AAT3687 Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – Lithium-Ion/Polymer Battery Charger

AAT3687
Lithium-Ion/Polymer Battery Charger
approximately 10kΩ at room temperature (25°C).
The TS pin has been specifically designed to
source 80μA of current to the thermistor. The volt-
age 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 current,
the thermistor will heat up and reduce in value,
pulling the TS pin voltage lower than the TS1
threshold, and the AAT3687 will signal the fault
condition.
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 AAT3687-1
and AAT3687-2, the TS pin may be left open.
Battery Charge Status Indication
The AAT3687 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 AAT3687 also
provides a bi-directional data reporting function so
that a system microcontroller can interrogate the
DATA pin and read any one of 12 system states.
Status Indicator Display
Simple system charging status states can be dis-
played using one or two LEDs in conjunction with
the STAT1 and STAT2 pins on the AAT3687.
These two pins are simple open drain switches
used to connect the 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 "charg-
ing" or "not charging."
This can be accomplished by using the STAT1 pin
and a single LED. Using two LEDs and both STAT
pins simply gives the user more information to the
charging states. Refer to Table 3 for LED display
definitions. The LED anodes should be connected
to V_ADP.
The LEDs should be biased with as little current as
necessary to create reasonable illumination; there-
fore, a ballast resistor should be placed between
the LED cathodes and the STAT1/2 pins. LED cur-
rent consumption will add to the overall thermal
power budget for the device package, hence it is
good to keep the LED drive current to a minimum.
2mA should be sufficient to drive most low-cost
green or red LEDs. It is not recommended to
exceed 8mA for driving an individual status LED.
The required ballast resistor values can be esti-
mated using the following formulas:
Eq. 2:
R = _B(STAT1/2)
(V_ADP - V_F(LED))
I_LED(STAT1/2)
Example:
Eq. 3:
R_B(STAT1) =
(5.5V - 2.0V)
2mA
=
1.75kΩ
Note: Red LED forward voltage (V_F) is typically
2.0V @ 2mA
Event Description
Charge Disabled or Low Supply
Charge Enabled Without Battery
Battery Charging
Charge Completed
Fault
STAT1
Off
Flash¹
On
Off
On
Table 3: LED Status Indicator.
STAT2
Off
Flash¹
Off
On
On
1. Flashing rate depends on output capacitance.
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3687.2006.10.1.7