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參數(shù)資料
| 型號： | AAT3682 |
| 廠商： | Advanced Analogic Technologies, Inc. |
| 英文描述： | Lithium-Ion/Polymer Linear Battery Charger |
| 中文描述： | 鋰離子線性電池充電器 |
| 文件頁數(shù)： | 14/18頁 |
| 文件大?。?/td> | 233K |
| 代理商： | AAT3682 |
AAT3682
Lithium-Ion/Polymer Linear Battery Charger
14
3682.2005.02.1.1
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 AAT3682 checks battery temperature before
starting the charge cycle, as well as during all stages
of charging. This is accomplished by monitoring the
voltage at the T
S
pin. Either a negative temperature
coefficient thermistor (NTC) or positive-temperature
coefficient thermistor (PTC) can be used because
the AAT3682 checks to see that the voltage at T
S
is
within a voltage window bounded by V
TS1
and V
TS2
.
Please see the following equations for specifying
resistors:
R
T1
and R
T2
for use with
NTC
Thermistor
R
T1
and R
T2
for use with
PTC
Thermistor
Where R
TC
is the thermistor's cold temperature
resistance and R
TH
is the thermistor's hot tempera-
ture resistance. See thermistor specifications for
additional information. To ensure there is no
dependence on the input supply changes, connect
divider between V
CC
and V
SS
. Disabling the tem-
perature-monitoring function is achieved by apply-
ing a voltage between V
TS1
and V
TS2
on the T
S
pin.
Capacitor Selection
Input Capacitor
In general, it is good design practice to place a
decoupling capacitor between the V
CC
and V
SS
pins. An input capacitor in the range of 1μF to 10μF
is recommended. If the source supply is unregu-
lated, it may be necessary to increase the capaci-
tance to keep the input voltage above the under-
voltage lockout threshold. If the AAT3682 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. Alarger input capacitor in this application will
minimize switching or power bounce effects when
the power supply is "hot plugged" in.
Output Capacitor
The AAT3682 does not need an output capacitor for
stability of the device itself. However, a capacitor
connected between BAT and V
SS
will control the
output voltage when the AAT3682 is powered up
when no battery is connected. The AAT3682 can
become unstable if a high impedance load is placed
across the BAT pin to V
SS
. Such a case is possible
with aging Li-Ion battery cells. As cells age through
repeated charge and discharge cycles, the internal
impedance can rise over time. A 10μF or larger out-
put capacitor will compensate for the adverse
effects of a high impedance load and assure device
stability over all operating conditions.
Power Dissipation
The voltage drop across the V
P
and BAT pins mul-
tiplied times the charge current is used to deter-
mine the internal power dissipation. The maximum
power dissipation occurs when the input voltage is
at a maximum and the battery voltage is at the min-
imum preconditioning voltage threshold. This
power is then multiplied times the package theta to
determine the maximum junction temperature. The
worst case power junction temperature is calculat-
ed as follows.
R
T1
= 3 (R
TC
-
R
TH
)
5 R
R
TC
TH
) - (7 R
TC
)
5 R
TH
R
TC
R
T2
= (2 R
R
T1
= 3 (R
TC
-
R
TH
)
5 R
R
TC
TC
) - (7 R
TH
)
5 R
TH
R
TC
R
T2
= (2 R
V
R
SENSE
0.2
2
0.1V
2
P = = = 50mW
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