Table 17. Design Parameters

9.2.1.2.1 Output Voltage Set Point

The output voltage of the bq5102x device can be set by adjusting a feedback resistor divider network. The resistor divider network is used to set the voltage gain at the VO_REG pin. The device is intended to operate where the voltage at the VO_REG pin is set to 0.5 V. This value is the default setting and can be changed through I²C (for the device bq51021). In Figure 14, R₆ and R₇ are the feedback network for the output voltage sense.

Figure 14. Voltage Gain for Feedback

Equation 10.

Equation 11.

Choose R₇ to be a standard value. In this case, take care to choose R₆ and R₇ to be large values in order to avoid dissipating excessive power in the resistors, and thereby lowering efficiency.

K_VO is set to be 0.5 / 5 = 0.1, choose R₇ to be 102 kΩ, and thus R₆ to be 11.3 kΩ.

9.2.1.2.2 Output and Rectifier Capacitors

Set C₄ between 1 and 4.7 µF. This example uses 1 µF.

Set C₃ between 4.7 and 22 µF. This example uses 20 µF.

9.2.1.2.3 Maximum Output Current Set Point

Figure 15. Current Limit Setting for bq5102x

The bq5102x device includes a means of providing hardware overcurrent protection by means of an analog current regulation loop. The hardware current limit provides a level of safety by clamping the maximum allowable output current (for example, a current compliance). The R_ILIM resistor size also sets the thresholds for the dynamic rectifier levels and thus providing efficiency tuning per each application’s maximum system current. The calculation for the total R_ILIM resistance is as follows:

Equation 12.

Equation 13.

The R_ILIM will allow for the ILIM pin to reach 1.2 V at an output current equal to I_ILIM. When choosing R_ILIM, two options are possible.

If the application requires an output current equal to or greater than external ILIM that the circuit is designed for (input current limit on the charger where the RX is delivering power to is higher than the external ILIM), ensure that the downstream charger is capable of regulating the voltage of the input into which the RX device output is tied to by lowering the amount of current being drawn. This will ensure that the RX output does not collapse.

Such behavior is referred to as VIN DPM in TI chargers. Unless such behavior is enabled on the charger, the charger will pull the output of the RX device to ground when the RX device enters current regulation.

If the applications are designed to extract less than the ILIM (1-A maximum), typical designs should leave a design margin of at least 20% so that the voltage at ILIM pin reaches 1.2 V when 20% more than maximum current of the system is drawn from the output of the RX. Such a design would have input current limit on the charger lower than the external ILIM of the RX device.

In both cases, the charger must be capable of regulating the current drawn from the device to allow the output voltage to stay at a reasonable value. This same behavior is also necessary during the WPC V1.1 Communication. See Communication Current Limit for more details. The following calculations show how such a design is achieved:

Equation 14.

Equation 15.

When referring to the application diagram shown in Figure 15, R_ILIM is the sum of the R₁ and R_FOD resistance (that is, the total resistance from the ILIM pin to GND). R_FOD is chosen according to the FOD application note that can be obtained by contacting your TI representative. This is used to allow the RX implementation to comply with WPC v1.1 requirements related to received power accuracy.

In many applications, the resistor R_OS is needed in order to comply with WPC V1.1 requirements. In such a case, the offset on the FOD pin from the voltage on R_FOD can cause a shift in the calculation that can reduce the expected current limit. Therefore, it is always a good idea to check the output current limit after FOD calibration is performed according to the FOD application note. Because the RECT voltage is not deterministic, and depends on transmitter operation to a certain degree, it is not possible to determine R₁ with R_OS present in a deterministic manner.

In this example, set maximum current for the example to be 1000 mA. Set I_ILIM = 1.2 A to allow for the 20% margin.

Equation 16.

9.2.1.2.4 TERM Pin

The term pin is not used for bq5102x. Leave the pin floating.

9.2.1.2.5 I²C

The I²C lines are used to communicate with the device. To enable the I²C, they can be pulled up to an internal host bus. The device address is 0x6C. I²C is enabled only for the device bq51021.

9.2.1.2.6 Communication Current Limit

Communication current limit allows the device to communicate with the transmitter in an error free manner by decoupling the coil from load transients on the OUT pin during WPC communication. This is done by setting the current limit in a manner that is consistent with Table 4. However, this will require the downstream charger to have a function such as VIN_DPM. In some cases this communication current limit feature is not desirable if the charger does not have this feature. In this design, the user enables the communication current limit by tying the CM_ILIM pin to GND. In the case that this is not needed, the CM_ILIM pin can be tied to OUT pin to disable the communication current limit. In this case, take care that the voltage on the CM_ILIM pin does not exceed the maximum rating of the pin which is 7 V.

9.2.1.2.7 Receiver Coil

The receiver coil design is the most open part of the system design. The choice of the receiver inductance, shape, and materials all intimately influence the parameters themselves in an intertwined manner. This design can be complicated and involves optimizing many different aspects; refer to the user's guide for the EVM (SLUUAX6).

The typical choice of the inductance of the receiver coil for a WPC only 5-V solution is between 8 to 11 µH depending on the mutual inductance between the transmitter coil and the receiver coil.

9.2.1.2.8 Series and Parallel Resonant Capacitors

Resonant capacitors C₁ and C₂ are set according to WPC specification.

The equations for calculating the values of the resonant capacitors are shown:

Equation 17.

9.2.1.2.9 Communication, Boot, and Clamp Capacitors

Set C_COMMx to a value ranging from C₁ / 8 to C₁ / 3. Note that higher capacitors lower the overall efficiency of the system. Make sure these are X7R ceramic material and have at least a minimum voltage rating of 25 V; TI recommends a minimum voltage rating of 50 V.

Set C_BOOTx to be 15 nF. Make sure these are X7R ceramic material and have at least a minimum voltage rating of 25 V; TI recommends a minimum voltage rating of 50 V.

Set C_CLAMPx to be 470 nF. Make sure these are X7R ceramic material and have at least a minimum voltage rating of 25 V; TI recommends a minimum voltage rating of 50 V.