APPLICATION NOTE
AN-886 REVISION 08/26/15
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©2015 Integrated Device Technology, Inc.
P9025AC FOD Tuning Guide
AN-886
Introduction
The P9025AC wireless power receiver is a WPC-compliant integrated circuit used to receive wireless power inductively. To
achieve WPC compliance, the Foreign Object Detection (FOD) feature must be enabled and tuned on both the transmitter (Tx)
and Receiver (Rx).
In any wireless power transfer system where a magnetic field is induced to transfer energy, the power transfer flux is also present
outside the Tx and Rx coils. Although this field decays proportionally to the inverse square of the distance, it is typical that these
fields intersect with other permeable materials (such as copper, steel or Lithium Ion batteries). These ferrous materials that are
unrelated to the power transfer but necessary to the product, absorb a portion of the transmitted energy and appear as a system
loss in efficiency. Other ferrous objects that are not part of the system may also be present, such as keys, coins, or other foreign
objects - and they too may absorb energy during operation. As a safety precaution, the Tx will disable power transfer whenever
the losses are measured to be higher than a pre-determined threshold.
Most wireless power systems will have some permeable materials present by design on the Tx and Rx side of the system (such
as the final product casing). This is normal and can be accounted for by tuning the FOD parameters of the system. The P9025AC
has internal OTP registers that are used to configure the FOD parameters required to ignore losses from known system design
objects and only report losses associated with actual foreign objects. Additionally, the P9025AC offers an external global FOD
offset adjustment resistor from FOD2 pin to GND. This resistor can add a positive or negative offset to the reported power by the
device based on the pre-programmed FOD curve.
For WPC-compliant systems, the loss allowed due to foreign objects is 350 mW, not including known losses due to the Tx and
Rx systems. For systems where WPC compliance is not a requirement, FOD can be programmed to accept losses greater than
350 mW, or can be disabled completely.
Getting Started
The P9025AC has two external resistors for changing (tuning) the received power value that is reported to the Tx. The resistor
from pin FOD1 to GND is used to select one of several pre-programmed banks of FOD values. The resistor from pin FOD2 to
GND is an additional offset adjustment and shifts the entire reported received power curve up or down. Details on how to adjust
these values is explained later in this guide.
Before starting to tune the system FOD, careful consideration should be taken to determine to what degree FOD is required in
the application. When WPC compliance is not required, increasing the FOD loss threshold or disabling FOD completely can
significantly reduce the effort and costs associated with tuning. The three options are as follows:
Option #1: Disabling FOD
– some applications will not need any form of FOD, such as fixed location chargers. This
eliminates the need for tuning completely. Skip to the Disabling FOD section for details on how to disable FOD.
Option #2: “Good Enough” FOD
– some applications want FOD to a degree, but do not require a strict FOD detection
threshold. This is typical of systems where the Tx and Rx are paired, and where WPC-compliance is not necessary. This
type of tuning can be accomplished in a short time with just a few simple measurements and two external resistors. This
guide will instruct users on how to achieve this.
Option #3: WPC-compliant FOD
– this is for applications that require Qi Certification from one of the WPC certification
centers. This typically requires precision tuning with the help of an FOD expert and special tools. This degree of tuning falls
outside the scope of this guide. IDT will refer customers to a third party support team for tuning and pre-certification testing.
