AAT3687_08 Datasheet, PDF (13/20 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

BatteryManager^TM
10000
1000
100
10
1
10
100
R_SET (kΩ)
Figure 2: I_FASTCHARGE vs. R_SET.
Protection Circuitry
Programmable Watchdog Timer
The AAT3687 contains a watchdog timing circuit to shut
down charging functions in the event of a defective battery
cell not accepting a charge over a preset period of time.
Typically, a 0.1µF ceramic capacitor is connected between
the CT pin and ground. When a 0.1µF ceramic capacitor
is used, the device will time out a shutdown condition if
the trickle charge mode exceeds 25 minutes (AAT3687
and AAT3687-1 only) and a combined trickle charge plus
fast charge mode of 3 hours. When the device transitions
to the constant voltage mode, the timing counter is reset
and will time out after an additional 3 hours if the charge
current does not drop to the charge termination level.
Mode
Trickle Charge (TC) Time Out
Trickle Charge (TC) + Fast
Charge (CC) Time Out
Constant Voltage (VC) Mode
Time Out
Time
25 minutes (AAT3687
and AAT3687-1 only)
3 hours
3 hours
Table 2: Summary for a 0.1µF Ceramic Capacitor
Used for the Timing Capacitor.
The CT pin is driven by a constant current source and will
provide a linear response to increases in the timing
capacitor value. Thus, if the timing capacitor were to be
doubled from the nominal 0.1µF value, the time-out
periods would be doubled.
If the programmable watchdog timer function is not
needed, it can be disabled by terminating the CT pin to
ground. The CT pin should not be left floating or un-
terminated, as this will cause errors in the internal tim-
ing control circuit.
PRODUCT DATASHEET
AAT3687
Lithium-Ion/Polymer Battery Charger
The constant current provided to charge the timing
capacitor is very small, and this pin is susceptible to
noise and changes in capacitance value. Therefore, the
timing capacitor should be physically located on the
printed circuit board layout as close as possible to the CT
pin. Since the accuracy of the internal timer is domi-
nated by the capacitance value, a 10% tolerance or bet-
ter ceramic capacitor is recommended. Ceramic capaci-
tor materials, such as X7R and X5R types, are a good
choice for this application.
Over-Voltage Protection
An over-voltage event is defined as a condition where
the voltage on the BAT pin exceeds the maximum bat-
tery charge voltage and is set by the over-voltage pro-
tection threshold (V_OVP). If an over-voltage condition
occurs, the AAT3687 charge control will shut down the
device until the voltage on the BAT pin drops below V_OVP.
The AAT3687 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, and the actual fault condition can be read via the
DATA pin signal.
Over-Temperature Shutdown
The AAT3687 has a thermal protection control circuit
which will shut down charging functions should the inter-
nal die temperature exceed the preset thermal limit
threshold.
Battery Temperature Fault Monitoring
In the event of a battery over-temperature condition, the
charge control will turn off the internal pass device and
report a battery temperature fault on the DATA pin func-
tion. The STAT LEDs will also display a system fault.
After the system recovers from a temperature fault, the
device will resume charging operation.
The AAT3687 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 with negative tem-
perature coefficient thermistors (NTC) which are typically
integrated into the battery package. Most of the com-
monly used NTC thermistors in battery packs are approx-
imately 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 resulting
from the resistive load should stay within a window of
335mV to 2.32V. If the battery becomes too hot during
charging due to an internal fault or excessive fast charge
3687.2008.04.1.12
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