BQ51221 datasheet(24/50 Pages) TI2

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Table 8-5. Adaptive Communication Limit

(continued)

IOUT

COMMUNICATION CURRENT LIMIT

> 300 mA

IOUT + 50 mA

The first mode is illustrated in Figure 7-18. In this plot, an output load pulse of 300 mA is periodically introduced

on a DC current level of 200 mA. Therefore, the 400 mA current limit is enabled. The pulses on VRECT indicate

that a communication packet event is occurring. When the output load pulse occurs, the regulator limits the pulse

to a constant 400 mA and, therefore, preserves communication. Note that VOUT drops to 4.5 V instead of GND.

A charger device with an input voltage regulation set to 4.5 V allows this to occur by offloading the load transient

support to the mobile device’s battery.

The second mode is illustrated in Figure 7-19. In this plot, an output pulse of 200 mA is periodically introduced

on a DC current level of 400 mA. Therefore, the tracking current mode (IOUT + 50 mA) is enabled. In this mode,

the BQ51013B measures the active output current and sets the regulator's current limit 50 mA above this

measurement. When the load pulse occurs during a communication packet event, the output current is regulated

to 450 mA. As the communication packet event has finished the output load is allowed to increase. Note that

during the time the regulator is in current limit VOUT is reduced to 4.5 V and 5 V when not in current limit.

8.3.12 Synchronous Rectification

The BQ51013B provides an integrated, self-driven synchronous rectifier that enables high-efficiency AC to DC

power conversion. The rectifier consists of an all NMOS H-Bridge driver where the backgates of the diodes are

configured to be the rectifier when the synchronous rectifier is disabled. During the initial start-up of the WPC

system the synchronous rectifier is not enabled. At this operating point, the DC rectifier voltage is provided by

the diode rectifier. Once VRECT is greater than VUVLO, half synchronous mode will be enabled until the load

current surpasses IBAT-SR. Above IBAT-SR the full synchronous rectifier stays enabled until the load current drops

back below the hysteresis level (IBAT-SRH) where half-synchronous mode is enabled re-enabled.

8.3.13 Temperature Sense Resistor Network (TS)

BQ51013B includes a ratiometric external temperature sense function. The temperature sense function has two

ratiometric thresholds which represent a hot and cold condition. An external temperature sensor is

recommended 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 (place the NTC resistor closest to where the user

would physically contact the end product).

Figure 8-12 allows for any NTC resistor to be used with the given VHOT and VCOLD thresholds.

NTC

TS/CTRL

20 lQ

VTSB

NTC

TS/CTRL

20 lQ

VTSB

Figure 8-12. NTC Circuit Options For Safe Operation of the Wireless Receiver Power Supply

BQ51013B

SLUSB62D – MARCH 2013 – REVISED SEPTEMBER 2020

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