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LTC4081

4081f

APPLICATIO S I FOR ATIO

Where VOUT is the regulated output of the switching

regulator, IOUT is the regulator load and η is the regulator

efﬁciency at that particular load.

It is not necessary to perform worst-case power dissipa-

tion scenarios because the LTC4081 will automatically

reduce the charge current to maintain the die temperature

at approximately 115°C. However, the approximate ambi-

ent temperature at which the thermal feedback begins to

protect the IC is:

TA = 115°C – PDθJA

TA = 115°C – (VCC – VBAT) • IBAT • θJA if the regulator

is off.

Example: Consider the extreme case when an LTC4081 is

operating from a 6V supply providing 250mA to a 3V Li-Ion

battery and the regulator is off. The ambient temperature

above which the LTC4081 will begin to reduce the 250mA

charge current is approximately:

TA = 115°C – (6V – 3V) • (250mA) • 43°C/W

TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C

TA = 82.75°C

If there is more power dissipation due to the regulator,

the thermal regulation will begin at a somewhat lower

temperature. In the above circumstances, the LTC4081

can be used above 82.75°C, but the charge current will be

reduced from 250mA. The approximate current at a given

ambient temperature can be calculated:

BAT

°−

−

()

115

•

Using the previous example with an ambient temperature

of 85°C, the charge current will be reduced to approxi-

mately:

C W

BAT =

°− °

−

() °

115

129

•/

32 6

.mA

Furthermore, the voltage at the PROG pin will change

proportionally with the charge current as discussed in the

Programming Charge Current section.

VCC Bypass Capacitor

Many types of capacitors can be used for input bypassing;

however, caution must be exercised when using multi-layer

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 con-

ditions, such as connecting the battery charger input to a live

power source. Adding a 1Ω series resistor in series with an

X5Rceramiccapacitorwillminimizestart-upvoltagetransients.

For more information, refer to Application Note 88.

Thermistors

The LTC4081 NTC trip points are designed to work with therm-

istors whose resistance-temperature characteristics follow

VishayDale’s“R-TCurve1.”TheVishayNTHS0603NO1N1002J

is an example of such a thermistor. However, Vishay Dale

has many thermistor products that follow the “R-T Curve 1”

characteristic in a variety of sizes. Furthermore, any thermis-

tor whose ratio of RCOLD to RHOT is about 5 will also work

(Vishay Dale R-T Curve 1 shows a ratio of RCOLD to RHOT of

3.266/0.5325 = 6.13).

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