ltc3586 Linear Technology Corporation, ltc3586 Datasheet - Page 27

• Manufacturer Part Number: ltc3586
• Description: High Ef? Ciency Usb Power Manager With Boost, Buck-boost And Dual Bucks
• Manufacturer: Linear Technology Corporation

By using a bias resistor, R
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
where r
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
at 60°C. Using the above equation, R
to 46.4k. With this value of R
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-
APPLICATIONS INFORMATION
R
R
V
BUS
NOM
NOM
T
R
100k
NOM
HOT
NTC
R
100k
5
NTC
=
=
r
0 536
and r
3 25
COLD
r
HOT
.
.
V
BUS
0.017 • V
0.349 • V
COLD
0.765 • V
•
•
R
R
25
25
BUS
are the resistance ratios at the de-
BUS
BUS
(6a)
NOM
–
+
–
+
+
–
NTC BLOCK
NOM
LTC3586
, different in value from
, the cold trip point is
NOM
TOO_COLD
TOO_HOT
NTC_ENABLE
should be set
HOT
is 0.2488
3586 F06a
Figure 6. NTC Circuits
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 6b. The following formulas can be used
to compute the values of R
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
the nearest 1% value is 105k:
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
the nearest 1% value is 12.7k. The final circuit is shown
in Figure 6b and results in an upper trip point of 45°C and
a lower trip point of 0°C.
R
R
R
V
NOM
1 0 536
NOM
BUS
=
T
R
105k
R1
12.7k
NOM
NTC
R
100k
5
NTC
.
=
=
r
3 266 0 4368
COLD
.
•
2 714
V
R
BUS
0.017 • V
.
2 714
0.349 • V
NOM
–
.
0.765 • V
– .
r
HOT
BUS
–
BUS
BUS
r
HOT
•
R
(6b)
NOM
25
•
–
+
–
+
+
–
•
100
NTC BLOCK
R R 25
LTC3586
and R1:
k
=
104 2
LTC3586
.
TOO_COLD
TOO_HOT
NTC_ENABLE
k
27
3586 F06b
3586f