LTC4097
16
4097f
APPLICATIONS INFORMATION
The LTC4097 can be used above 64°C ambient, but the
charge current will be reduced from 500mA. The ap-
proximate current at a given ambient temperature can be
approximated by:
I
CT
VV
BAT
A
IN BAT JA
=
°115 –
(– •
)
θ
Using the previous example with an ambient temperature
of 75°C, the charge current will be reduced to approxi-
mately:
I
CC
VV CW
C
CA
BAT
=
°°
°
=
°
°
115 75
53360
40
102
–
(–.)• / /
IImA
BAT
= 392
It is important to remember that LTC4097 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 by forcing the
LTC4097 into shut down mode.
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 LTC4097 package is properly soldered
to the PC board ground. When correctly soldered to a
2500mm
2
double sided 1oz copper board, the LTC4097
has a thermal resistance of approximately 60°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 60°C/W. As an example,
a correctly soldered LTC4097 can deliver over 500mA to
a battery from a 5V supply at room temperature. Without
a good backside thermal connection, this number would
drop to much less than 300mA.
Alternate NTC Thermistors and Biasing
The LTC4097 provides temperature qualifi ed charging if
a grounded thermistor and a bias resistor are connected
to NTC. By using a bias resistor whose value is equal to
the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are pre-programmed
to approximately 40°C and 0°C, respectively (assuming
a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be ad-
justed by either a modifi cation of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modifi ed but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, used
in the following examples, has a nominal value of 100k
and follows the Vishay “Curve 1” resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the Thermistor at 25°C
R
NTC|COLD
= Value of thermistor at the cold trip point
R
NTC|HOT
= Value of the thermistor at the hot trip
point
r
COLD
= Ratio of R
NTC|COLD
to R25
r
HOT
= Ratio of R
NTC|HOT
to R25
R
NOM
= Primary thermistor bias resistor (see Fig-
ure 4)
R1 = Optional temperature range adjustment resistor
(see Figure 5)
The trip points for the LTC4097’s temperature qualifi cation
are internally programmed at 0.349 • VNTC for the hot
threshold and 0.765 • VNTC for the cold threshold.
Therefore, the hot trip point is set when:
R
RR
VNTC VNTC
NTC HOT
NOM NTC HOT
|
|
•.•
+
= 0 349
