Table 19. Design Parameters

10.2.1.2.1 Output Voltage Set Point

The output voltage of the bq51221 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. In Figure 17, R₆ and R₇ are the feedback network for the output voltage sense.

Figure 17. Voltage Gain for Feedback

Equation 10.

Equation 11.

Choose R₇ to be a standard value. In this case, care should be taken to choose R₆ and R₇ to be fairly large values so as to not dissipate excessive amount of power in the resistors and thereby lower 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.

After R₆ and R₇ are chosen, the same values should be used on R₈ and R₉. This allows the device to regulate the rectifier in the PMA mode to accurately track the output voltage when the output voltage is changed through I²C.

10.2.1.2.2 Output and Rectifier Capacitors

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

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

10.2.1.2.3 Maximum Output Current Set Point

Figure 18. Current Limit Setting for bq51221

The bq51221 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 however, 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 18, 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.

Also note that 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 section shown below. Unfortunately, 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. To set I_ILIM = 1.2 A to allow for the 20% margin.

Equation 16.

10.2.1.2.4 TERM Resistor

The TERM resistor is used to set the termination threshold on the RX. The device will send an EPT Charge Complete, or EOC message to the transmitter and thus allow for the system to go into a low standby mode. This is also mandated through PMA specification.

By picking a resistor to ground from the TERM pin the system designer can set the termination threshold. The device will send the EPT/EOC message, when the voltage on the ILIM pin goes below the voltage on the TERM pin. The designer can therefore set a resistor on the TERM pin that will determine the threshold.

Equation 17.

Typically, one can use R_ILIM to set R₅ resistor such that at the desired current, on OUT pin, V_{ILIM_TERM} can be reached. However, this can be made indeterministic because of the presence of the R_os resistor that is used to comply with WPC v1.1 FOD requirements. Therefore, the system designer is suggested to measure the voltage on the ILIM pin at the output current where he would like to set the termination. This voltage on the ILIM pin is termed as V_{ILIM_TERM}. In the design example, to set 50 mA, measure V_{ILIM_TERM}. After this is done, set the resistor R₅ using the equation Equation 17.

10.2.1.2.5 Setting LPRB1 and LPRB2 Resistors

Figure 19. Setting Low Power Rectifier Boost

LPRB1 and LPRB2 are multifunction pins. Depending on whether the termination resistor is used or not, the LPRB pins will change function. This allows the designer to optimize the PMA design for efficiency or transient performance.

For more information on how to set the TERM resistor, see TERM Resistor.

The LPRBx boosts the rectifier voltage to a higher voltage, and thus it sets the transmitter in PMA mode to operate in frequency or load line that can sustain load step which is part of the PMA certification process. LPRB1 is used to boost the rectifier voltage at low power (output current below about 95 mA). LPRB2 is used to boost the rectifier voltage when output current is below about 310 mA). Both pins are connected to VIREG through resistors, R₃ and R₄ as shown in Setting LPRB1 and LPRB2 Resistors. These two values depend on the coil and the output voltage choice. Also, the allowable voltage drop also defined by the board manufacturer can allow you to set the voltage in these modes to optimize the efficiency and transient response. To design R₃ and R₄, set a window of V_RECT to boost the operating frequency of the TX a 0-mA load and 100 mA

Good starting points are: 7.3 to 7.8 V for 0 to 100 mA and 6.7 to 7.3 V for 100 to 400 mA

Now, find the values of R₃ and R₄ that can provide the chosen window. The lower and upper reference of VIREG is 0.4906 and 0.5318 V

Calculate V_RECT as follows using the TI tool provided in the product folder under the "Tools & sofware" tab.

Figure 20. LPRB Resistor Calculations

10.2.1.2.6 I²C

The I²C lines are used to communicate with the device. In order to enable the I²C, they can be pulled up to an internal host bus. When not in use as in Figure 41, tie them to GND. The device address is 0x6C.

10.2.1.2.7 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. In some cases this communication current limit feature is not desirable. In this design, the user enables the communication current limit. This is done 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.

10.2.1.2.8 Receiver Coil

The receiver coil design is the most open and interesting 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 dual mode 5-V solution is between 6 to 8 µH.

10.2.1.2.9 Series and Parallel Resonant Capacitors

Resonant capacitors C₁ and C₂ are set according to WPC specification. Although this is a dual mode solution, the PMA does not specify an exact resonance frequency for the resonant capacitors and in fact does not specify that resonant capacitors are indeed needed.

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

Equation 18.

10.2.1.2.10 Communication, Boot and Clamp Capacitors

Set C_COMMx to a value ranging from C₁ / 8 to C₁ / 3. The higher the value of the communication capacitors, the easier it is to comply with PMA specification. However, higher capacitors do lower the overall efficiency of the system. Make sure these are X7R ceramic material and have a minimum voltage rating of 25 V.

Set C_BOOTx to be 15 nF. Make sure these are X7R ceramic material and have a minimum voltage rating of 25 V.

Set C_CLAMPx to be 470 nF. Make sure these are X7R ceramic material and have a minimum voltage rating of
25 V.