AAT3687 Datasheet, PDF (13/22 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3687
Lithium-Ion/Polymer Battery Charger
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 defec-
tive 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 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 cur-
rent 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 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
3687.2006.06.1.5
pin to ground. The CT pin should not be left float-
ing or un-terminated, as this will cause errors in the
internal timing control circuit.
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 physical-
ly located on the printed circuit board layout as
close as possible to the CT pin. Since the accura-
cy of the internal timer is dominated by the capaci-
tance value, a 10% tolerance or better ceramic
capacitor is recommended. Ceramic capacitor
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 battery charge voltage and is set by the
over-voltage protection 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 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 charg-
ing operation.
The AAT3687 checks battery temperature before
starting the charge cycle, as well as during all
stages of charging. This is accomplished by moni-
toring the voltage at the TS pin. This system is
intended for use with negative temperature coeffi-
cient thermistors (NTC) which are typically inte-
grated into the battery package. Most of the com-
monly used NTC thermistors in battery packs are
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