LTC1733

11

sn1733 1733fs

resistance of the NTC thermistor rises. The LTC1733 is

designed to go into hold mode when the value of the NTC

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.

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.

NTC Trip Point Errors

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) =

±3°C, gives the temperature error of the hot trip point.

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

°Canditsvalue

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 %/

the value of the thermistor at 50

Temperature Error (

°C)=

1

7

1 100

+









TOL R

R

TC

COLD

HOT

•–

•

For example, the Vishay NTHS0603N02N1002J thermistor

with a tolerance of

Temperature Error (

°C) =

10 10

7

6 89 1 100

45

±









.

•.

–

•

–.

= –1.8

°C, +2.5°C

APPLICATIO S I FOR ATIO

–

+

–

+

–

+

1733 F03

10k

1%

TOO COLD

TOO HOT

DISABLE NTC

LTC1733

NTC

Figure 3.

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

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