bq51010
bq51011
bq51013
SLVSAT9B –APRIL 2011–REVISED AUGUST 2011
www.ti.com
AC1
AC2
22nF
22nF
COMM1
COMM2
COMM_DRIVE
Figure 26. Capacitive Load Modulation
Synchronous Rectification
The bq5101x 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 startup 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 UVLO, half synchronous mode will be enabled until the load current
surpasses 250mA. Above 250mA the synchronous rectifier will stay enabled until the load current drops back
below 250mA where half synchronous mode will be enabled instead.
Rectifier Tracking Mode (Fold-Back)
The bq51011 is a 5V power supply intended to run efficiently in current limit. In order to optimize the efficiency
and power dissipation, the rectifier must track the output voltage within 250mV. This feature is termed
VRECT-TRACK where the bq51011 monitors the status of the programmed current limit and the output voltage value.
When the output current breaches the current limit of the power supply the controller sets the rectifier target
voltage to the output voltage plus 250mV. This feature is illustrated in Figure 22. When the output current is
equal to the current limit and the output voltage is transitioned from 4.8V to 3.5V the rectifier voltage will follow
the transition. This is possible via the WPC system control loop where the bq51011 communicates to the Tx to
adjust the operating point. This feature ensures that the internal LDO is always running near dropout for
optimized efficiency when the output current is equal to the current limit of the power supply
Communication Current Limit (Comm. ILIM
)
The bq51011 employs a 400mA current limit during the time it takes to send a communication packet to the Tx.
This feature adds robustness to communication link between the Tx and Rx when the rectifier is in fold-back
mode. Communication can be compromised while in fold-back mode because of less headroom (gain) across the
internal LDO. When the current limit is reduced at a fixed operating frequency, the rectifier voltage increases (see
Figure 22 where the output current reduces from the power supply current limit). This will increase the headroom
across the LDO adding more gain between the output and the rectifier; therefore, increasing immunity to
communication failure.
18
Submit Documentation Feedback
Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): bq51010 bq51011 bq51013
TI [ TEXAS INSTRUMENTS ]
