ꢀ ꢁꢂ ꢃ ꢄ ꢅ ꢂ ꢆ ꢀꢁ ꢂꢃ ꢄꢅ ꢇ
SLUS553D − MAY 2003 − REVISED JULY 2005
ADAPTER SELECT SWITCH
ADAPTER
INPUT
SYSTEM
LOAD
(bq24702)
PWM
BATTERY
CHARGER
BATTERY
SELECT
SWITCH
BAT
ACDRV
(bq24702) 24
BATTERY
SELECTOR
CONTROL 23
BATDRV
UDG−00119
Figure 6. Selector Control Switches
Autonomous Selection Operation
Adapter voltage information is sensed at the ACDET pin via a resistor divider from the adapter input. The voltage
on the ACDET pin is compared to an internally fixed threshold. An ACDET voltage less than the set threshold
is considered as a loss of adapter power regardless of the actual voltage at the adapter input. Information
concerning the status of adapter power is fed back to the host controller through ACPRES. The presence of
adapter power is indicated by ACPRES being set high. A loss of adapter power is indicated by ACPRES going
low regardless of which power source is powering the system. During a loss of adapter power, the
bq24702/bq24703 obtains operating power from the battery through the body diode of the P-channel battery
select MOSFET. Under a loss of adapter power, ACPRES (normally high) goes low, if adapter power is selected
to power the system, the bq24702/bq24703 automatically switches over to battery power by commanding
ACDRV high and BATDRV low. During the switch transition period, battery power is supplied to the load via the
body diode of the battery select P-channel MOSFET. When adapter power is restored, the bq24702/bq24703
configures the selector switches according to the state of signals; ACSEL, and ACPRES. If the ACSEL pin is
left high when ac power is restored, the bq24702/bq24703 automatically switches back to ac power. To remain
on battery power after ac power is restored, the ACSEL pin must be brought low.
Conversely, if the battery is removed while the system is running on battery power and adapter power is present,
the bq24702/bq24703 automatically switches over to adapter power by commanding BATDRV high and
ACDRV low.
NOTE: For the bq24702 any fault condition that results in the selector MOSFET switches not
matching their programmed states is indicated by the ALARM pin momentarily going high. Refer
to Battery Depletion Detection Section for more information on the ALARM discrete.
When switching between the ac adapter and battery the internal logic monitors the voltage at pins ACDRV and
BATDRV to implement a break-before-make function, with typical dead time on the order of 150 nsec.
The turnon times for the external ac/battery switches can be increased to minimize inrush peak currents; that
can be accomplished by adding external resistors in series with the MOSFET gates(R18 and R26). Note,
however, that adding those resistors effectively disables the internal break-before-make function for
ac/battery-switches, as the MOSFET gate voltages can not be monitored directly. If external resistors are added
to increase the rise/fall times for battery/ac switches the break-before-make has to be implemented with discrete
external components, to avoid shoot-through currents between ac adapter and battery pack. This functionality
can be implemented by adding diodes (D2/D9) that bypass the external resistors when turning off the external
FETs.
23
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