LTC4075 datasheet(13/16 Pages) LINER

13 / 16 page

LTC4075/LTC4075X

4075Xfa

PD is the power dissipated, VIN is the input supply volt-

age (either DCIN or USBIN), VBAT is the battery voltage

and IBAT is the charge current. The approximate ambient

temperature at which the thermal feedback begins to

protect the IC is:

TA = 105°C – PD • θJA

TA = 105°C – (VIN – VBAT) • IBAT • θJA

Example: An LTC4075 operating from a 5V wall adapter (on

the DCIN input) is programmed to supply 800mA full-scale

current to a discharged Li-Ion battery with a voltage of 3.3V.

Assuming

θJA is 40°C/W (see Thermal Considerations),

the ambient temperature at which the LTC4075 will begin

to reduce the charge current is approximately:

TA = 105°C – (5V – 3.3V) • (800mA) • 40°C/W

TA = 105°C – 1.36W • 40°C/W = 105°C – 54.4°C

TA = 50.6°C

The LTC4075 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:

BAT

105

–

(–

•

)

Using the previous example with an ambient temperature

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

mately:

C W

ImA

BAT

°°

105

53 3

662

–

(– .

)•

It is important to remember that LTC4075 applications do

not need to be designed for worst-case thermal conditions,

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 LTC4075 package is properly soldered

to the PC board ground. When correctly soldered to a

2500mm2 double sided 1oz copper board, the LTC4075

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

thermal resistances far greater than 40°C/W. As an example,

a correctly soldered LTC4075 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.

Protecting the USB Pin and Wall Adapter Input from

Overvoltage Transients

Caution must be exercised when using ceramic capacitors

to bypass the USBIN pin or the wall adapter inputs. High

voltage transients can be generated when the USB or wall

adapter is hot plugged. When power is supplied via the

USB bus or wall adapter, the cable inductance along with

the self resonant and high Q characteristics of ceramic

capacitors can cause substantial ringing which could

exceed the maximum voltage pin ratings and damage the

LTC4075. Refer to Linear Technology Application Note 88,

entitled “Ceramic Input Capacitors Can Cause Overvoltage

Transients” for a detailed discussion of this problem. The

long cable lengths of most wall adapters and USB cables

APPLICATIO S I FOR ATIO

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