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LTC4069

4069fb

APPLICATIONS INFORMATION

4069 F06

RNOM

8.87k

RNTC

10k

VCC

–

TOO COLD

TOO HOT

NTC_ENABLE

604Ω

0.76 • VCC

0.35 • VCC

0.016 • VCC

NTC

Figure 6. NTC Circuits

Thermistors

The LTC4069 NTC trip points are designed to work with

thermistors whose resistance-temperature characteristics

follow Vishay Dale’s “R-T Curve 1.” The Vishay

NTHS0603N01N1003J 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 thermistor whose ratio of RCOLD to RHOT

is about 6 will also work (Vishay Dale R-T Curve 1 shows

a ratio of RCOLD to RHOT of 3.266/0.5325 = 6.13).

Designers may want to use thermistors whose room

temperature value is different than 100k. Vishay Dale

has a number of values of thermistor from 32k to 100k

that follow the “R-T Curve 1.” Using different R-T curves,

such as Vishay Dale “R-T Curve 2”, is also possible. This

curve, combined with LTC4069 internal thresholds, gives

temperature trip points of approximately –3°C (falling) and

42°C (rising), a delta of 45°C. This delta in temperature

can be moved in either direction by changing the value of

RNOM with respect to RNTC. Increasing RNOM will move

both trip points to higher temperatures. To calculate

RNOM for a shift to lower temperature for example, use

the following equation:

Rat

NOM

COLD

NTC

=°

3 266

•

where RCOLD is the resistance ratio of RNTC at the desired

cold temperature trip point. If you want to shift the trip points

to higher temperatures use the following equation:

Rat

NOM

HOT

NTC

=°

0 5325

•

where RHOT is the resistance ratio of RNTC at the desired

hot temperature trip point.

Here is an example using a 10k R-T Curve 2 thermistor

from Vishay Dale. The difference between the trip points

is 45°C, from before, and we want the cold trip point to

be 0°C, which would put the hot trip point at 45°C. The

RNOM needed is calculated as follows:

Rat

NOM

COLD

NTC

=°

3 266

2 816

•

•.

266

8 62

The nearest 1% value for RNOM is 8.66k. This is the value

used to bias the NTC thermistor to get cold and hot trip

points of approximately 0°C and 45°C respectively. To

extend the delta between the cold and hot trip points, a

resistor, R1, can be added in series with RNTC (see Figure

6). The values of the resistors are calculated as follows:

NOM

COLD

HOT

−

3 266 0 5325

0 5325

3 266

−

⎛

⎝⎜

⎞

⎠⎟

−

() −

0 5325

•R

COLD

HOT

where RNOM is the value of the bias resistor and RHOT and

RCOLD are the values of RNTC at the desired temperature

trip points. Continuing the example from before with a

desired trip point of 50°C:

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