P9415 Datasheet

13

December 19, 2019

7.1 Overview

The simplified internal block diagram of the P9415 is shown in Figure 3. External Rx coil(s) and CS capacitor(s) as shown in Figure 1 transfer

energy wirelessly using the P9415 AC1 and AC2 pins to be full-wave-rectified (AC-to-DC).

The wireless power is stored on a capacitor(s) connected to VRECT. Until the voltage across the VRECT capacitor exceeds the UVLO threshold,

the rectification is performed by the body diodes of the Synchronous Full Bridge Rectifier FETs. After the internal biasing circuit is enabled, the

Driver and Control block operate the MOSFET switches of the rectifier in various modes to maintain reliable connections at optimal efficiency.

An internal ADC monitors the voltage at VRECT and the load current, the P9415 sends instructions to the wireless power transmitter to increase

or decrease the amount of power transferred or to terminate power transmission based on these readings. The voltage at the output of the

P9415 Main Low-Drop-Out (LDO) regulator is programmed up to 20V using I2C commands. The internal temperature is continuously monitored

to ensure proper operation.

The voltage at VRECT and the current through the rectifier are sampled periodically and digitized by the ADC. The digital equivalents of the

voltage and current are supplied to the internal control logic, which decides whether the loading conditions on VRECT indicate that a change in

the operating point is required. If the load is heavy enough to bring the voltage at VRECT below its target, the transmitter is instructed to move

its frequency lower, closer to resonance. If the voltage at VRECT is higher than its target, the transmitter is instructed to increase its frequency.

To maximize efficiency, the voltage at VRECT is programmed to decrease as the LDO’s load current increases.

7.2 WPC Mode Characteristics

7.2.1 Startup

When a mobile device containing the P9415 is placed on a WPC “Qi” charging pad, it responds to the transmitter’s “ping” signal by rectifying

the AC power from the transmitter and storing it on a capacitor connected to VRECT. During the "Ping" phase, once the rectifier provides voltage

at the VRECT pin above the UVLO threshold, the digital section of the P9415 enables communication. The control loop of the P9415 adjusts

the rectifier voltage by sending error packets to the transmitter before and after it enables the VOUT LDO.

The VOUT LDO is enabled when the power transfer mode is initiated and the voltage at VRECT, the output of the full-wave synchronous rectifier

reaches the target voltage that includes headroom in addition to the LDO VOUT target voltage. For example, if the VOUT voltage target is 12V,

the target VRECT voltage is VOUT + headroom, where headroom is a function of the output current.

7.2.2 Power Transfer

Once the “identification and configuration” phase is completed and successful “negotiation and calibration” is made, then the transmitter initiates

power transfer mode. The P9415 control circuit measures the rectifier voltage and sends error packets to the transmitter to adjust the rectifier

voltage to the level required to maximize the efficiency of the main LDO linear regulator and to notify the Tx of the current Rectified Power

Packet for Foreign Object Detection (FOD) to guarantee safe efficient power transfer. The P9415 is compatible with WPC (latest specification)

and can use compatible Rx coils. Each receiver coil type has a unique inductance value. As such, a unique resonant capacitor (CS) is used for

a given type of receiver coil.

7.2.3 Advanced Foreign Object Detection (FOD) WPC MODE

When metallic objects are exposed to an alternating magnetic field, eddy currents cause such objects to heat up. Examples of parasitic metal

objects are coins, keys, paperclips, etc. The amount of heating depends on the amplitude and frequency of the magnetic field coupled, as well

as on the characteristics of the object such as its resistivity, size, and shape. In a wireless power transfer system, the heating manifests itself

as a power loss, and therefore a reduced power transfer efficiency. Moreover, if no appropriate measures are taken, the heating could lead to

unsafe situations if the objects reach high temperatures.

WPC power transmitters and receivers also need to compensate for the power loss due to parasitic metals intentionally designed into the final

product: i.e., metals that are neither part of the power transmitter, nor of the power receiver, but which absorb power from magnetic field coupling

during power transfer, such as Li-ion batteries, metallic cases, etc.