AAT3680 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

AAT3680
Lithium-Ion Linear Battery Charge Controller
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_SENSE, programs the charge
current according to the following equation:
R_SENSE =
(V_P -V_CSI)
I_CHARGE(REG)
Where I_CHARGE(REG) is the desired typical charge cur-
rent during constant-current charge mode. V_P-V_CSI
is the voltage across R_SENSE, shown in the Electrical
Characteristic table as V_CS. 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:
P
=
(V_CS)²
R_SENSE
P
=
(0.1)²
0.2
P = 50mW
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
be used because the AAT3680 checks to see that
the voltage at TS is within a voltage window bound-
ed by V_TS1 and V_TS2. Please see equations below
for specifying resistors:
R_T1 and R_T2 for use with NTC Thermistor
R_T1 =
5 · R_TH · R_TC
3 · (R_TC - R_TH)
R_T2 =
5 · R_TH · R_TC
(2 · R_TC) - (7 · R_TH)
R_T1 and R_T2 for use with PTC Thermistor
R_T1 =
5 · R_TH · R_TC
3 · (R_TC - R_TH)
R_T2 =
5 · R_TH · R_TC
(2 · R_TH) - (7 · R_TC)
Where R_TC is the thermistor's cold temperature
resistance, and R_TH 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 V_P
and V_SS. Disabling the temperature-monitoring
function is achieved by applying a voltage between
V_TS1 and V_TS2 on the TS pin.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between V_P and V_SS 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 C_IN 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.
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