Datasheet

LTC4081
18
4081f
APPLICATIO S I FOR ATIO
WUU
U
Where V
OUT
is the regulated output of the switching
regulator, I
OUT
is the regulator load and η is the regulator
effi ciency 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:
T
A
= 115°C – P
D
θ
JA
T
A
= 115°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
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:
T
A
= 115°C – (6V – 3V) • (250mA) • 43°C/W
T
A
= 115°C – 0.75W • 43°C/W = 115°C – 32.25°C
T
A
= 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:
I
CT
VV
BAT
A
CC BAT JA
=
°−
−
()
115
• θ
Using the previous example with an ambient temperature
of 85°C, the charge current will be reduced to approxi-
mately:
I
CC
VV CW
C
CA
BAT
=
°− °
−
()
°
=
°
°
=
115 85
63 43
30
129
2
•/ /
332 6.mA
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in the
Programming Charge Current section.
V
CC
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 1
Ω series resistor in series with an
X5R ceramic capacitor will minimize start-up voltage transients.
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
Vishay Dale’s “R-T Curve 1.” The Vishay NTHS0603NO1N1002J
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 R
COLD
to R
HOT
is about 5 will also work
(Vishay Dale R-T Curve 1 shows a ratio of R
COLD
to R
HOT
of
3.266/0.5325 = 6.13).