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AAT3681A

500mA USB Port or AC Adapter Lithium-Ion/Polymer Battery Charger

Analog Technology Inc 

AAT3681A

500mA USB Port or AC Adapter

Lithium-Ion/Polymer Battery Charger

Protection Circuitry

Over-Voltage Protection

An over-voltage event is defined as a condition

where the voltage on the BAT pin exceeds the

maximum battery charge voltage and is set by the

over-voltage protection threshold (VOVP). If an over-

voltage condition occurs, the AAT3681A charge

control will shut down the device until voltage on

the BAT pin drops below VOVP. The AAT3681A will

resume normal charging operation after the over-

voltage condition is removed.

Over-Temperature Shutdown

The AAT3681A has a thermal protection control cir-

cuit which will shut down charging functions should

the internal die temperature exceed the preset ther-

mal limit threshold. Once the internal die tempera-

ture falls below the thermal limit, normal operation

will resume the previous charging state.

Charge Status Output

The AAT3681A provides battery charge status via a

status pin. This pin is internally connected to an N-

channel open drain MOSFET, which can be used to

drive an external LED. The status pin can indicate

the following conditions:

Event Description

No battery charging activity

Battery charging via adapter

or USB port

Charging completed1

Status

OFF

ON

OFF

Table 2: LED Status Indicator.

The LED should be biased with as little current as

necessary to create reasonable illumination; there-

fore, a ballast resistor should be placed between

the LED cathode and the STAT pin. LED current

consumption will add to the overall thermal power

budget for the device package, hence it is good to

keep the LED drive current to a minimum. 2mA

should be sufficient to drive most low-cost green or

red LEDs. It is not recommended to exceed 8mA

for driving an individual status LED.

The required ballast resistor values can be esti-

mated using the following formulas:

Example:

R1=

(VADP - VF(LED))

ILED

R1 =

(5.5V - 2.0V)

2mA

= 1.75kΩ

Note: Red LED forward voltage (VF) is typically

2.0V @ 2mA.

Thermal Considerations

The AAT3681A is offered in a SC70JW-8 package

which can provide up to 687mW of power dissipa-

tion when it is properly bonded to a printed circuit

board and has a maximum thermal resistance of

160°C/W. Many considerations should be taken

into account when designing the printed circuit

board layout, as well as the placement of the

charger IC package in proximity to other heat gen-

erating devices in a given application design. The

ambient temperature around the charger IC will

also have an effect on the thermal limits of a bat-

tery charging application. The maximum limits that

can be expected for a given ambient condition can

be estimated by the following discussion.

First, the maximum power dissipation for a given

situation should be calculated:

PD(MAX) =

(TJ(MAX) - TA)

θJA

Where:

PD(MAX) = Maximum Power Dissipation (W)

θJA

= Package Thermal Resistance (°C/W)

TJ(MAX) = Maximum Device Junction Temperature

(°C) [135°C]

TA

= Ambient Temperature (°C)

1. AAT3681A-3 does not terminate charging. However, the status pin turns off when IBAT ≤ ITERM and turns back on when VBAT ≤

VBAT_EOC - 0.1V.

12

3681A.2007.10.1.2

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