P9242-R Datasheet

© 2018 Integrated Device Technology, Inc

14

January 16, 2018

7. Theory of Operation

A wireless power charging system has a base station with one or more transmitters that make power available via DC-to-AC inverter(s) and

transmit the power over a strongly-coupled inductor pair to a receiver in a mobile device. The amount of power transferred to the mobile device

is controlled by the wireless power receiver by sending communication packets to the transmitter to increase, decrease, or maintain the power

level. The communication from receiver to transmitter is purely digital and consists of 1’s and 0’s that ride on top of the power link that exists

between the transmitter (TX) and receiver (RX) coil. Communication from transmitter to receiver is achieved by frequency shift keying (FSK)

modulation over the power signal frequency and amplitude shift keying (ASK) is used for the communication protocol from receiver to transmitter.

A feature of the wireless charging system is the fact that when it is not delivering power, the transmitter is in Standby Mode. The transmitter

remains in Standby Mode and periodically pings until it detects the presence of a receiver. Once an Extended Power Profile Receiver is detected,

such as the P9221-R or equivalent, the transmitter will provide with up to 15W of output power. If a Baseline Power Profile Receiver is present,

the transmitter will deliver only up to 5W of output power.

The P9242-R contains features that ensure a high level of functionality and compliance with the WPC requirements, such as a power path that

efficiently achieves power transfer, a simple and robust communication demodulation circuit, safety and protection circuits, configuration, and

status indication circuits.

7.1 Foreign Object Detection

When metallic objects, such as coins, keys, and paperclips, are exposed to alternating magnetic fields, the eddy current flowing through the

object will heat up. The amount of heat generated is a function of the amplitude and frequency of the magnetic field, as well as the characteristics

of the object, such as resistivity, size, and shape. In any wireless power system, the heat generated by the eddy current manifests itself as a

power loss reducing the overall system efficiency. If appropriate measures are not taken, the heating could lead to unsafe situations.

In the Extended Power Profile, there are two stages of foreign object detection (FOD). One is by measuring the system quality factor prior to

entering the power-transfer phase, and the other is to measure the power loss difference between the received power and the transmitted

power during the power-transfer phase. Prior to entering the power-transfer phase, the P9242-R detects a change in the coil’s quality factor

(the Q-factor) when a wireless power receiver or metal object has been placed on its surface. The transmitter measures the Q-factor and

compares it with the reference Q-factor provided by the receiver. If the difference is higher than the reference Q-factor, the P9242-R will identify

it as an FOD and shut down the system. See section 7.9 for details for enabling Q-factor detection.

The second stage of the foreign object detection is during the power transfer where the power loss difference between the received power and

transmitted power is constantly measured and compared to the WPC-1.2-specified threshold. If the difference is higher than the threshold set

by the WPC specification, the system will shut down to avoid over-heating.

See Table 6, Table 7, and Table 8 in section 7.2 for details for configuring the FOD threshold via external resistors.

7.2 Configuring Foreign Object Detection and Over-Current Limit – ILIM/FOD Pin

The over-current protection (OCP) is designed to protect the half-bridge and wireless receiver unit from becoming exposed to operating

conditions that could potentially cause damage or unexpected behavior from the system. The input current is continuously monitored during the

power transfer stage. If the input current goes above the OCP threshold of 2.5A (typical), the P9242-R will increase the switching frequency or

reduce the duty cycle in order to keep the input current below the OCP value.

External resistors can be used to select a different OCP threshold (ILIM). The same resistors are used to set the FOD threshold (refer to section

7.1 for FOD details). See Table 6, Table 7, and Table 8 for the resistor values. See the typical application schematic in Figure 25 for the location

of the resistors.

Note: The default current limit setting on the P9242-R-EVK is set to 2.5A. It is highly recommended that the current limit to be set at 2.5A for

15W applications.