AAT3680
Lithium-Ion Linear Battery Charge Controller
be used because the AAT3680 checks to see that
the voltage at TS is within a voltage window bound-
Choosing a Sense Resistor
The charging rate recommended by Lithium Ion
cell vendors is normally 1C, with a 2C absolute
maximum rating. Charging at the highest recom-
mended rate offers the advantage of shortened
charging time without decreasing the battery's lifes-
pan. This means that the suggested fast charge
rate for a 500mAH battery pack is 500mA. The cur-
rent sense resistor, RSENSE, programs the charge
current according to the following equation:
ed by VTS1 and VTS2. Please see equations below
for specifying resistors:
RT1 and RT2 for use with NTC Thermistor
5 · RTH · RTC
RT1
RT2
=
=
3 · (RTC - RTH)
5 · RTH · RTC
(2 · RTC) - (7 · RTH)
R
T1 and RT2 for use with PTC Thermistor
(VP -VCSI
ICHARGE(REG)
)
RSENSE
=
5 · RTH · RTC
RT1
RT2
=
=
3 · (RTC - RTH)
Where ICHARGE(REG) is the desired typical charge cur-
rent during constant-current charge mode. VP-VCSI
is the voltage across RSENSE, shown in the Electrical
Characteristic table as VCS. To program a nominal
500mA charge current during fast-charge, a 200mΩ
value resistor should be selected. Calculate the
worst case power dissipated in the sense resistor
according to the following equation:
5 · RTH · RTC
(2 · RTH) - (7 · RTC)
Where RTC is the thermistor's cold temperature
resistance, and RTH is the thermistor's hot temper-
ature resistance. See thermistor specifications for
info. To ensure there is no dependence on the
input supply changes, connect divider between VP
and VSS. Disabling the temperature-monitoring
function is achieved by applying a voltage between
VTS1 and VTS2 on the TS pin.
2
(VCS)
P =
RSENSE
2
(0.1)
P =
0.2
P = 50mW
Capacitor Selection
Input Capacitor
A 500mW LRC type sense resistor from IRC is
adequate for this purpose. Higher value sense
resistors can be used, decreasing the power dissi-
pated in the sense resistor and pass transistor.
The drawback of higher value sense resistors is
that the charge cycle time is increased, so tradeoffs
should be considered when optimizing the design.
In general, it is good design practice to place a
decoupling capacitor between VP and VSS pins. An
input capacitor in the range of 0.1µF to 4.7µF is rec-
ommended. If the source supply is unregulated, it
may be necessary to increase the capacitance to
keep the input voltage above the undervoltage lock-
out threshold.
If the AAT3680 is to be used in a system with an
external power supply source, such as a typical AC to
DC wall adaptor, then a CIN capacitor in the range of
10µF should be used. A larger input capacitor in this
application will minimize switching or power bounce
effects when the power supply is "hot plugged" in.
Thermistor
The AAT3680 checks battery temperature before
starting the charge cycle as well as during all
stages of charging. This is accomplished by mon-
itoring the voltage at the TS pin. Either a negative-
temperature coefficient thermistor (NTC) or posi-
tive-temperature coefficient thermistor (PTC) can
12
3680.2003.4.0.91
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