ꢀ ꢁ ꢂ ꢃꢄ ꢅ ꢂꢆ ꢀ ꢁ ꢂ ꢃꢄꢅ ꢇ
SLUS553D − MAY 2003 − REVISED JULY 2005
Smart Learn Cycles When Adapter Power Is Present
Smart learn cycles can be conducted when adapter power is present by asserting and maintaining the ACSEL
pin low. The adapter power can be reselected at the end of the learn cycle by a setting ACSEL to a logic high,
provided that adapter power is present. Battery charging is suspended while selected as the system power
source.
NOTE: On the bq24703 the ac adapter is switched to the load when the battery voltage reaches
the battery depleted threshold; it can be used when the learn cycle does not require the battery
voltage to go below the battery depleted threshold. If the learn cycle algorithm requires the battery
voltage to go lower than the battery depleted voltage, the bq24702 should be used, as it does not
switch the ac adapter to load upon battery depleted detection.
System Break Before Make Function
When selecting the battery as the system primary power source, the adapter power select MOSFET turns off,
in a break-before-make fashion, before the battery select MOSFET turns on. To ensure that this happens under
all load conditions, the system voltage (load voltage) can be monitored through a resistor divider on the VS pin.
This function provides protection against switching over to battery power if the adapter selector switch were
shorted and adapter power present. Setting the VS resistive divider gain with the same gain selected for the
BATP resistive divider assures the battery switch is turned on only when the system voltage is equal or less than
the battery voltage. This function can be eliminated by grounding the VS pin.
The ACDET function senses the adapter voltage via a resistive divider (refer to application circuit).The divider
can be connected either to the anode of the input blocking diode (directly to the adapter supply) or to the cathode
of the input blocking diode (bq24702/3 VCC pin). When the divider is connected to the adapter supply, the
adapter power removal is immediately identified and the sleep mode is entered, disabling the
break-before-make function for system voltage (see section for system power switching) and coupling system
voltage to the battery line. In normal operation with a battery present, the battery low impedance prevents any
over-voltage conditions. However, if a pack is not present or the pack is open, the battery line voltage has a
transient equal to the adapter voltage. The bq24703 SRP/SRN pins are designed to withstand this over-voltage
condition, but avoid connection to the battery line of any external devices that are not rated to withstand the
adapter voltage.
Connecting the ACDET resistive divider input to the VCC node keeps the system break-before-make function
enabled until the voltage at pin VS is lower than the voltage at pin BATP. However, note that when using this
topology the VCC pin voltage can be held by capacitive loads at either the VCC or system (ac switch is on) nodes
when the ac adapter is removed. As the ACDET divider is connected to the VCC line there is a time delay from
ac adapter removal to ac adapter removal detection by the IC. This time is dependent on load conditions and
capacitive load values at VCC and system lines.
Battery Depletion Detection
The bq24702/bq24703 provides the host controller with a battery depletion discrete, the ALARM pin, to alert
the host when a depleted battery condition occurs. The battery depletion level is set by the voltage applied to
the BATDEP pin through a voltage divider network. The ALARM output asserts high and remains high as long
as the battery deplete condition exists, regardless of the power source selected.
For the bq24702, the host controller must take appropriate action during a battery deplete condition to select
the proper power source. The bq24702 remains on the selected power source. The bq24703, however,
automatically reverts over to adapter power, provided the adapter is present, during a deep discharge state. The
battery is considered as being in a deep discharge state when the battery voltage is less than (0.8 × depleted
level).
Feature sets for the bq24702 and bq24703 are detailed in Table 1.
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