7.1.1 Fundamentals

The principle of wireless power transfer is simply an open-cored transformer consisting of a transmitter and receiver coils. The transmitter coil and electronics are typically built into a charger pad and the receiver coil and electronics are typically built into a portable device, such as a cell phone.

When the receiver coil is positioned on the transmitter coil, magnetic coupling occurs once the transmitter coil is driven. The flux is coupled into the secondary coil which induces a voltage and current flows. The secondary voltage is rectified, and power can be transferred effectively to a load, wirelessly. Power transfer can be managed through any of various familiar closed-loop control schemes.

7.1.2 Wireless Power Consortium (WPC)

The Wireless Power Consortium (WPC) is an international group of companies from diverse industries. The WPC Standard was developed to facilitate cross-compatibility of compliant transmitters and receivers. The standard defines the physical parameters and the communication protocol to be used in wireless power. For more information, or to download a copy of the WPC specification, go to http://www.wirelesspowerconsortium.com.

7.1.3 Power Transfer

Power transfer depends on coil coupling. Coupling is dependant on the distance between coils, alignment, coil dimensions, coil materials, number of turns, magnetic shielding, impedance matching, frequency, and duty cycle.

Most importantly, the receiver and transmitter coils must be aligned for best coupling and efficient power transfer. The closer the space between the coils is, the better the coupling. However, the practical distance is set to be less than 5 mm, as defined within the WPC Specification, to account for housing and interface surfaces.

Shielding is added as a backing to both the transmitter and receiver coils to direct the magnetic field to the coupled zone. Magnetic fields outside the coupled zone do not transfer power. Thus, shielding also serves to contain the fields to avoid coupling to other adjacent system components.

Regulation can be achieved by controlling any one of the coil coupling parameters. However, for WPC compatibility, the transmitter-side coils and capacitance are specified and the resonant frequency point is fixed. Power transfer is thus regulated by changing the frequency along the resonance curve from 112 kHz to 205 kHz, (that is, the higher the frequency is, the lower the power). Duty cycle remains constant at 50% throughout the power band and is reduced only once 205 kHz is reached.

The WPC standard describes the dimensions, materials of the coils and information regarding the tuning of the coils to resonance. The value of the inductor and resonant capacitor are critical to proper operation and system efficiency.

7.1.4 Communication

Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to send power and how much. In order to regulate, the receiver must communicate with the transmitter whether to increase or decrease frequency. The receiver monitors the rectifier output and using Amplitude Modulation (AM), sends packets of information to the transmitter. A packet is comprised of a preamble, a header, the actual message, and a checksum, as defined by the WPC standard.

The receiver sends a packet by modulating an impedance network. This AM signal reflects back as a change in the voltage amplitude on the transmitter coil. The signal is demodulated and decoded by the transmitter-side electronics and the frequency of its coil-drive output is adjusted to close the regulation loop. The bq500410A device features internal digital demodulation circuitry.

The modulated impedance network on the receiver can either be resistive or capacitive. Figure 1 shows the resistive modulation approach, where a resistor is periodically added to the load, Figure 2 shows the resulting amplitude change in the transmitter voltage. Figure 2 shows the capacitive modulation approach, where a capacitor is periodically added to the load and the resulting amplitude change in the transmitter voltage.

Figure 1. Receiver Resistive Modulation Circuit

Figure 2. Receiver Capacitive Modulation Circuit

7.3.1 Capacitor Selection

Capacitor selection is critical to proper system operation. The total capacitance value of 2 nF × 68 nF (+5.6-nF center coil) is required in the resonant tank. This is the WPC system compatibility requirement, not a guideline.

The capacitors chosen must be rated for at least 100 V and must be of a high quality C0G dielectric (sometimes also called NP0). These are typically available in a 5% tolerance, which is adequate. The use of X7R types or below is not recommended if WPC compliance is required because critical WPC Certification Testing, such as the minimum modulation or ensured power requirements, might fail.

The designer can combine capacitors to achieve the desired capacitance value. Various combinations can work depending on market availability. All capacitors must be of C0G types, not mixed with any other dielectric types.

7.3.2 A6 Coil Specification

The coil and matching capacitor specification for the A6 transmitter has been established by WPC Standard. This is fixed and cannot be changed on the transmitter side.

The bq500410A is primarily intended to drive a 3-coil array but it can also be used to drive a single coil. For single coil operation the two outer coils and associated electronics are simply omitted. Please refer to Figure 6.

Figure 3. Coil Specification Drawing

For a current list of coil vendors, please see:

• bqTESLA Transmitter Coil Vendors, SLUA649

7.3.3 Option Select Pins

Two pins (pin 43 and pin 44) on the bq500410A are allocated to program the Loss Threshold and the LED mode of the device. At power up, a bias current is applied to pins LED_MODE and LOSS_THR and the resulting voltage measured in order to identify the value of the attached programming resistor. The values of the operating parameters set by these pins are determined using Table 3. For LED_MODE, the selected bin determines the LED behavior based on Table 2; for the LOSS_THR, the selected bin sets a threshold used for parasitic metal object detection (see Parasitic Metal Detection (PMOD) and Foreign Object Detection (FOD) section).

Figure 4. Option Programming

7.3.4 LED Modes

The bq500410A device can directly drive two LED outputs (pin 7 and pin 8) through a simple current limit resistor (typically 470 Ω), based on the mode selected. The two current limit resistors can be individually adjusted to tune or match the brightness of the two LEDs. Do not exceed the maximum output current rating of the device.

The selection resistor connected between pin 44 and GND selects one of the desired LED indication schemes presented in Table 2.

7.3.5 Parasitic Metal Object Detect (PMOD) and Foreign Object Detection (FOD)

The bq500410A is WPC1.1 ready and supports both enhanced PMOD and FOD features by continuously monitoring the input voltage and current to calculate input power. Combining input power, known losses, and the value of power reported by the RX device being charged, the bq500410A can estimate how much power is unaccounted for and presumed lost due to metal objects placed in the wireless power transfer path. If this unexpected loss exceeds the threshold set by the LOSS_THR resistor, a fault is indicated and power transfer is halted. Whether the PMOD or the FOD algorithm is used is determined by the ID packet of the receiver being charged.

PMOD has certain inherent weaknesses as rectified power is not ensured to be accurate per WPC1.0 Specification. The user has the flexibility to adjust the LOSS_THR resistor or to disable PMOD by leaving pin 43 open should issues with compliance or interoperability arise.

The FOD algorithm uses information from an in-system characterized and WPC1.1 certified RX and it is therefore more accurate. Where the WPC1.0 specification requires merely the Rectified Power packet, the WPC1.1 specification additionally uses the Received Power packet which more accurately tracks power used by the receiver.

As default, PMOD and FOD share the same LOSS_THR setting resistor for which the recommended starting point is 400 mW (selected by a 56.2-kΩ resistor on the LOSS_THR option pin 43). If, for some reason, the application requires disabling one or the other or setting separate PMOD and FOD thresholds, Figure 5 can be used.

Resistor R39 sets the FOD threshold and R24 sets the PMOD threshold in this configuration. The control lines (FOD and PMOD) are driven briefly at power-up when the resistor values are read.

To selectively disable PMOD support, R24 and Q8-B should be omitted from the above design.

A. Either one of these circuits is connected to LOSS_THR, but not both.

Figure 5. LOSS_THR Connection Circuits

7.3.6 Shut Down by Thermal Sensor or Trigger

Typical applications of the bq500410A does not require additional thermal protection. This shutdown feature is provided for enhanced applications and is not limited to thermal shutdown. The key parameter is the 1.0-V threshold on pin 2. Voltage below 1.0 V on pin 2 causes the device to shut down.

The application of thermal monitoring via a Negative Temperature Coefficient (NTC) sensor, for example, is straightforward. The NTC forms the lower leg of a temperature dependant voltage divider. The NTC leads are connected to the bq500410A device, pin 2 and GND. The threshold on pin 2 is set to 1.0 V, below which the system shuts down and a fault is indicated (depending on LED mode chosen).

To implement this feature follow these steps:

1. Consult the NTC datasheet and find the resistence vs temperature curve.
2. Determine the actual temperature where the NTC will be placed by using a thermal probe.
3. Read the NTC resistance at that temperature in the NTC datasheet, that is R_NTC.
4. Use the following formula to determine the upper leg resistor (R_Setpoint):

Equation 1.

The system restores normal operation after approximately five minutes or if the receiver is removed. If the feature is not used, this pin must be pulled high.

7.3.7 Fault Handling and Indication

The following is a table of End Power Transfer (EPT) packet responses, fault conditions, the duration how long the condition lasts until a retry in attempted. The LED mode selected determines how the LED indicates the condition or fault.

7.3.8 Power Transfer Start Signal

The bq500410A features two signal outputs to indicate that power transfer has begun. Pin 23 outputs a 400-ms duration, 4-kHz square wave for driving low-cost AC type ceramic buzzers. Pin 24 outputs logic high, also for 400 ms, which is suitable for DC type buzzers with built-in tone generators, or as a trigger for any type of customized indication scheme. Do not exceed 4-mA loading from either of these pins which is more than adequate for small signaling and actuation. If not used, these pins should be left open.

7.3.9 Power-On Reset

The bq500410A has an integrated Power-On Reset (POR) circuit which monitors the supply voltage and handles the correct device startup sequence. Additional supply voltage supervisor or reset circuits are not needed.

7.3.10 External Reset, RESET Pin

The bq500410A can be forced into a reset state by an external circuit connected to the RESET pin. A logic low voltage on this pin holds the device in reset. For normal operation, this pin is pulled up to 3.3 V_CC with a 10-kΩ pull-up resistor.

7.3.11 Trickle Charge and CS100

CS100 is supported. If CS100 is reported by the RX, the bq500410A indicates that charge is complete.

The WPC specification provides an End-of-Power Transfer message (EPT) to indicate charge complete. Upon receipt of the charge complete message, the bq500410A changes the LED indication to solid green LED output and halt power transfer for 5 seconds. Subsequently, transmitters pings the receiver again to see if its status has changed, assuming it receives another EPT, the LED mode stays the same.

The WPC specification also provides reporting of the level of battery charge (Charge Status). In some battery charging applications, there is a benefit to continue the charging process in trickle-charge mode to top off the battery. The bq500410A changes the LED indication to reflect charge complete when a 'Charge Status 100%' message is received, but unlike the response to an EPT message, it does not halt power transfer while the LED is solid green. The RX, the mobile device being charged, uses a CS100 packet to enable trickle charge mode.

7.3.12 Current Monitoring Requirements

The bq500410A is WPC1.1 ready. In order to enable the PMOD or FOD features, current monitoring must be provided in the design.

Current monitoring is optional however, it is used for the foreign metal protection features and over current protection. The system designer can choose not to include the current monitor and remain WPC1.0 compliant. Alternately, the additional current monitoring circuitry can be added to the hardware design but not loaded. This would enable a forward migration path to future WPC1.1 compatibility.

For proper scaling of the current monitor signal, the current sense resistor should be 20 mΩ and the current shunt amplifier should have a gain of 50, such as the INA199A1. The current sense resistor has a temperature stability of ±200 PPM. Proper current sensing techniques in the application hardware should also be observed.

7.3.13 Overcurrent Protection

The bq500410A has an integrated current protection feature which monitors the input current reported by the current sense resistor and amplifier. If the input current exceeds a safety threshold, a fault is indicated and power transfer is halted for one minute.

If this feature is desired, the sense resistor and amplifier are required. If this feature is not desired, the I_SENSE input pin to the bq500410A (pin 42) should be grounded.

7.3.14 MSP430G2101 Low Power Supervisor

This is an optional low-power feature. By adding the MSP430G2101, as recommended in Figure 6, the bq500410A device is periodically shut down to conserve power, yet all relevant states are recalled and all running LED status indicators remain active.

Since the bq500410A needs an external low-power mode to significantly reduce power consumption, the most direct way to reduce power is to remove its supply and completely shut it down. In doing so, however, the bq500410A goes through a reset and any data in memory would be lost. Important information regarding charge state, fault condition, operating mode, and indicator pins driven would be cleared.

The MSP430G2101, in its role as a low-power supervisor, is used to provide accurate 'ping' timing, retains charge state, operating mode, fault condition and all relevant operation states. The LEDs are now driven and controlled by the MSP430, not the bq500410A, which directly drives and maintains the LED status indication during the bq500410A reset periods. Since the LED indicators are now driven by the MSP430G2101, care should be taken not to exceed the pin output current drive limit.

Using the suggested circuitry, a standby power reduction from 300 mW to less than 90 mW can be expected making it possible to achieve Energy Star rating.

The user does not need to program the MSP430G2101, an off-the-shelf part can be used. The required MSP430G2101 firmware is embedded in the bq500410A and is boot loaded at first power up, similar to a field update. The MSP430G2101 code cannot be modified by the user.

7.3.15 All Unused Pins

All unused pins can be left open unless otherwise indicated. Please refer to . Grounding of unused pins, if it is an option, can improve PCB layout.