AAT3682(2005) 查看數據表（PDF） - Advanced Analogic Technologies

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AAT3682
(Rev.:2005)

Lithium-Ion/Polymer Linear Battery Charger

Advanced Analogic Technologies 

AAT3682

Lithium-Ion/Polymer Linear Battery Charger

Calculate the worst case power dissipated in the

sense resistor according to the following equation:

P=

VCS2

RSENSE

=

0.1V2

0.2Ω

=

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 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 TS 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 TS is

within a voltage window bounded by VTS1 and VTS2.

Please see the following equations for specifying

resistors:

RT1 and RT2 for use with NTC Thermistor

RT1

=

5 • RTH • RTC

3 • (RTC - RTH)

RT2

=

5 • RTH • RTC

(2 • RTC) - (7 • RTH)

RT1 and RT2 for use with PTC Thermistor

RT1

=

5 • RTH • RTC

3 • (RTC - RTH)

RT2

=

5 • RTH • RTC

(2 • RTH) - (7 • RTC)

Where RTC is the thermistor's cold temperature

resistance and RTH 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 VCC and VSS. Disabling the tem-

14

perature-monitoring function is achieved by apply-

ing a voltage between VTS1 and VTS2 on the TS pin.

Capacitor Selection

Input Capacitor

In general, it is good design practice to place a

decoupling capacitor between the VCC and VSS

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 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.

Output Capacitor

The AAT3682 does not need an output capacitor for

stability of the device itself. However, a capacitor

connected between BAT and VSS 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 VSS. 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 VP 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.

3682.2005.02.1.1

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