BQ51221 Datasheet, PDF (16/49 Pages) Texas Instruments – Single Chip Wireless Power Receiver

bq51221
SLUSBS9A – FEBRUARY 2014 – REVISED JULY 2014
www.ti.com
When the communication current limit is enabled, the amount of current that the load can draw is limited. If the
charger in the system does not have a VIN-DPM feature, the output of the receiver will collapse if communication
current limit is enabled. In order to disable Communication Current Limit, pull CM_ILIM pin high.
9.3.8 PD_DET and TMEM
PD_DET is only available in WPC mode. This is an open-drain pin that goes low based on the voltage of the
TMEM pin. When the voltage of TMEM is higher than 1.6 V, PD_DET will be low. The voltage on the TMEM pin
depends on capturing the energy from the digital ping from the transmitter and storing it on the C₅ capacitor in
Figure 10. After the receiver sends an EPT (charge complete), the transmitter shuts down and goes into a low-
power mode. However, it will continue to check if the receiver would like to renegotiate a power transfer by
periodically performing the digital ping. The energy from the digital ping can be stored on the TMEM pin until the
next digital ping refreshes the capacitor. A bleedoff resistor R_MEMcan be chosen in parallel with C₅ that sets the
time constant so that the TMEM pin will fall below 1.6 V once the next ping timer expires. The duration between
digital pings is indeterminate and depends on each transmitter manufacturer.
TMEM
C₅
R_MEM
Figure 10. TMEM Configuration
Set capacitor on C₅ = TMEM to 2.2 µF. Resistor R_MEM across C₅ can be set by understanding the duration
between digital pings (t_ping). Set the resistor such that:
R_MEM

t_ping
C₅
(8)
9.3.9 TS, Both WPC and PMA
The bq51221 device includes a ratio metric external temperature sense function. The temperature sense function
has a low ratio metric threshold which represents a hot condition. TI recommends an external temperature
sensor in order to provide safe operating conditions for the receiver product. This pin is best used for monitoring
the surface that can be exposed to the end user (for example, place the negative temperature coefficient (NTC)
resistor closest to the user touch point on the back cover). A resistor in series or parallel can be inserted to
adjust the NTC to match the trip point of the device. The implementation in Figure 11 shows the series-parallel
resistor implementation for setting the threshold at which V_TS-HOT is reached. Once V_TS-HOT is reached, the device
will send an EPT – overtemperature signal for a WPC transmitter or an EOC signal to a PMA transmitter
depending on the mode the device is operating in. An Excel tool to assist with defining the correct resistor values
is available on the bq51221 web folder under 'Tools & Software'. The Excel file can be found at
www.ti.com/product/bq51221/toolssoftware.
VTSB
(1.8 V)
R_{2 20 k}
TS/CTRL
R₁
NTC R₃
16
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Figure 11. NTC Resistor Setup
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