LTC4076

13

4076fa

Power Dissipation

When designing the battery charger circuit, it is not neces-

sary to design for worst-case power dissipation scenarios

because the LTC4076 automatically reduces the charge

current during high power conditions. The conditions

that cause the LTC4076 to reduce charge current through

thermal feedback can be approximated by considering the

power dissipated in the IC. Most of the power dissipation

is generated from the internal MOSFET pass device. Thus,

the power dissipation is calculated to be:

temperature at which the thermal feedback begins to

protect the IC is:

Example:AnLTC4076operatingfroma5Vwalladapter(on

theDCINinput)isprogrammedtosupply800mAfull-scale

currenttoadischargedLi-Ionbatterywithavoltageof3.3V.

the ambient temperature at which the LTC4076 will begin

to reduce the charge current is approximately:

The LTC4076 can be used above 50.6°C ambient, but

the charge current will be reduced from 800mA. The ap-

proximate current at a given ambient temperature can be

approximated by:

I

C T

V

V

BAT

A

IN

BAT

JA

=

°

105 –

(

–

•

Using the previous example with an ambient temperature

of 60°C, the charge current will be reduced to approxi-

mately:

I

C

C

V

V

C W

C

C A

I

mA

BAT

BAT

=

°

°

°

=

°

°

=

105

60

5

3 3

40

45

68

662

–

( – . )•

/

/

It is important to remember that LTC4076 applications do

notneedtobedesignedforworst-casethermalconditions,

since the IC will automatically reduce power dissipation

when the junction temperature reaches approximately

105°C.

Thermal Considerations

In order to deliver maximum charge current under all

conditions, it is critical that the exposed metal pad on the

backside of the LTC4076 package is properly soldered

to the PC board ground. When correctly soldered to a

has a thermal resistance of approximately 40°C/W. Failure

to make thermal contact between the exposed pad on the

backside of the package and the copper board will result in

thermalresistancesfargreaterthan40°C/W.Asanexample,

a correctly soldered LTC4076 can deliver over 800mA to

a battery from a 5V supply at room temperature. Without

a good backside thermal connection, this number would

drop to much less than 500mA.

APPLICATIO S I FOR ATIO