AAT3687
Lithium-Ion/Polymer Battery Charger
10000
1000
100
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.
10
1
10
100
RSET (kΩ)
Figure 2: IFASTCHARGE vs. RSET
.
Protection Circuitry
Programmable Watchdog Timer
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 (VOVP). If an
over-voltage condition occurs, the AAT3687 charge
control will shut down the device until the voltage
on the BAT pin drops below VOVP. 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.
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.
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.
Mode
Time
Trickle Charge (TC)
Time Out
25 minutes (AAT3687
(and AAT3687-1 only)
3 hours
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.
Trickle Charge (TC) + Fast
Charge (CC) Time Out
Constant Voltage (VC)
Mode Time Out
3 hours
Table 2: Summary for a 0.1μF Ceramic
Capacitor Used for the Timing Capacitor.
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
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
3687.2006.10.1.7
13
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
