Examples of such applications are extremely common in the fields of remote sensors, metrology, automotive diagnostics, and medical diagnostics. For example, if the remote sensor does not require continuous power, it does not require a battery, and the battery needs to be replaced periodically (if it is a primary battery) or charged (if it is a rechargeable battery). If the remote sensor only needs to give a reading when the user is nearby, wireless power can be supplied as needed.
Figure 2. Wireless Rx battery charger with back-end load connected to a) battery, b) PowerPath ideal diode and c) regulator output.
Figure 3. WPT uses the LTC3588-1 to provide a stable 3.3 V rail.
Let's look at the LTC3588-1 nanopower power harvesting power solution. Although the LTC3588-1 was originally designed for energy harvesting (EH) applications that supply sensors (such as piezoelectric, solar, etc.), it can also be used in wireless power applications. Figure 3 shows the complete transmitter and receiver WPT solution using the LTC3588-1. On the transmit side, a simple open-loop wireless transmitter based on the LTC6992 TimerBlox® silicon oscillator is used. In this design, the drive frequency is set to 216 kHz, which is lower than the resonant frequency of the LC resonant circuit of 266 kHz. The exact ratio of fLC_TX to fDRIVE is best determined empirically to minimize the M1 switching losses caused by zero voltage switching (ZVS). The design considerations for transmitter coil selection and operating frequency are no different from other WPT solutions, that is, there is nothing unique about using the LTC3588-1 at the receiving end.
At the receiving end, the resonant frequency of the LC resonant circuit is set equal to the driving frequency of 216 kHz. Since many EH applications require AC-to-DC rectification (like the WPT), the LTC3588-1 has this feature built in, allowing the LC resonant circuit to be directly connected to the PZ1 and PZ2 pins of the LTC3588-1. This rectification is broadband rectification: DC to >10 MHz. Similar to the VCC pin of the LTC4123/LTC4124/LTC4126, adjust the VIN pin of the LTC3588-1 to a level suitable for powering the back-end output. For the LTC3588-1, it is the output of a hysteretic step-down DC-DC regulator rather than the output of a battery charger. Four output voltages are selectable via pin: 1.8 V, 2.5 V, 3.3 V, and 3.6 V, with continuous output currents up to 100 mA. As long as the average output current does not exceed 100 mA, an appropriately sized output capacitor can be selected to provide a higher short-term burst current. Of course, the ability to fully achieve 100 mA output current depends on whether it has the proper size of the transmitter, the pair of coils, and whether it is fully coupled.
If the load demand is below the supported available wireless input power, the VIN voltage will increase. Although the LTC3588-1 integrates an input-protected shunt to provide up to 25 mA of current when the VIN voltage rises to 20 V, this feature is not required. As the VIN voltage rises, the peak AC voltage on the receiving coil also rises, which is equivalent to the drop in the amount of AC that can be supplied to the LTC3588-1, not just in the receiving resonant circuit. If the open-circuit voltage (VOC) of the receiving coil is reached before VIN rises to 20 V, the back-end circuit is protected and no heat is generated in the receiving-end IC to cause energy consumption.
Test Results: For an application with an air gap of 2 mm as shown in Figure 3, the maximum output current available at 3.3 V is measured to be 30 mA, while the VIN voltage measured at no load is 9.1 V. When the air gap is near zero, the maximum available output current is increased to approximately 90 mA, while the VIN voltage at no load is only increased to 16.2 V, well below the input protection shunt voltage (see Figure 4).
Figure 4. Maximum output current available at various distances at 3.3 V.
For battery-free applications with wireless power, the LTC3588-1 provides a simple, integrated solution that provides a low current regulated voltage rail with full input protection.