5W, QI WIRELESS POWER RECEIVER WITH INTEGRATED RECTIFIER AND LDO OUTPUT

10

09/01/15

P9025AC DATASHEET

Advanced Foreign Object Detection (FOD)

When metallic objects are exposed to an alternating magnetic

field, eddy currents cause such objects to heat up. Examples

of parasitic metal objects as such are coins, keys, paper clips,

etc. The amount of heating depends on the strength of

coupled magnetic field, 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 be sufficient that the foreign objects may become

heated to an undesirable temperature.

During Power Transfer state, the receiver periodically will

communicate to the transmitter the amount of power received

by means of a Received Power packet. The transmitter will

compare this power with the amount of power transmitted

during the same time period. If there is a significant

unexplained loss of power, then the transmitter will shut off

power delivery, because a possible foreign object may be

absorbing too much energy.

For a WPC system to perform this function with sufficient

accuracy, both the transmitter and receiver need each to

account for and compensate for all of their known losses.

Such losses could be to resistive losses, nearby metals that

are part of the transmitter or receiver, etc. Because the system

accurately measures its power and accounts for all known

losses, it can thereby detect foreign objects owing to their

creation of an unknown loss.

The P9025AC employs advanced FOD techniques to both

accurately measure its received power, and to accurately

compensate all of its known losses. This compensation is

implemented by means of a curve fitting table. This table

supports up to 10 different curves stored in OTP (One Time

Programmable) memory. The 10 programmed OTP settings

are externally selected by means of the FOD1 selection

resistor value (see page 14) which selects the settings that

best match a particular system characteristic. Additionally, the

that can be selected that will override the OTP memory

settings. A further enhancement is selected by the FOD2

offset resistor which adds a -300 to +300mW power offset to

the values selected by the FOD1 resistor. This is useful to tune

the selected compensation curve up or down to optimally

match the actual known receiver system losses.

For more information about how to determine and program the

FOD settings, refer to application note AN-886 P9025AC FOD

Tuning Guide.

Over-Voltage Protection

In the event that the input voltage increases above 15V, the

control loop disables the LDO, and sends error packets to the

transmitter in an attempt to bring the rectifier voltage below 7V.

This is accomplished through loading the receiver resonant

tank with extra capacitance shorted to ground through the

above mentioned internal FETs. This changes the parallel

resonance of the tank circuit which causes the Tx to Rx gain

to decrease dramatically. The clamp is released when the

must not be directly loaded.

Over-Current Protection and Thermal Shutdown

The P9025AC employs over-current and thermal protection

by sending an End of Power packet to the transmitter when

the output current reaches the current limit level or the die

temperature exceeds the thermal shutdown level. The LDO

output is also disabled during these conditions.

The current limit level is programmable with an external

resistor:

For any value of RLIM below 25k

, the output current is

limited to 2-Amps. However, RLIM is not recommended to be

set higher than 60 k



rectifier switches to half synchronous or full synchronous

mode (depending on the loading conditions) to efficiently

of the P9025AC maintains the rectifier voltage between 5.45V

.

I

LIM

45000

R

LIM

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=

I

SET

38000

R

LIM

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=