BQ51221 datasheet(34/50 Pages) TI2

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EN1 and EN2 to low (or float) states. The next ping of the BQ500212A will power the BQ51013B which will now

communicate that it is time to transfer power. The TX and RX communication resumes and power transfer is

reinitiated.

The TS/CTRL pin will be 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 will be in high impedance for normal NTC

(Temperature Sense) control.

The CHG 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 will light during power transfer.

9.2.1.2.6 Current Limit and FOD

The current limit and foreign object detection functions are related. The current limit is set by R1 + RFOD. RFOD

and Ros are determined by FOD calibration. Default values of 20 kΩ for Ros and 196 Ω for RFOD are used. The

final values need to be determined based on the FOD calibration. The tool for FOD calibration can be found on

the BQ51013B web folder under "Tools & software". Good practice is to set the layout with 2 resistors for Ros

and 2 for RFOD to allow for precise values once the calibration is complete.

After setting RFOD, R1 can be calculated based on the desired current limit. The maximum current for this

solution under normal operating conditions (IMAX) is 1 A. Using Equation 2 to calculate the maximum current

yields a value of 262 Ω for RILIM. With RFOD set to 196 Ω the remaining resistance for R1 is 66 Ω. This also sets

the hardware current limit to 1.2 A to allow for temporary current surges without system performance concerns.

9.2.1.2.7 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 1-A IMAX, select two 10-µF capacitors and one 0.1-µF capacitor. These should 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.

9.2.1.3 Application Curves

Figure 9-4 shows wireless power start-up when the RX is placed on the TX. In this case, the BQ500212A is used

as the transmitter. When the rectifier voltage stabilizes, the output is enabled and current is passed. In this case,

the load is resistive generating 900 mA. The pulses on the RECT pin indicate communication packets being

transferred from the RX to the TX.

Figure 9-5 shows a current transition. The plot shows a 1-A load removed then added again. Note the stability of

VOUT.

OUT

RECT

OUT

Figure 9-4. Start-Up With 900-mA Load

OUT

RECT

OUT

Figure 9-5. Load Transitions (1 A to 0 A to 1 A)

BQ51013B

SLUSB62D – MARCH 2013 – REVISED SEPTEMBER 2020

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