AAT3680 Datasheet, PDF(13/18 Page) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

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AAT3680 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – Lithium-Ion Linear Battery Charge Controller

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AAT3680

Lithium-Ion Linear Battery Charge Controller

Output Capacitor

The AAT3680 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 AAT3680 is powered up

when no battery is connected. The AAT3680 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 10ÂµF or larger out-

put capacitor will compensate for the adverse

effects of a high impedance load and assure device

stability over all operating conditions.

Operation Under No-Load

Under no-load conditions, that is when the

AAT3680 is powered with no battery connected

between the BAT pin and VSS, the output capacitor

is charged up very quickly by the trickle charge

control circuit to the BAT pin until the output reach-

es the recharge threshold (VRCH). At this point the

AAT3680 will drop into the sleep mode. The output

capacitor will discharge slowly by the capacitor's

own internal leakage until the voltage seen at the

BAT pin drops below the VRCH threshold. This

100mV cycle will continue at approximately 3Hz

with a 0.1ÂµF capacitor connected. A larger capaci-

tor value will produce a slower voltage cycle. This

operation mode can be observed by viewing the

STAT LED blinking on and off at the rate estab-

lished by the COUT value.

For Desk Top Charger applications where it might

not be desirable to have a "charger ready" blinking

LED, a large COUT capacitor in the range of 100ÂµF

or more would prevent the operation of this mode.

Reverse Current Blocking Diode

Bi-Polar Circuit Application

When using the AAT3680 with a PNP transistor, a

reverse-blocking diode is not required because

there is no current path from BAT to VP. However,

it is advisable to still place a blocking diode

between the bipolar transistor collector and the

BAT pin connection to the circuit output. In the

event where the input supply is interrupted or

removed during the constant current or constant

voltage phases of the charging cycle, the battery

under charge will discharge through the circuit

pass transistor rendering it impossible to turn off. If

the circuit is unable to turn off, the reverse leakage

will eventually discharge the battery. A blocking

diode will prevent this undesirable effect.

MOSFET Circuit Application

An reverse-blocking diode is generally required for

the circuit shown in Figure 5. For this application,

the blocking diode gives the system protection

from a shorted input, when the AAT3680 is used

with a P-Channel MOSFET. If there is no other pro-

tection in the system, a shorted input could dis-

charge the battery through the body diode of the

pass MOSFET. If a reverse-blocking diode is

added to the system, a device should be chosen

which can withstand the maximum constant- cur-

rent charge current at the maximum system ambi-

ent temperature.

Diode Selection

Typically, a Schottky diode is used in reverse cur-

rent blocking applications with the AAT3680. Other

lower cost rectifier type diodes may also be used to

save cost if sufficient input power supply head

room is available.

The blocking diode selection should based on mer-

its of the device forward voltage (VF), current rat-

ing, input supply level versus the maximum battery

charge voltage and cost.

First, one must determine what the minimum diode

forward voltage drop must be. Refer to the follow-

ing equation where:

VIN(MIN) = Minimum input supply level

VBAT(MAX) = Maximum battery charge voltage

required

VF(TRAN) = Pass transistor forward voltage drop

VF(DIODE) = Blocking diode forward voltage

VIN(MIN) = VBAT(MAX) + VF(TRAN) + VF(DIODE)

Based on the maximum constant current charge

level set for the system, the next step is to determine

the minimum current rating and power handling

capacity for the blocking diode. The constant cur-

rent charge level itself will dictate what the minimum

3680.2003.4.0.91

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