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LTC4061

4061fd

It is important to remember that LTC4061 applications do

notneedtobedesignedforworst-casethermalconditions,

since the IC will automatically reduce power dissipation if

the junction temperature reaches approximately 105°C.

Thermistors

The LTC4061 NTC comparator trip points were 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 also works (Vishay Dale R-T

Curve 1 shows a ratio of RCOLD to RHOT of 3.266/0.5325

= 6.13).

Power conscious designers may want to use thermistors

whose room temperature value is greater than 10k

Ω.

Vishay Dale has a number of values of thermistor from

10k

Ω to 100kΩ that follow the “R-T Curve 1.” Using dif-

ferent R-T curves, such as Vishay Dale “R-T Curve 2,” is

also possible. This curve, combined with LTC4061 internal

thresholds,givestemperaturetrippointsofapproximately

0°C (falling) and 40°C (rising), a delta of 40°C. This delta in

temperature can be moved in either direction by changing

the value of RNOM with respect to RNTC. Increasing RNOM

moves both trip points to lower temperatures. Likewise

a decrease in RNOM with respect to RNTC moves the trip

pointstohighertemperatures.TocalculateRNOMforashift

to lower temperatures, use the following equation:

NOM

COLD

NTC

3 266

•

where RCOLD is the resistance ratio of RNTC at the desired

coldtemperaturetrippoint.Ifyouwanttoshiftthetrippoints

to higher temperatures, use the following equations:

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 10k

Ω R-T Curve 2 thermistor

from Vishay Dale. The difference between the trip points

applicaTions inForMaTion

is 40°C, from before, and we want the cold trip point to

be 0°C, which would put the hot trip point at 40°C. The

RNOM needed is calculated as follows:

NOM

COLD

NTC

3 266

2 816

3 266

8 62

•

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

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

trip points of approximately 0°C and 40°C respectively.

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

resistor, R1, can be added in series with RNTC. The values

of the resistors are calculated as follows:

NOM

COLD

HOT

–

– .

- .

3 266 0 5325

0 5325

3 266 0 532









•(

–

) –

COLD

HOT

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

RCOLD are the values of RNTC at the desired temperature

trip points. Continuing the example from before with a

desired hot trip point of 50°C:

NOM

COLD

HOT

–

– .

•( .

– .

)

3 266 0 5325

2 816 0 4086

3..

– .

, .

266 0 5325

8 8

8 87

k is the nearest

value

1 10

0 5325

3 266 0 5325

2 816 0 4086









•

– .

•( .

– .

) – 00 4086

604 604

is the nearest

value

The final solution is RNOM = 8.87kΩ, R1 = 604Ω and

RNTC = 10kΩ at 25°C.

NTCTripPointError

When a 1% resistor is used for RHOT, the major error

in the 40°C trip point is determined by the tolerance of

the NTC thermistor. A typical 100k

Ω NTC thermistor has

±10% tolerance. By looking up the temperature coef-

ficient of the thermistor at 40°C, the tolerance error can

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