9.2.1.2.1 Using the bq51010B as a Wireless Power Supply

Figure 27 is the schematic of a system which uses the bq51010B as a power supply.

When the system shown in Figure 27 is placed on the charging pad, the receiver coil is inductively coupled to the magnetic flux generated by the coil in the charging pad, which consequently induces a voltage in the receiver coil. The internal synchronous rectifier feeds this voltage to the RECT pin, which has the filter capacitor C3.

The bq51010B identifies and authenticates itself to the primary using the COMM pins by switching on and off the COMM FETs and hence switching in and out C_COMM. If the authentication is successful, the transmitter remains powered on. The bq51010B measures the voltage at the RECT pin, calculates the difference between the actual voltage and the desired voltage V_RECT-REG, (threshold 1 at no load) and sends back error packets to the primary. Dynamic V_RECT thresholds are shown in Electrical Characteristics. This process goes on until the input voltage settles at V_RECT-REG. During a load transient, the dynamic rectifier algorithm sets the targets specified by V_RECT-REG thresholds 1, 2, 3, and 4. This algorithm is termed dynamic rectifier control and is used to enhance the transient response of the power supply.

During power up, the LDO is held off until the V_RECT-REG threshold 1 converges. The voltage control loop ensures that the output voltage is maintained at V_OUT-REG to power the system. The bq51010B meanwhile continues to monitor the input voltage and maintains sending error packets to the primary every 250 ms. If a large overshoot occurs, the feedback to the primary speeds up to every 32 ms to converge on an operating point in less time.

9.2.1.2.2 Series and Parallel Resonant Capacitor Selection

Shown in Figure 27, the capacitors C1 (series) and C2 (parallel) make up the dual resonant circuit with the receiver coil. These two capacitors must be sized correctly per the WPC v1.1 specification. Figure 28 illustrates the equivalent circuit of the dual resonant circuit.

Figure 28. Dual Resonant Circuit With the Receiver Coil

Section 4.2 (Power Receiver Design Requirements) in Part 1 of the WPC v1.1 specification highlights in detail the sizing requirements. To summarize, the receiver designer is required take inductance measurements with a fixed test fixture. Figure 29 shows the test fixture.

Figure 29. WPC v1.1 Receiver Coil Test Fixture for the Inductance Measurement Ls’ (Copied from System Description Wireless Power Transfer, Volume 1: Low Power, Part 1 Interface Definition,
Version 1.1)

The primary shield is to be 50 mm × 50 mm × 1 mm of Ferrite material PC44 from TDK Corp. The gap d_Z is to be 3.4 mm. The receiver coil, as it is placed in the final system (for example, the back cover and battery must be included if the system calls for this), is to be placed on top of this surface and the inductance is to be measured at 1-V RMS and a frequency of 100 kHz. This measurement is termed Ls’. The same measurement is to be repeated without the test fixture shown in Figure 8. This measurement is termed Ls or the free-space inductance. Each capacitor can then be calculated using Equation 5.

Equation 5.

where

• f_S is 100 kHz +5/-10%
• f_D is 1 MHz ±10%

C1 must be chosen first prior to calculating C2.

The quality factor must be greater than 77 and can be determined by Equation 6.

Equation 6.

where

• R is the DC resistance of the receiver coil

All other constants are defined above.

9.2.1.2.3 COMM, CLAMP, and BOOT Capacitors

For most applications, the COMM, CLAMP, and BOOT capacitance values is chosen to match the bq51010B.

The BOOT capacitors are used to allow the internal rectifier FETs to turn on and off properly. These capacitors are from AC1 to BOOT1 and from AC2 to BOOT2 and must have a minimum 25-V rating. A 10-nF capacitor with a 25-V rating is chosen.

The CLAMP capacitors are used to aid in the clamping process to protect against overvoltage. These capacitors are from AC1 to CLAMP1 and from AC2 to CLAMP2 and must have a minimum 25-V rating. A 0.47-µF capacitor with a 25-V rating is chosen.

The COMM capacitors are used to facilitate the communication from the RX to the TX. This selection can vary a bit more than the BOOT and CLAMP capacitors. In general, TI recommends a 22-nF capacitor. Based on the results of testing of the communication robustness in the final solution, a change to a 47-nF capacitor may be in order. The larger the capacitor the larger the deviation is on the coil which sends a stronger signal to the TX. This also decreases the efficiency somewhat. In this case, a 22-nF capacitor with a 25-V rating is chosen.

9.2.1.2.4 Control Pins and WPG

This section discusses the pins that control the functions of the bq51010B (AD, AD_EN, EN1, EN2, and TS or CTRL).

This solution uses wireless power exclusively. The AD pin is tied low to disable wired power interaction. The output pin AD_EN is left floating.

EN1 and EN2 are tied to the system controller GPIO pins. This allows the system to control the wireless power transfer. Normal operation leaves EN1 and EN2 low or floating (GPIO low or high impedance). EN1 and EN2 have internal pulldown resistors. With both EN1 and EN2 low, wireless power is enabled and power can be transferred whenever the RX is on a suitable TX. The RX system controller can terminate power transfer and send an EPT 0x01 (Charge Complete) by setting EN1=EN2=1. The TX terminates power when the EPT 0x01 is received. The TX continues to test for power transfer, but not engage until the RX requests power. For example, if the TX is the bq500212A, the TX sends digital pings approximately once per 5 seconds. During each ping, the bq51010B resends the EPT 0x01. Between the pings, the bq500212A goes into low power sleep mode reducing power consumption. When the RX system controller determines it is time to resume power transfer (for example, the battery voltage is below its recharge threshold) the controller simply returns EN1 and EN2 to low (or float) states. The next ping of the bq500212A powers the bq51010B which now communicates that it is time to transfer power. The TX and RX communication resumes and power transfer is reinitiated.

The TS or CTRL pin is used as a temperature sensor (with the NTC) and maintain the ability to terminate power transfer through the system controller. In this case, the GPIO is in high impedance for normal NTC (Temperature Sense) control.

The WPG pin is used to indicate power transfer. A 2.1-V forward bias LED is used for D₁ with a current limiting 1.5-kΩ series resistor. The LED and resistor are tied from OUT to PGND and D₁ lights during power transfer.

9.2.1.2.5 Current Limit and FOD

The current limit and foreign object detection functions are related. The current limit is set by R₁ + R_FOD. R_FOD and Ros are determined by FOD calibration. Default values of 20 kΩ for Ros (to RECT, Ros2. Ros1 is not populated). 200 Ω for R_FOD are used. The final values required to be determined based on the FOD calibration. The tool for FOD calibration can be found on the bq51010B web folder under Tools & Software. Good practice is to set the layout with 2 resistors for Ros and 2 for R_FOD to allow for precise values once the calibration is complete.

After setting R_FOD, R₁ can be calculated based on the desired current limit. The maximum current for this solution under normal operating conditions (I_MAX) is 714 mA. Using Equation 1 to calculate the maximum current yields a value of 367 Ω for R_ILIM. With R_FOD set to 200 Ω the remaining resistance for R₁ is 167 Ω. Choose the closest standard resistor of 165 Ω. This also sets the hardware current limit to 856 mA to allow for temporary current surges without system performance concerns.

9.2.1.2.6 RECT and OUT Capacitance

RECT capacitance is used to smooth the AC to DC conversion and to prevent minor current transients from passing to OUT. For this 714-mA I_MAX, select two 10-µF capacitors and one 0.1-µF capacitor. These must be rated to 16 V.

OUT capacitance is used to reduce any ripple from minor load transients. For this solution, a single 10-µF capacitor and a single 0.1-µF capacitor are used.