Datasheet

LTC3553
19
3553fb
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
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
NOM
=
r
HOT
0.538
•R25
R
NOM
=
r
COLD
3.17
•R25
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 independently set, the other is determined by the de-
fault 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 temperature
gain of the thermistor as absolute temperature increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 2. The following formulas can
be used to compute the values of R
NOM
and R1:
R
NOM
=
r
COLD
–r
HOT
2.714
•R25
R1 = 0.536 • R
NOM
–r
HOT
•R25
For example, to set the trip points to 0°C and 45°C with
a Vishay curve 1 thermistor choose:
R
NOM
=
3.266 – 0.4368
2.714
• 100k =104.2k
the nearest 1% value is 105k:
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
The nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 2 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
OPERATION
Figure 2. NTC Thermistor Circuit With Additional Bias Resistor
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
V
BUS
NTC_ENABLE
3553 F02
TOO_COLD
TOO_HOT
0.76 • V
BUS
(NTC RISING)
0.35 • V
BUS
(NTC FALLING)
0.017 • V
BUS
(NTC FALLING)
–
+
20
15
The trip points for the LTC3553’s temperature qualifi ca-
tion are internally programmed at 0.35 • V
BUS
for the hot
threshold and 0.76 • V
BUS
for the cold threshold.
Therefore, the hot trip point is set when:
R
NTC|HOT
R
NOM
+R
NTC|HOT
•V
BUS
= 0.35 • V
BUS
and the cold trip point is set when:
R
NTC|COLD
R
NOM
+R
NTC|COLD
•V
BUS
= 0.76 • V
BUS
Solving these equations for R
NTC|COLD
and R
NTC|HOT
results in the following:
R
NTC|HOT
= 0.538 • R
NOM
and
R
NTC|COLD
= 3.17 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in r
HOT
= 0.538 and r
COLD
= 3.17. 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.