BQ500211A Datasheet, PDF (13/27 Pages) Texas Instruments – 5-V, WPC1.1 Compliant Wireless Power Transmitter Manager

bq500211A
www.ti.com
SLUSBB1 – DECEMBER 2012
Parasitic Metal Object Detect (PMOD) and Foreign Object Detection (FOD)
The bq500211A is WPC1.1 compliant and supports both enhanced PMOD and the new FOD features by
continuously monitoring the input voltage and current to calculate input power. Combining input power, known
losses, and the value of power reported by the RX device being charged, the bq500211A can estimate how
much power is unaccounted for and presumed lost due to metal objects placed in the wireless power transfer
path. If this unexpected loss exceeds the threshold set by the LOSS_THR resistor, a fault is indicated and power
transfer is halted. Whether the PMOD or the FOD algorithm is used is determined by the ID packet of the
receiver being charged.
PMOD has certain inherent weaknesses as rectified power is not ensured to be accurate per WPC1.0
Specification. The user has the flexibility to adjust the LOSS_THR resistor to suit the application. Should issues
with compliance or interoperability arise, the PMOD feature can be selectively disabled as explained below.
The FOD algorithm uses information from an in-system characterized and WPC1.1 certified RX and it is therefore
more accurate. Where the WPC1.0 specification merely requires the Rectified Power packet, the WPC1.1
specification additionally uses the Received Power packet which more accurately tracks power used by the
receiver.
As the default, PMOD and FOD share the same LOSS_THR setting resistor for which the recommended starting
point is 400 mW (selected by a 56.2-kΩ resistor on the LOSS_THR option pin 43). That value has been
empirically determined using standard WPC disc, ring and foil FOD test objects. Some tuning might be required
in the final system as every system will be different. This tuning is best done by trial and error, use the set
resistor values given in the table to increase or decrease the loss threshold and retry the system with the
standard test objects. The ultimate goal of the FOD feature is safety, to protect misplaced metal objects from
becoming hot. Reducing the loss threshold and making the system too sensitive will lead to false trips and a bad
user experience. Find the balance which best suits the application.
If the application requires disabling one or the other or setting separate PMOD and FOD thresholds, a setting
resistor of appropriate value can be connected directly from the LOSS_THR (pin43) to the FOD (pin16) or PMOD
(pin17) pins, as needed. These pins are then read at power up and the correct respective values are set. To
selectively disable PMOD, for example, only the chosen FOD resistor value would be connected between
LOSS_THR (pin43) and FOD (pin 16) and PMOD (pin17) would left open.
Resistors of 1% tolerance must be used for proper detection of the desired bin.
Table 2. Option Select Bins
BIN NUMBER
0
1
2
3
4
5
6
7
8
9
10
11
12
13
RESISTANCE (kΩ)
<36.5
42.2
48.7
56.2
64.9
75.0
86.6
100
115
133
154
178
205
>237
LOSS THRESHOLD
(mW)
250
300
350
400
450
500
550
600
650
700
750
800
850
Feature Disabled
Copyright © 2012, Texas Instruments Incorporated
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