LTC3556_15 Datasheet, PDF (30/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
APPLICATIONS INFORMATION
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_NTC|HOT
R_NOM + R_NTC|HOT
•
V_BUS
=
0.349 •
V_BUS
and the cold trip point is set when:
R_NTC|COLD
R_NOM + R_NTC|COLD
•
V_BUS
=
0.765
•
V_BUS
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
30
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
0.536
•
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 “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_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 solution is shown
in Figure 7b and results in an upper trip point of 45°C and
a lower trip point of 0°C.
3556f