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LTC4066/LTC4066-1

4066fc

APPLICATIONS INFORMATION

Power conscious designs may want to use thermistors

whose room temperature value is greater than 10k. Vishay

Dale has a number of values of thermistor from 10k to

100k that follow the “R-T Curve 2.” Using these directly

in the manor spelled out previously in the NTC Thermistor

section will give temperature trip points of approximately

3°C and 47°C, a delta of 44°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 lower temperatures. Likewise a decrease

in RNOM with respect to RNTC will move the trip points to

higher temperatures. To calculate RNOM for a shift to lower

temperature for example, use the following equation:

Rat

NOM

COLD

NTC

=°

2 815

•

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 4086

•

where RHOT is the resistance ratio of RNTC at the desired

hot temperature trip point.

Here is an example using a 100k R-T Curve 1 Thermistor

from Vishay Dale. The difference between the trip points

is 44°C, from before, and we want the cold trip point to

be 0°C, which would put the hot trip point at 44°C. The

RNOM needed is calculated as follows:

Rat

NOM

COLD

NTC

=°

=Ω =

2 815

3 266

2 815

100

116

•

The nearest 1% value for RNOM is 115k. This is the value

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

points of approximately 0°C and 44°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 3b). The values of the resistors are calculated as

follows:

NOM

COLD

HOT

COLD

HOT

= ⎛

⎝⎜

⎞

⎠⎟ ()

–

.– .

•–

–

2 815 0 4086

0 4086

2 815 0 4086

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:

k nearest

k is nearest

NOM

COLD

HOT

()

=Ω

⎛

⎝⎜

⎞

⎠⎟ ()

⎡

⎣

⎢

⎤

⎦

⎥

=Ω

–

.– .

•.

– .

.– .

•

.– .

•

–.

2 815 0 4086

100

3 266 0 3602

2 815 0 4086

120 8

121

1 100

0 4086

2 815 0 4086

3 266 0 3602

0 3602

13 3

The ﬁnal solution is as shown if Figure 3b where RNOM =

121k, R1 = 13.3k and RNTC = 100k at 25°C.

Gas Gauge

The extremely low impedance of the ideal diode between

BAT and OUT (typically 50mΩ) allows users to connect

all of their loads to the OUT pin. Such a conﬁguration puts

the LTC4066/LTC4066-1 in a unique position whereby it

can monitor all of the current that ﬂows into and out of the

battery. Two output pins, ISTAT and POL, are provided to

enable users to monitor and integrate the battery current

for a true gas gauge function.

LTC4066 Datasheet(PDF) 21 Page - Linear Technology

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