Table 20. Design Parameters

9.2.1.2.1 Output Voltage Set Point

The output voltage of the bq51025 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 15, R₆ and R₇ are the feedback network for the output voltage sense.

Figure 15. Voltage Gain for Feedback

Equation 12.

Equation 13.

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

K_VO is set to be 0.5 / 7 = 1/14, choose R₇ to be 130 kΩ, and thus R₆ to be 10 kΩ.

After R₆ and R₇ are chosen, the same values should be used on the VI_REG resistor divider (R₉ and R₈). This allows the device to regulate the rectifier voltage properly and accurately track the output voltage.

9.2.1.2.2 Output and Rectifier Capacitors

Set C₄ between 1 and 4.7 µF. This example uses 3.3 µF.

Set C₃ between 22 and 44 µF. This example uses 44 µF to minimize output ripple.

9.2.1.2.3 TMEM

Set C₅ to 2.2 µF. To determine the bleedoff resistor, the WPC transmitters (for which the PD_DET is being set for) needs to be determined. After the ping timing (time between two consecutive digital pings after EPT charge complete is sent) is determined, the bleedoff resistor R_MEM can be determined. This example uses TI transmitter EVMs as the use case. In this case, the time between pings is 5 s. To set the time constant using Equation 9, R_MEM is set to 560 kΩ.

9.2.1.2.4 Maximum Output Current Set Point

Figure 16. Current Limit Setting for bq51025

The bq51025 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 14.

Equation 15.

The R_ILIM allows for the ILIM pin to reach 1.2 V at an output current equal to I_ILIM in 10-W mode and reach 0.6 V in 5-W mode. When choosing I_ILIM, consider two possible operating conditions:

• If the application requires an output current equal to or greater than external I_LIM that the circuit is designed for (input current limit on the charger where the RX is delivering power to is higher than the external I_LIM), 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 ensures 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 pulls 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 I_MAX, 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 (I_MAX) is drawn from the output of the RX. Such a design would have input current limit on the charger lower than the external current limit 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 v1.2 Communication. See Communication Current Limit for more details. The following calculations show how such a design is achieved:

Equation 16.

Equation 17.

When referring to the application diagram shown in Figure 16, 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 v1.2 requirements related to received power accuracy.

Also note that in many applications, the resistor R_OS is necessary to comply with WPC v1.2 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. 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 1.4 A at 10 W and 700 mA at 5-W mode. Set I_ILIM = 1.7 A to allow for the 20% margin.

Equation 18.

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.

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. In some cases, this communication current limit feature is not desirable. In this design, the user enables the communication current limit by tying the CM_ILIM pin to GND. If this is not needed, the CM_ILIM pin can be tied to the 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.

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 EVM user's guide (SLUUB55).

The typical choice of the inductance of the receiver coil for a 10-W, 7-V solution is between 15 and 16 µH.

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 19.

Because the bq51025 can provide up to 10 W of output power, TI highly recommends that the resonant capacitors have very-low ESR and dissipate as little power as possible for better thermal performance. TI highly recommends NP0/C0G ceramic material capacitors.

9.2.1.2.9 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 the WPC 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 as 15 nF. Make sure these are X7R ceramic material and have a minimum voltage rating of 25 V.

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

Table 21. Design Parameters

9.2.3.2.1 Output Voltage Set Point

The output voltage of the bq51025 device can be set by adjusting a feedback resistor divider network as described in Output Voltage Set Point. The ratio of VO_REG and VIREG resistor dividers are chosen to achieve 10-V based on the 0.5-V feedback voltage. Following Equation 12 and Equation 13, R6 and R7 are selected to be 11.3KΩ- and 215-kΩ, respectively. The same values are used on R9 and R8 in the VIREG divider.

9.2.3.2.2 Output and Rectifier Capacitors

Set C₄ to at least 3.3 µF.

Set C₃ to at least 44 µF to minimize output ripple. Use capacitors rated for 25 V or higher.

9.2.3.2.3 TMEM

Follow procedure described in TMEM.

9.2.3.2.4 Maximum Output Current Set Point

Follow the procedure described in Maximum Output Current Set Point.

9.2.3.2.5 I²C

Connect I²C lines to ground.

9.2.3.2.6 Communication Current Limit

Communication current limit must be disabled. Connect CM_ILIM pin to voltage supply making sure it does not exceed maximum absolute rating for the pin. If only the battery voltage is available, use a 5-V Zener diode (D₁) to clamp the voltage.

9.2.3.2.7 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. The typical choice of the inductance of the receiver coil for a 10-W, 10-V solution is between 15 and 16 µH.

9.2.3.2.8 Series Resonant Capacitors

In order for the bq51025 to work properly as a supply to a 2S charger, the bq51025 output voltage must not drop below the minimum input charging voltage of the charger, which may be around 9 V depending on the charger IC. In a WPC tuned Rx/Tx system, the power delivered to the load may change during Rx/Tx communication due to the capacitive modulation when C_COMM1 and C_COMM2 are connected from AC1 and AC2 pins to ground. If the power delivered to the load decreases, the V_RECT voltage will drop and so may V_OUT if the charger IC does not have VIN-DPM function. If the power delivered to the load does not change or increases for a given current, the V_RECT voltage will increase and the bq51025 will regulate the voltage to a fixed value. The following section discusses the tuning procedure to ensure that the output voltage level is maintained during communication when operating with a bq51025 based transmitter.

9.2.3.2.9 Communication, Boot, and Clamp Capacitors

Set C_COMMx to a value ranging from C₁ / 8 to C₁ / 3. Make sure these are X7R ceramic material and have a minimum voltage rating of 25 V. For this example 56-nF capacitors are chosen.

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

C_CLAMPx is not populated since a external clamping diode is used.

9.2.3.2.10 V_RECT Clamp

Connect a 12-V Zener diode (D_CLAMP) from V_RECT to ground. This diode prevents the rectifier voltage from overshoot above V_RECT-OVP level, preventing unwanted resets during large load transients during communication.