AAT3685 Datasheet, PDF (15/23 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3685
Lithium-Ion/Polymer Battery Charger
10000
1000
SETL
100
SETH
10
1
10
100
R_SET (kΩ)
1000
Figure 3: I_FASTCHARGE vs. R_SET.
Protection Circuitry
Over-Voltage Protection
An over-voltage event is defined as a condition
where the voltage on the BAT pin exceeds the
maximum battery charge voltage and is set by the
over-voltage protection threshold (V_OVP). If an
over-voltage condition occurs, the AAT3685 charge
control will shut down the device until voltage on
the BAT pin drops below the over-voltage protec-
tion threshold (V_OVP). The AAT3685 will resume
normal charging operation after the over-voltage
condition is removed. During an over-voltage
event, the STAT LEDs will report a system fault; the
actual fault condition may also be read via the
DATA pin signal.
Over-Temperature Shutdown
The AAT3685 has a thermal protection control cir-
cuit which will shut down charging functions should
the internal die temperature exceed the preset
thermal limit threshold.
Battery Temperature Fault Monitoring
In the event of a battery over-temperature condi-
tion, the charge control will turn off the internal pass
device and report a battery temperature fault on the
DATA pin function. The STAT LEDs will also dis-
play a system fault. After the system recovers
from a temperature fault, the device will resume
charging operation.
The AAT3685 checks battery temperature before
starting the charge cycle, as well as during all stages
of charging. This is accomplished by monitoring the
voltage at the TS pin. This system is intended for
use negative temperature coefficient (NTC) thermis-
tors which are typically integrated into the battery
package. Most commonly used NTC thermistors
used in battery packs are approximately 10kΩ at
room temperature (25°C). The TS pin has been
specifically designed to source 80μA of current to
the thermistor. The voltage on the TS pin that results
from the resistive load should stay within a window
from 335mV to 2.32V. If the battery becomes too hot
during charging due to an internal fault, the thermis-
tor will heat up and reduce in value, thus pulling the
TS pin voltage lower than the TS1 threshold, and the
AAT3685 will halt charging and signal the fault con-
dition. If the use of the TS pin function is not
required by the system, it should be terminated to
ground using a 10kΩ resistor. Alternatively, on the
AAT3685-1, the TS pin may be left open.
Battery Charge Status Indication
The AAT3685 indicates the status of the battery
under charge with two different systems. First, the
device has two status LED driver outputs. These
two LEDs can indicate simple functions such as no
battery charge activity, battery charging, charge
complete, and charge fault. The AAT3685 also
provides a bi-directional data reporting function so
that a system microcontroller may interrogate the
DATA pin and read any one of 14 system states.
Status Indicator Display
Simple system charging status may be displayed
using one or two LEDs in conjunction with the
STAT1 and STAT2 pins on the AAT3685. These
two pins are simple switches to connect the display
LED cathodes to ground. It is not necessary to use
both display LEDs if a user simply wants to have a
single lamp to show "charging" or "not charging."
This can be accomplished by just using the STAT1
pin and a single LED. Using two LEDs and both
STAT pins simply gives the user more information
for charging states. Refer to Table 2 for LED dis-
play definitions.
3685.2006.10.1.3
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