Datasheet

LTC3556
30
3556f
in the following examples, has a nominal value of 100k
and follows the Vishay “Curve 1” resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
R
NTC|COLD
= Value of thermistor at the cold trip point
R
NTC|HOT
= Value of thermistor at the hot trip point
r
COLD
= Ratio of R
NTC|COLD
to R25
r
HOT
= Ratio of R
NTC|COLD
to R25
R
NOM
= Primary thermistor bias resistor (see Figure 7a)
R1 = Optional temperature range adjustment resistor
(see Figure 7b)
The trip points for the LTC3556’s temperature qualification
are internally programmed at 0.349 • V
BUS
for the hot
threshold and 0.765 • V
BUS
for the cold threshold.
Therefore, the hot trip point is set when:
R
RR
VV
NTCHOT
NOM NTCHOT
BUS BUS
|
|
•.•
+
= 0 349
and the cold trip point is set when:
R
RR
VV
NTCCOLD
NOM NTCCOLD
BUS BUS
|
|
•.•
+
= 0 765
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results
in the following:
R
NTC|HOT
= 0.536 • R
NOM
and
R
NTC|COLD
= 3.25 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in r
HOT
= 0.536 and r
COLD
= 3.25. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, R
NOM
, different in value from
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
APPLICATIONS INFORMATION
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
r
R
R
r
R
NOM
HOT
NOM
COLD
=
=
0 536
25
325
25
.
•
.
•
where r
HOT
and r
COLD
are the resistance ratios at the de-
sired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
From the Vishay Curve 1 R-T characteristics, r
HOT
is 0.2488
at 60°C. Using the above equation, R
NOM
should be set
to 46.4k. With this value of R
NOM
, the cold trip point is
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in “tem-
perature gain” of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 7b. The following formulas can be used
to compute the values of R
NOM
and R1:
R
rr
R
RRr
NOM
COLD HOT
NOM HOT
=
=
–
.
•
.• – •
2 714
25
1 0 536 RR25
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
Rkk
NOM
==
3 266 0 4368
2 714
100 104 2
.–.
.
•.
The nearest 1% value is 105k.
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
The nearest 1% value is 12.7k. The final solution is shown
in Figure 7b and results in an upper trip point of 45°C and
a lower trip point of 0°C.