BQ51221 datasheet(32/50 Pages) TI2

32 / 50 page

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Mobile

Device

Interface

Surface

Magnetic

Attractor

(example)

Secondary Coil

Shielding (optional)

Primary Shielding

Figure 9-3. WPC V1.2 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 dZ is to be

3.4 mm. The receiver coil, as it will be 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 9-3. This measurement is termed Ls or the free-space

inductance. Each capacitor can then be calculated using Equation 6:

(

)

(

)

´ p

(6)

where

• fS is 100 kHz +5/-10%.

• fD 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 7:

p ×

(7)

where

• R is the DC resistance of the receiver coil.

All other constants are defined above.

For this application, the selected coil inductance, Ls, is 11 µH and the Ls' is 16 µH with a DC resistance of 191

mΩ. Using Equation 6, the C1 resolves to 158.3 nF (with a range of 144 nF to 175 nF). For an optimum solution

of 3 capacitors in parallel, the chosen capacitors are 68 nF, 47 nF, and 39 nF for a total of 154 nF, well within the

BQ51013B

SLUSB62D – MARCH 2013 – REVISED SEPTEMBER 2020

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