LTC3586-1_15 Datasheet, PDF (27/36 Pages) Linear Technology – High Efficiency USB Power Manager with Boost, Buck-Boost and Dual Bucks

LTC3586/LTC3586-1
Applications Information
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 non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R_NOM
=
r_HOT
0.536
•
R25
R_NOM
=
r_COLD
3.25
•
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 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
“temperature gain” of the thermistor as absolute tem-
perature 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_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 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.
V_BUS
R_NOM
100k
NTC
5
V_BUS
LTC3586/LTC3586-1
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
T
R_NTC
100k
–
0.349 • V_BUS
+
TOO_HOT
+
0.017 • V_BUS
–
NTC_ENABLE
V_BUS
R_NOM
105k
NTC
5
V_BUS
LTC3586/LTC3586-1
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
R1
12.7k
T
R_NTC
100k
–
0.349 • V_BUS
+
+
0.017 • V_BUS
–
TOO_HOT
NTC_ENABLE
(6a)
3586 F06a
Figure 6. NTC Circuits
(6b)
3586 F06b
3586fb
27