LTC1733 Linear Technology, LTC1733 Datasheet - Page 11

• Manufacturer Part Number: LTC1733
• Description: Monolithic Linear Lithium-Ion Battery Charger with Thermal Regulation
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
resistance of the NTC thermistor rises. The LTC1733 is
designed to go into hold mode when the value of the NTC
thermistor increases to seven times the value of R
a Vishay NTHS0603N02N1002J thermistor, this value is
28.2k which corresponds to approximately 0 C. The hot
and cold comparators each have approximately 2 C of
hysteresis to prevent oscillation about the trip point. The
NTC function can be disabled by grounding the NTC pin.
Thermistors
The LTC1733 NTC trip points were designed to work with
thermistors whose resistance-temperature characteris-
tics follow Vishay Dale’s “R-T Curve 2”. The Vishay
NTHS0603N02N1002J is an example of such a ther-
mistor. However, Vishay Dale has many thermistor prod-
ucts that follow the “R-T Curve 2” characteristic in a variety
of sizes. Futhermore, any thermistor whose ratio of R
to R
2 shows a ratio of R
NTC Layout Considerations
It is important that the NTC thermistor not be in close
thermal contact with the LTC1733. Because the LTC1733
package can reach temperatures in excess of the 50 C trip
point, the NTC function can cause a hysteretic oscillation
which turns the charge current on and off according to the
R
NTC
10k
R
HOT
HOT
1%
V
is about 7.0 will also work (Vishay Dale R-T Curve
CC
NTC
LTC1733
3/160 V
7/8 V
1/2 V
U
COLD
CC
CC
CC
Figure 3.
to R
U
–
–
–
+
+
+
HOT
of 2.816/0.4086 = 6.9).
W
TOO COLD
TOO HOT
DISABLE NTC
U
HOT
1733 F03
COLD
. For
package temperature rather than the battery temperature.
This problem can be eliminated by thermally coupling the
NTC thermistor to the battery and not to the LTC1733.
Furthermore, it is essential that the V
R
niques. Since V
it is essential to minimize voltage drops between the V
input pin and the top of R
NTC Trip Point Errors
When a 1% resistor is used for R
the 50 C trip point is determined by the tolerance of the
NTC thermistor. A typical 10k NTC thermistor has a 10%
tolerance. By looking up the temperature coefficient of the
thermistor at 50 C, the tolerance error can be calculated
in degrees centigrade. Consider the Vishay
NTHS0603N02N1002J thermistor which has a tempera-
ture coefficient of –3.3%/ C at 50 C. Dividing the toler-
ance by the temperature coefficient, 10%/(–3.3%/ C) =
The cold trip point is a little more complicated because its
error depends on the tolerance of the NTC thermistor and
the degree to which the ratio of its value at 0 C and its value
at 50 C varies from 7 to 1. Therefore, the cold trip point
error can be calculated using the tolerance, TOL, the
temperature coefficient of the thermistor at 0 C, TC
(in %/ C), the value of the thermistor at 0 C, R
the value of the thermistor at 50 C, R
For example, the Vishay NTHS0603N02N1002J thermistor
with a tolerance of 10%, TC of –4.5%/ C, and R
R
HOT
3 C, gives the temperature error of the hot trip point.
HOT
Temperature Error ( C) =
Temperature Error ( C) =
of 6.89, has a cold trip point error of:
is made according to standard Kelvin sense tech-
CC
is a high current path into the LTC1733,
= –1.8 C, +2.5 C
HOT
.
1
1 0 10
7
TOL R
HOT
7
.
HOT
, the major error in
•
CC
• .
LTC1733
R
– .
. The formula is:
6 89 1 100
TC
COLD
HOT
4 5
connection to
–
–
COLD
sn1733 1733fs
1 100
11
•
•
COLD
, and
CC
/