BQ24721 Datasheet, PDF (27/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR

bq24721
www.ti.com
SLUS683 – NOVEMBER 2005
Asymmetrical gate drives (100 Ω turn-off; 10 kΩ turn-on) for the ACDRV and BATDRV drivers provide fast
turn-off and slow turn-on of the ACFET and BATFET to help the break-before-make logic and to allow a soft-start
at turn-on of either FET. The soft-start time can be further increased by putting a capacitor from gate to source of
the p-channel power MOSFETs.
Input Overcurrent Protection (ACOC)
For solutions using the selector functions, an input overcurrent protection function (ACOC) is provided which
disconnects the ACFET by turning off the ACDRV pin, whenever the sensed input current exceeds the
programmed ACOC threshold. The ACOC threshold is programmed through the SMBus Charge Mode (0x12)
control register, bits b6, b7, b8. The ACOC function is automatically disabled upon power-on-reset. The host
needs to enable it by setting the ACOC bit (b6) HI. The ACOC threshold is set by the SET_ACOC bits (b7, b8),
to thresholds of 130%, 150%, 170%, or 190% of the input current (DPM) regulation limit threshold from the Input
Current (0x3F) register.
The ACFET turns off when the sensed current exceeds the threshold after a 200-µs deglitch time. The ACFET
automatically turns on after 2 ms, to limit the on-time duty cycle, and limit the power dissipation on the ACFET.
Care must be taken to ensure system load power does not exceed the power-up allowable power when the
ACOC function is used.
The function is not intended for system short-circuits, as usually the adapter self protects. Instead, the function is
intended for long-term overcurrent protection of the selector power devices, and to limit start-up peak current.
Bypass FET
The BYPASS pin is used to control an input FET that is off to prevent reverse discharge from the battery to the
adapter, and is on during input current draw to the system or battery, to minimize the power dissipation, as
compared to using a Schottky diode. If no adapter is detected, the BYPASS FET is off, by setting the BYPASS
pin to the PVCC pin. When the adapter is detected there is a 500-ms delay, then an ACP-to-BAT voltage
comparator is used to control the BYPASS pin. The BYPASS driver is set to the PVCC pin voltage when the
adapter voltage (ACP pin) is not more than 250 mV (ACP voltage falling-edge) above the battery voltage (BAT
pin), in order to turn off the external BYPASS p-channel power MOSFET. There is a 50 mV (ACP voltage
rising-edge) hysteresis, to protect from noise and prevent chatter. When adapter is detected and the ACP pin
voltage is greater than 300 mV above the BAT pin voltage, the BYPASS pin voltage is set to PVCC pin voltage
minus 6 V, in order to turn on the external BYPASS p-channel power MOSFET.
The ACP-to-BAT comparator also prevents the battery voltage from holding-up the ACDET sensed value and
falsely detecting ACDET when the adapter is removed, this prevents the system power selector from getting
stuck in an adapter always detected state. When ACP gets near to BAT, the external BYPASS p-channel power
MOSFET is turned off. This isolates the ACDET network from the battery, and allows the adapter input node to
discharge to PGND.
The BYPASS driver has a symmetrical gate drive of 1 kΩ turn-on and turn-off and does not need to be slowed
down.
Enabling Charge
Charge can only be enabled 500 ms after adapter is detected. To initiate charge the CHGEN pin must be low,
and the Charger Mode (0x12) register END CHARGE bit (b0) must be set LO. The power-on-reset default for the
END CHARGE bit is HI, disabling charge.
The Charger Status (0x13) register NOT READY TO CHARGE bit (b0) indicates whether the charger is ready to
charge. A HI indicates the charger is not ready to charge, while a LO indicates the charger is ready to charge.
The NOT READY TO CHARGE bit (b0) must be LO in order for the charger to be enabled. The conditions that
make the charger not ready to charge are: adapter not detected, 500-ms delay after adapter detected not over,
REGN voltage not up, or VREF5 voltage not up.
The Charger Status (0x13) register CHARGER NOT ON bit (b1) indicates whether the charger is not on (HI), or
the charger is on (LO). For charger to be on, the CHGEN pin must be low, the Charger Mode (0x13) register
END CHARGE bit (b0) must be LO, the, IC junction temperature must be below the TSHUT threshold,
overcurrent detected, or Thermistor Sense (TS) indicates battery pack is out of programmed permissible charge
temperature range (bq24721 only).
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