LTC4000-1
21
40001fa
For more information www.linear.com/LTC4001-1
applicaTions inForMaTion
if the user finds that a negative value is needed for R
D
,
the two temperature thresholds selected are too close to
each other and a higher sensitivity thermistor is needed.
For example, this method can be used to set the hot
and cold thresholds independently to 60°C and –5°C.
Using a Vishay Curve 2 thermistor whose nominal value
at 25°C is 100k, the formula results in R3 = 130k and
R
D
= 41.2k for the closest 1% resistors values.
To increase thermal sensitivity such that the valid charging
temperature band is much smaller than 40°C, it is pos-
sible to put a PTC (positive thermal coefficient) resistor
in series with R3 between the BIAS pin and the NTC pin.
This PTC resistor also needs to be thermally coupled with
the battery. Note that this method increases the number of
thermal sensing connections to the battery pack from one
wire to three wires. The exact value of the nominal PTC
resistor required can be calculated using a similar method
as described above, keeping in mind that the threshold at
the NTC pin is always 75% and 35% of V
BIAS
.
Leaving the NTC pin floating or connecting it to a capacitor
disables all
NTC functionality.
Battery V
oltage Temperature Compensation
Some battery chemistries have charge voltage require-
ments that vary with temperature. Lead-acid batteries in
particular experience a significant change in charge volt-
age requirements as temperature changes. For example,
manufacturers of large lead-acid batteries recommend a
float charge of 2.25V/cell at 25°C. This battery float voltage,
however, has a temperature coefficient which is typically
specified at –3.3mV/°C per cell.
The LTC4000-1 employs a resistor feedback network to
program the battery float voltage. manipulation of this
network makes for an efficient implementation of vari-
ous temperature compensation schemes of battery float
voltage.
A simple solution for tracking such a linear voltage de-
pendence on temperature is to use the LM234 3-terminal
temperature sensor. This creates an easily programmable
linear temperature dependent characteristic.
equal to 0.179 • R
NTC
at cold_threshold. Similarly, if the
value of R3 is set to adjust the hot threshold, the value
of the NTC resistor at the cold threshold is then equal to
5.571 • R
NTC
at cold_threshold.
Note that changing the value of R3 to be larger than R25
will move both the hot and cold threshold lower and vice
versa. For example, using a Vishay
Curve 2 thermistor
whose nominal value at 25°C is 100k, the user can set
the cold temperature to be at 5°C by setting the value of
R3 = 75k, which automatically then sets the hot threshold
at approximately 50°C.
It is possible to adjust the hot and cold threshold indepen-
dently by introducing another resistor as a second degree
of freedom (Figure 10). The resistor R
D
in effect reduces
the sensitivity of the resistance between the NTC pin and
ground. Therefore, intuitively this resistor will move the hot
threshold to a hotter temperature and the cold threshold
to a colder temperature.
Figure 10. NTC Thermistor Connection with
Desensitizing Resistor R
D
NTC RESISTOR
THERMALLY COUPLED
WITH BATTERY PACK
R3
R
NTC
NTC
BIAS
BAT
LTC4000-1
R
D
40001 F10
C
BIAS
The value of R3 and R
D
can now be set according to the
following formula:
R3 =
NTC
NTC
2.461
R
D
= 0.219 • R
NTC
at cold_ threshold –
1.219 • R
NTC
at hot_ threshold
Note the important caveat that this method can only be
used to desensitize the thermal effect on the thermistor
and hence push the hot and cold temperature thresholds
apart from each other. When using the formulas above,
