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LTC4085

4085fc

Example: Consider an LTC4085 operating from a wall

adapter with 5V at VOUT providing 0.8A to a 3V Li-Ion

battery. The ambient temperature above which the

LTC4085 will begin to reduce the 0.8A charge current, is

approximately

TA = 105°C – (5V – 3V) • 0.8A • 37°C/W

TA = 105°C – 1.6W • 37°C/W = 105°C – 59°C = 46°C

The LTC4085 can be used above 46°C, but the charge cur-

rent will be reduced below 0.8A. The charge current at a

given ambient temperature can be approximated by:

BAT

OUT

BAT

()

105

–

–•

Consider the above example with an ambient temperature

of 55°C. The charge current will be reduced to approxi-

mately:

C W

BAT

°°

() °

105

0 675

–

–•

Board Layout Considerations

In order to be able to deliver maximum charge current

under all conditions, it is critical that the Exposed Pad on

the backside of the LTC4085 package is soldered to the

board. Correctly soldered to a 2500mm2 double-sided

1oz. copper board the LTC4085 has a thermal resistance

of approximately 37°C/W. Failure to make thermal contact

between the Exposed Pad on the backside of the package

and the copper board will result in thermal resistances far

greater than 37°C/W. As an example, a correctly soldered

LTC4085 can deliver over 1A to a battery from a 5V supply

at room temperature. Without a backside thermal connec-

tion, this number could drop to less than 500mA.

VIN and Wall Adapter Bypass Capacitor

Many types of capacitors can be used for input bypassing.

However, caution must be exercised when using multilayer

ceramic capacitors. Because of the self resonant and high

Q characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up

conditions, such as connecting the charger input to a hot

power source. For more information, refer to Application

Note 88.

Stability

The constant-voltage mode feedback loop is stable without

any compensation when a battery is connected. However,

a 4.7μF capacitor with a 1

Ω series resistor to GND is

recommended at the BAT pin to keep ripple voltage low

when the battery is disconnected.

APPLICATIONS INFORMATION

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