AAT2554 datasheet(21/33 Pages) ANALOGICTECH

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AAT2554

Total Power Solution for Portable Applications

2554.2007.01.1.2

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:

Note: Red LED forward voltage (VF) is typically

2.0V @ 2mA.

Thermal Considerations

The AAT2554 is offered in a TDFN34-16 package

which can provide up to 2W of power dissipation

when it is properly bonded to a printed circuit board

and has a maximum thermal resistance of 50°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 generating devices in a given

application design. The ambient temperature around

the IC will also have an effect on the thermal limits of

a battery 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:

Where:

PD(MAX) = Maximum Power Dissipation (W)

θJA

= Package Thermal Resistance (°C/W)

TJ(MAX) = Maximum Device Junction Temperature

(°C) [135°C]

= Ambient Temperature (°C)

Figure 3 shows the relationship of maximum

power dissipation and ambient temperature of the

AAT2554.

Figure 3: Maximum Power Dissipation.

Next, the power dissipation of the battery charger

can be calculated by the following equation:

Where:

= Total Power Dissipation by the Device

VADP = ADP/USB 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]

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

TA (

°°C)

500

1000

1500

2000

2500

3000

100

120

(TJ(MAX) - TA)

PD(MAX) =

(5.5V - 2.0V)

R1 =

= 1.75kΩ

2mA

(VADP - VF(LED))

R1=

ILED

Event Description

Status

No battery charging activity

OFF

Battery charging via adapter

or USB port

Charging completed

OFF

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