AAT3697 데이터 시트보기 (PDF) - Advanced Analogic Technologies

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AAT3697

2A Lithium-Ion/Polymer Battery Charger

Advanced Analogic Technologies 

BatteryManagerTM

Thermal Considerations

The AAT3697 is offered in a 3x3mm TDFN package which

can provide up to 2.0W of power dissipation when it is

properly bonded to a printed circuit board and has a

maximum thermal resistance of 50°C/W. Many consider-

ations 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 ambi-

ent temperature around the charger IC will also have an

effect on the thermal limits of a battery charging applica-

tion. The maximum limits that can be expected for a

given ambient condition can be estimated by the follow-

ing discussion.

First, the maximum power dissipation for a given situa-

tion should be calculated:

Eq. 7: PD = [(VIN - VBAT) · ICC + (VIN · IOP)]

Where:

PD = Total Power Dissipation by the Device

VIN = Input Voltage Amplitude, VADP

VBAT = Battery Voltage as Seen at the BAT Pin

ICC = Maximum Constant Fast Charge Current Programmed

for the Application

IOP = Quiescent Current Consumed by the Charger IC for

Normal Operation

Next, the maximum operating ambient temperature for

a given application can be estimated based on the ther-

mal resistance of the 3x3mm TDFN package when suf-

ficiently mounted to a PCB layout and the internal ther-

mal loop temperature threshold.

Where:

Eq. 8: TA = TJ - (θJA · PD)

TA = Ambient Temperature in °C

TJ = -Maximum Device Junction Temperature Below the

Thermal Loop Threshold

PD = Total Power Dissipation by the Device

θJA = Package Thermal Resistance in °C/W

PRODUCT DATASHEET

AAT3697

2A Lithium-Ion/Polymer Battery Charger

Example:

For an application where the fast charge current for the

adapter mode is set to 1A, VADP = 5.0V and the battery

voltage at 3.6V, what is the maximum ambient tempera-

ture at which the thermal loop will become active?

Given:

VADP = 5.0V

VBAT = 3.6V

ICC = 1A

IOP = 0.75mA

TJ = 110°C

θJA = 50°C/W

Using Equation 7, calculate the device power dissipation

for the stated condition:

Eq. 9: PD = (5.0V - 3.6V)(1A) + (5.0V · 0.75mA)

= 1.40375W

The maximum ambient temperature before the AAT3697

thermal loop becomes active can now be calculated

using Equation 8:

Eq. 10: TA = 110°C - (50°C/W · 1.40375W)

= 39.8125°C

Therefore, under the stated conditions for this worst

case power dissipation example, the AAT3697 will enter

the thermal loop and lower the fast charge constant cur-

rent when the ambient operating temperature rises

above 39.8°C.

Capacitor Selection

Input Capacitor

In general, it is good design practice to place a decou-

pling capacitor between the ADP pin and ground. An

input capacitor in the range of 1µF to 22µF is recom-

mended. If the source supply is unregulated, it may be

necessary to increase the capacitance to keep the input

voltage above the under-voltage lockout threshold during

device enable and when battery charging is initiated.

If the AAT3697 adapter input is to be used in a system

with an external power supply 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.

3697.2008.02.1.3

www.analogictech.com

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