AAT3680 Advanced Analogic Technologies, AAT3680 Datasheet - Page 12

• Manufacturer Part Number: AAT3680
• Description: Lithium-Ion Linear Battery Charge Controller
• Manufacturer: Advanced Analogic Technologies

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, R
current according to the following equation:
Where I
rent during constant-current charge mode. V
is the voltage across R
Characteristic table as V
500mA charge current during fast-charge, a 200m
value resistor should be selected. Calculate the
worst case power dissipated in the sense resistor
according to the following equation:
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.
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
CHARGE(REG)
R
SENSE
is the desired typical charge cur-
P =
P =
P = 50mW
=
SENSE
SENSE
CS
R
(0.1)
I
(V
CHARGE(REG)
0.2
(V
SENSE
. To program a nominal
, shown in the Electrical
CS
, programs the charge
P
2
)
-V
2
CSI
)
Lithium-Ion Linear Battery Charge Controller
P
-V
CSI
be used because the AAT3680 checks to see that
the voltage at TS is within a voltage window bound-
ed by V
for specifying resistors:
R
R
Where R
resistance, and R
ature resistance. See thermistor specifications for
info. To ensure there is no dependence on the
input supply changes, connect divider between V
and V
function is achieved by applying a voltage between
V
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between V
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 C
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.
TS1
T1
T1
and R
and R
and V
SS
TS1
.
TC
T2
T2
TS2
and V
R
R
R
R
Disabling the temperature-monitoring
for use with NTC Thermistor
for use with PTC Thermistor
is the thermistor's cold temperature
T1
T2
T1
T2
on the TS pin.
=
=
=
=
TS2
TH
(2 · R
(2 · R
. Please see equations below
is the thermistor's hot temper-
3 · (R
3 · (R
5 · R
5 · R
5 · R
5 · R
TC
TH
IN
) - (7 · R
) - (7 · R
TC
TC
TH
TH
TH
TH
capacitor in the range of
· R
- R
· R
· R
- R
· R
P
TC
TH
TC
TC
TH
TC
AAT3680
and V
)
)
TH
TC
)
)
3680.2003.4.0.91
SS
pins. An
P