BQ24721 Datasheet, PDF (26/48 Pages) Texas Instruments – ADVANCED MULTI-CHEMISTRY AND MULTI-CELL SYNCHRONOUS SWITCH-MODE CHARGER AND SYSTEM POWER SELECTOR

bq24721
SLUS683 – NOVEMBER 2005
www.ti.com
Power Up
When the adapter is not detected, the REGN output voltage is 4.6 V and the VREF5 LDO regulator is off, to
lower the power consumption from the battery. The VREF5 LDO is pulled-down to AGND when the adapter is
not detected. The REGN LDO regulator begins to regulate at 4.6 V when the input VCC voltage is greater than 6
V. The REGN output voltage is then 6 V when the adapter is detected, and the VCC is greater than 7 V. If
adapter is detected, but VCC is less than 7 V, then the REGN is in dropout, and REGN output voltage depends
on the VCC voltage and REGN load current. The VREF5 LDO is allowed to turn-on and regulate to 5 V, 5 ms
after REGN is 6 V and the adapter is detected. There is a 500-ms delay from the time the adapter is detected,
until the ACFET from the system power selector is allowed to turn-on, and until the charger is allowed to turn-on.
The battery continues to be connected to the system during this 500-ms delay.
Adapter Detect
The adapter detect threshold is programmed by an external voltage divider resistor from the adapter to the
ACDET pin. The internal ACDET comparator has a 1.2-V rising-edge threshold and a 15-mV falling-edge
hysteresis. The adapter detect value should typically be programmed to a value greater than the maximum
battery voltage, and lower than the minimum allowed adapter voltage. The ACDETECT divider should be placed
before the BYPASS FET in order to sense the true adapter input voltage whether the BYPASS is on or off.
The VREF5 LDO output can also be used to indicate when the adapter is detected, for both the bq24721 and the
bq24721, since the VREF5 LDO only comes up when the adapter is detected.
System Power Selector
The bq24721 can automatically switch between adapter power or battery power to the system load. The battery
is connected to the system when there is no adapter detected. The adapter is connected to the system when the
adapter is detected. An automatic break-before-make logic prevents shoot-through currents when the selector
switches.
When no adapter is detected the ACDRV pin is pulled to the PVCC pin to keep the external ACFET p-channel
power MOSFET off, disconnecting the adapter from system. The break-before-make logic waits until the ACFET
is off and the System to battery voltage comparator indicates the system voltage is within 250 mV of the Battery
(SYS voltage falling-edge, with a 50-mV hysteresis SYS voltage rising-edge). This prevents shoot-through
currents or large discharge currents from going into the battery. The BATDRV pin is then set to the SYS pin
voltage minus 6 V by an internal regulator in order to turn on the external BATFET p-channel power MOSFET,
connecting the battery pack to the system.
When adapter is detected there is a 500-ms delay; then, the BATDRV is set to the SYS pin voltage to turn off the
external BATFET p-channel power MOSFET, in order to disconnect the battery. The break-before-make logic
waits until the BATFET is off to prevent shoot-through currents. The ACDRV pin is then set to the PVCC pin
voltage minus 6 V by an internal regulator in order to turn on the external ACFET p-channel power MOSFET,
connecting the adapter to the system.
The host can override the adapter to system connection when adapter is present, by setting the charger into
Learn Mode. The host can then induce a learn cycle in which the battery is allowed to discharge. A learn cycle is
used to recalibrate the fuel gauge for Ni-based batteries, or for clearing the memory effect of a Ni-cd battery.
After discharging the battery, Learn Mode can be disabled allowing the adapter to be reconnected to the system,
then resuming a normal charge cycle.
Whenever the battery is connected to the system (whether in learn cycle with adapter present, or no adapter
present), there is a Low Battery comparator that monitors the battery voltage, and alerts the host and charge
controller that the battery has been depleted. The BAT_DEP threshold can be programmed through the Charger
Mode control (0x12) register through bits b9-b11. The three bits can program the threshold between 2.2-V per
cell to 2.9-V per cell, in 100-mV increments. The number of cells is determined by the programmed battery
regulation voltage (0x15) register. If the battery voltage falls below the BAT_DEP threshold, then the battery is
disconnected from the system and the adapter is connected to the system. The Battery Voltage Low bit (b11) is
set in the Charger Status (0x13) register, and the ALARM pin is pulled low to alert the host. There is a 1 second
deglitch time to prevent false triggering.
The Charger Status (0x13) register bits b6 and b7 also always indicate whether the battery is connected to the
system (b6), or the adapter is connected to the system (b7). An alarm is triggered whenever these states
change.
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