AAT3681AIJS-4.2-T1 データシートの表示（PDF） - Analog Technology Inc

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AAT3681AIJS-4.2-T1

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

Figure 3 shows the relationship of maximum power

dissipation and ambient temperature of AAT3681A.

1000

800

600

400

200

0

0

20

40

60

80

100

TA (°C)

Figure 3: Maximum Power Dissipation.

Next, the power dissipation can be calculated by

the following equation:

PD = [(VIN - VBAT) · ICH + (VIN · IOP)]

Where:

PD = Total Power Dissipation by the Device

VIN = Input Voltage

VBAT = Battery Voltage as Seen at the BAT Pin

ICH = Constant Charge Current Programmed for

the Application

IOP = Quiescent Current Consumed by the

Charger IC for Normal Operation [0.5mA]

By substitution, we can derive the maximum

charge current before reaching the thermal limit

condition (thermal cycling). The maximum charge

current is the key factor when designing battery

charger applications.

ICH(MAX) =

(PD(MAX) - VIN · IOP)

VIN - VBAT

ICH(MAX) =

(TJ(MAX) -

θJA

TA)

-

VIN

·

IOP

VIN - VBAT

In general, the worst condition is the greatest volt-

age drop across the charger IC, when battery volt-

age is charged up to the preconditioning voltage

threshold. Figure 4 shows the maximum charge

current in different ambient temperatures without

PCB thermal enhancements.

500

450

400

350

TA = 25°C

300

TA = 45°C

250

200

150

100

50

TA = 85°C

TA = 60°C

0

4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 6.75 7

VIN (V)

Figure 4: Maximum Charging Current Before

Thermal Cycling Becomes Active.

Capacitor Selection

Input Capacitor

In general, it is good design practice to place a

decoupling capacitor between the ADP pin and

GND. An input capacitor in the range of 1µF to 22µF

is recommended. If the source supply is unregulated,

it may be necessary to increase the capacitance to

keep the input voltage above the under-voltage lock-

out threshold during device enable and when battery

charging is initiated. If the AAT3681A adapter input is

to be used in a system with an external power sup-

ply source, such as a typical AC-to-DC wall adapter,

then a CIN capacitor in the range of 10µF should be

used. A larger input capacitor in this application will

minimize switching or power transient effects when

the power supply is "hot plugged" in.

Output Capacitor

The AAT3681A only requires a 1µF ceramic capac-

itor on the BAT pin to maintain circuit stability. This

value should be increased to 10µF or more if the

battery connection is made any distance from the

charger output. If the AAT3681A is to be used in

applications where the battery can be removed

from the charger, such as with desktop charging

cradles, an output capacitor greater than 10µF may

be required to prevent the device from cycling on

and off when no battery is present.

3681A.2007.10.1.2

13

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