7.3.1 A29 Coil Specification

The bq500215 controller supports A29 TX coil type. The coil and matching capacitor specification for A29 transmitter has been established by the WPC Standard. This is fixed and cannot be changed on the transmitter side.

For a current list of coil vendors, see bqTESLA Transmitter Coil Vendors, SLUA649.

7.3.2 Option Select Pins

There are two option select pins (pin 60, LOSS_THR, and pin 61, LED_MODE) on the bq500215 and five selector outputs (pins 19, 20, 22, 23, and 24) used to read multiple voltage thresholds . All the pin voltages will be read by bq500215 at power-up.

• Pin 60 is used to program the loss threshold and calibrate the FOD algorithms.
• Pin 61 is used to select the LED mode of the device.
• Pins 19, 20, 22, 23, and 24 are used to sequentially bias the five programming resistors shown in Figure 4.

At power-up, a bias current is applied to pins LED_MODE and LOSS_THR, and the resulting voltage is measured to identify the value of the attached programming resistor. For LED_MODE, the selected bin determines the LED behavior based on Table 1. For the LOSS_THR, the selected bin sets a threshold based on Table 2. See FOD and Parasitic Metal Object Detect (PMOD) Calibration for more information.

Figure 4. Pin 60 LOSS_THR and Pin 61 LED_MODE Connections

7.3.3 LED Modes

The bq500215 can directly drive three LED outputs (pin 12, pin 13, and pin 25) through a simple current limit resistor (typically 470 Ω), based on the mode selected. The three current limit resistors can be individually adjusted to tune or match the brightness of the LEDs. Do not exceed the maximum output current rating of the device.

The selection resistor, connected between pin 61 and GND, selects one of the desired LED indication schemes presented in Table 1.

7.3.4 FOD and Parasitic Metal Object Detect (PMOD) Calibration

The bq500215 supports multiple levels of protection against heating metal objects placed in the magnetic field. An initial analysis of the impulse response to a short ping (FOD ping) detects most metal objects before any power transfer is initiated. If a foreign metallic object is detected by the FOD ping, an FOD warning is issued (see Table 1) for up to 6 seconds after the object is removed. In the case where a bq51025 receiver with a potential foreign object is detected, the bq500215 transmitter will not configure the receiver in proprietary 10-W mode in order to limit the losses in the foreign object. After power transfer has started, improved FOD (WPC1.2) and enhanced PMOD (WPC 1.0) features continuously monitor input power, known losses, and the value of power reported by the RX device being charged. Using these inputs, the bq500215 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 FOD or PMOD resistors, a fault is indicated and power transfer is halted. The ID packet of the receiver being charged determines whether the FOD or PMOD algorithm is used. The ultimate goal of the FOD feature is safety, to protect misplaced metal objects from becoming hot. Reducing the loss threshold and making the system too sensitive leads to false trips and a bad user experience. Find the balance which best suits the application. If the application requires disabling one function or the other (or both), it is possible by leaving the respective FOD/PMOD terminal open. For example, to selectively disable the PMOD function, PMOD should be left open. A final level of protection is provided with an optional temperature sensor to detect any large increase in temperature in the system (see Shut Down Through External Thermal Sensor or Trigger).

Resistors of 1% tolerance should be used for a reliable selection of the desired threshold. The FOD and PMOD resistors (pin 23 and pin 24) program the permitted power loss for the FOD and PMOD algorithms respectively. The FOD_CAL resistor (pin 22), can be used to compensate for any load-dependent effect on the power loss. Using a calibrated FOD reference receiver with no foreign objects present, the FOD_CAL resistor should be selected such that the calculated loss across the load range is substantially constant (within approximately 100 mW). After correcting for the load dependence, the FOD and PMOD thresholds should be re-set above the resulting average by approximately 400 mW for the transmitter to satisfy the WPC requirements on tolerated heating.

Contact TI for the TX tuning tool to set appropriate FOD, PMOD, and FOD_CAL resistor values for your design.

7.3.5 FOD Ping Calibration

The bq500215 is able to detect most metal objects in the charging pad by analyzing the impulse response to a short ping (FOD ping) sent before any power transfer is initiated. The bq500215 does this analysis by measuring the change in resonant frequency and decay of the pulse response and comparing it to given threshold values that are set by resistor in FP_RES and FP_DECAY pins.

Resistors of 1% tolerance should be used for a reliable selection of the desired threshold. The FP_RES and FP_DECAY resistors (pin 19 and pin 20) program the boundary conditions to determine is a receiver or a metal object is detected. The recommended resistor value for both FP_RES and FP_DECAY pins is 86.6 kΩ.

Contact TI for inquiries regarding FOD ping calibration.

7.3.6 Shut Down Through External Thermal Sensor or Trigger

Typical applications of the bq500215 do 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-V threshold on pin 2, T_SENSE. Voltage below 1-V on pin 2 causes the device to shut down.

The application of thermal monitoring through 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 bq500215 device, pin 2 and GND. The threshold on pin 2 is set to 1-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 data sheet and find the resistance versus 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 data sheet, 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.

Figure 5. NTC Application

7.3.7 Fault Handling and Indication

Table 3 shows end power transfer (EPT) packet responses, fault conditions, and the duration of 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 bq500215 features two signal outputs to indicate that power transfer has begun. Pin 31 BUZ_AC outputs a 400-ms duration, 4-kHz square wave for driving low cost AC type ceramic buzzers. Pin 32 BUZ_DC 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 bq500215 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 bq500215 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 supply with a 10-kΩ pullup resistor.

7.3.11 Trickle Charge and CS100

The WPC specification provides an EPT message (EPT–01) to indicate charge complete. Upon receipt of the charge complete message, the bq500215 disables the output and changes the LED indication. The exact indication depends on the LED_MODE chosen.

In some battery charging applications, there is a benefit to continue the charging process in trickle-charge mode to top off the battery. The WPC specification provides for an informational 'Charge Status' packet that conveys the level of battery charger. The bq500215 uses this command to enable top-off charging. The bq500215 changes the LED indication to reflect charge complete when a Charge Status message is 100% received, but unlike the response to an EPT, it will not halt power transfer while the LED indicates charge complete. The mobile device can use a CS100 packet to enable trickle charge mode.

If the reported charge status drops below 90%, normal charging indication is resumed.

7.4.1 Power Transfer

Power transfer depends on coil coupling. Coupling depends 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 smaller the space between the coils is, the better the coupling.

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 regulated by changing the supply voltage to the full-bridge power stage; higher voltage delivers more power. Duty cycle remains constant at 50% throughout the power band and frequency also remains constant at 130 kHz.

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.

7.4.2 Communication

Communication within the WPC is from the receiver to the transmitter, where the receiver tells the transmitter to send power and how much. 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 voltage on the inverter is adjusted to close the regulation loop. The bq500215 features internal digital demodulation circuitry.

The modulated impedance network on the receiver can either be resistive or capacitive. Figure 6 shows the resistive modulation approach, where a resistor is periodically added to the load, and the resulting amplitude change in the transmitter voltage. Figure 7 shows the capacitive modulation approach, where a capacitor is periodically added to the load and the resulting amplitude change in the transmitter voltage.

Figure 6. Receiver Resistive Modulation Circuit

Figure 7. Receiver Capacitive Modulation Circuit

The bq500215 also supports a proprietary handshake with the bq51025 in which minimal communication from the TX to the RX is used. This proprietary handshake enables the bq500215 to deliver power to the bq51025 receiver at levels higher than 5 W. The transmitter-to-receiver communication is achieved through frequency modulation of the power signal.

7.4.3 Power Trains

The bq500215 drives a full-bridge power stage, which drives the coil assembly. TI recommends the CSD97374CQ4M as the driver-plus-MOSFET device for this application. The supply voltage (Vrail) is controlled by the bq500215 device.

7.4.4 Power Train Voltage Control

The bq500215 controls power delivery by modulating the supply voltage (Vrail) of the power stage driving the coil assembly. The bq500215 device generates a PWM control signal in the PWM_RAIL terminal that controls an external power stage circuit (TI recommends CSD97374CQ4M). The switching frequency for this DC-DC controller signal is 520 kHz.

7.4.5 Signal Processing Components

The COMM signal used to control power transfer is derived from the coil voltage. The AC coupled coil voltage is scaled down to a manageable level and biased to a 1-V offset. Series connected diodes are provided for protection from any possible transients.