REVISION 08/26/15
7
P9025AC FOD TUNING GUIDE
AN-886
Option #2: “Good Enough” FOD
In many applications, it is not required to be WPC-compliant. This is especially true for applications where the transmitter and
receiver will be designed as a pair, and no guarantee for interoperability with other devices is needed. By removing the 350 mW
hard requirement for foreign object losses, it provides tremendous flexibility to use and adjust one of the pre-programmed FOD
curves. This eliminates the need for costly testing and device configuration.
To aid in the selection of the most appropriate FOD curve and make offset adjustments, a working spreadsheet is available. The
spreadsheet is titled P9025-R-EVK FOD Calculator and can be downloaded at
www.idt.com/P9025AC-R-EVK
. The spreadsheet
should be used in conjunction with following these steps:
1. Create a bench setup that allows for accurate current measurements and voltage measurements of the system under
design.
2. P9025AC measurements should take place directly across the OUT capacitor and the current should be measured as
it leaves the P9025AC OUT node (see
Figure 4
).
3. P9038 current measurements should take place at any point along the input power line feeding the sense resistor and
across the REG_IN pin capacitor (see
Figure 4
).
4. Find the optimal alignment location by placing the Rx on the Tx and moving around the Tx surface until the input current
to the Tx reaches the minimum RMS value while the Rx is connected to the Tx and the output is present and stable (This
should typically occur at the point when the center Rx coil opening (inner diameter) is directly above the center of the
opening in the Tx coil (inner diameter).
5. Move the Rx coil exactly 5mm away from the Optimal Alignment Location in any direction.
6. Measure and record the input current, input voltage, and output voltage while sweeping the output load from 0 mA to
1000 mA in approximately 50mA steps. While doing so, fill in Columns B, C and D at each load current level listed in
column A on the Measurements tab. The input and output power levels will be calculated for you. Ideally, this should be
performed with the final assembled wireless Tx and Rx products being built and the opposite Tx or Rx IDT EVK. Enter
these values in the left hand input table labeled First data set.
7. Move the Rx coil exactly 5mm away from the Optimal Alignment Location in any direction that is 90? Clock-wise or
counter-clockwise from the direction chosen for step 5. For example, if the Rx was move +5mm in the x-direction in step
5, move the Rx ±5mm in the y-direction in step 7. Repeat step 6, with the exception of entering the data in the right hand
data entry table (Columns: N,O, and P) labeled Second data set.
a. For fixed alignment systems, enter the same data collected in step 6 in the second table indicated in step 7 (enter
the same data in both tables, First and Second data sets are identical).
8. Make sure that no cell in the Output Voltage (V) columns D and P turns RED in after entering all the data. If the Output
Voltage columns turn RED the system is not performing properly and the new product should be debugged to determine
the source of the excessive losses and they should be corrected before proceeding.
9. Check the fit of the resulting input power curve from of the system under test to the default curves displayed on the
FOD_Curves tab of the spreadsheet or check the 'Delta from Pre-programmed Banks' table on the Measurements tab
for a column that has very few or no red cells. (see
Figure 5
)
a. If all the cells in at least one bank are within -350 mW of the difference of input power and reported power, the cells
will be white with black text (no red). That is the best fit FOD curve bank and should be selected. The necessary
FOD2 offset value will be displayed in green below the table and the appropriate FOD2 resistor should be installed
to result in that offset (if no FOD2 cell is green, it means the required offset shift is beyond the trim range).
In this case, use the determined FOD1 bank that has the green highlighted number beneath the Bank # by setting
the specified FOD1 resistance (found above the Bank#). Then, set the FOD2 resistor value as shown for that bank.
None of the available pre-programmed Banks meet the FOD thresholds at all load levels.
b. If no banks are free from red cells (or cells that are white with orange text), that means one or more points spanning
the load has a variation greater than the input power transmitted or less than -350 mW from the input power
compared to the pre-configured curves. The engineer has two options: look for a "good enough" fit where the
variation exceeds 350 mW, or, come up with a custom set of values that will need to programmed into the device.
