LTC4096 datasheet(13/16 Pages) LINER

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LTC4096/LTC4096X

4096xf

Power Dissipation

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

sary to design for worst-case power dissipation scenarios

because the LTC4096 automatically reduces the charge

current during high power conditions. The conditions

that cause the LTC4096 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:

PD = (VIN – VBAT) • IBAT

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 = 115°C – PD • θJA

TA = 115°C – (VIN – VBAT) • IBAT • θJA

Example: An LTC4096 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 LTC4096 will begin

to reduce the charge current is approximately:

TA = 115°C – (5V – 3.3V) • (800mA) • 40°C/W

TA = 115°C – 1.36W • 40°C/W = 115°C – 54.4°C

TA = 60.6°C

The LTC4096 can be used above 60.6°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 70°C, the charge current will be reduced to approxi-

mately:

C W

C A

ImA

BAT

°°

105

53 3

662

–

(– .

)•

It is important to remember that LTC4096 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

115°C. Moreover a thermal shut down protection circuit

around 150°C safely prevents any damage putting LTC4096

into shut down mode.

APPLICATIONS INFORMATION

LTC4096

DCIN

USBIN

IUSB

IDC

BAT

ITERM

RISET

RITERM

WALL

ADAPTER

USB

PORT

µF

500mA

4096 F03

GND

4.2V

1-CELL

Li-Ion

BATTERY

LTC4096

DCIN

USBIN

IUSB

IDC

RIDC

1.24k

WALL

ADAPTER

USB

PORT

µF

800mA (WALL)

500mA (USB)

4096 F04

GND

RIUSB

RITERM

4.2V

1-CELL

Li-Ion

BATTERY

BAT

ITERM

PWR

CHRG

Figure 3. Dual Input Charger Circuit. The

Wall Adapter Charge Current and USB Charge

Current are Both Programmed to be 500mA

Figure 4. Full Featured Dual Input Charger Circuit

LTC4096 Datasheet(PDF) 13 Page - Linear Technology

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