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LTC4085

4085fd

Consider the above example with an ambient temperature of

55°C. The charge current will be reduced to approximately:

BAT =

105

°C– 55°C

5V – 3V

()•37°C/W

°C

°C/A

= 0.675A

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

USB Power Control Application with Wall Adapter Input

SUSP

HPWR

SUSPEND USB POWER

500mA/100mA SELECT

OUT

PROG

LTC4085

CLPROG

GND

BAT

GATE

VNTC

NTC

Li-Ion

CELL

TO LDOs

REGs, ETC

CHRG

ACPR

WALL

TIMER

0.15μF

4085 TA02

RNTCBIAS

10k

RNTC

10k

510Ω

RPROG

71.5k

*SERIES 1W RESISTOR ONLY

NEEDED FOR INDUCTIVE

INPUT SUPPLIES

RCLPROG

2.1k

4.7μF

510Ω

5V WALL

ADAPTER

INPUT

5V (NOM)

FROM USB

CABLE VBUS

1Ω*

4.7μF

1Ω*

TYPICAL APPLICATION

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