P9025AC FOD TUNING GUIDE
4
REVISION 08/26/15
AN-886
Typically, some offsets are included in the reported received power value in order to avoid false FOD disconnects due to coil
misalignments. These offsets are incorporated by shifting the FOD curve up above the actual power in order to guarantee that
the Tx power level and the reported power levels are within the 350 mW threshold even when the Rx is off-center by 5 mm from
the optimal alignment location in any direction accept the z-direction. The bank used to determine the reported power vs. output
load current is selectable by installing a resistor from pin FOD1 to GND. The following table lists the resistance (±1%) needed
to select each internal FOD1 bank:
Table 1: FOD1 Bank Selection Table
Power offsets are described in more detail in the next section.
Global Offset (FOD2)
The P9025AC offers an additional layer of flexibility that allows a global power offset to be added to the RX
POWER_FOD
reported
level. This offset is applied by connecting an external resistor from pin FOD2 to GND and directly affects the received power
level that is reported to the transmitter across the load spectrum. The adjustment range allows for ±300 mW shifts to the reported
power value at all load levels across the spectrum. The magnitude of the FOD2 constant offset adjustment is dependent upon
the value of the FOD2 resistor and is added to the output power. The valid range for the FOD2 offset resistance is 20k
up to
open circuit where 20 k
results in a +300mW offset and open (FOD2 pin floating) results in -300mW offset.
The following graph indicates the theoretical FOD curve needed to be WPC V1.1.2 Pre-Certified with the P9025AC when using
the recommended layout and bill-of-materials in the P9025-R-EVK reference kit and shows the reported power shifts that would
occur at the positive and negative ends of the FOD2 adjustment range.
Figure 2. P9025AC-R-EVK with Wurth FOD Curve (Bank 1), ± FOD2 Global Offset Minimum and Maximum
Variation Allowance
Bank #
1
2
3
4
5
6
7
8
9
10
R
FOD1
(k
Ω
)
150 to open
113
97.6
75
64.9
56.2
49.9
45.3
40.2
20 to 33.2
