LTC3577-3 Datasheet, PDF (27/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
The trip points for the LTC3577-3/LTC3577-4’s temperature
qualification are internally programmed at 0.35 • V_NTC for
the hot threshold and 0.76 • V_NTC for the cold threshold.
Therefore, the hot trip point is set when:
R_{NTC|HOT • NTCBIAS = 0.35 • NTCBIAS}
R_NOM + R_NTC|HOT
and the cold trip point is set when:
R_{NTC|COLD • NTCBIAS = 0.76 • NTCBIAS}
R_NOM + R_NTC|COLD
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.
By using a bias resistor, R_NOM, different in value from R25,
the hot and cold trip points can be moved in either direc-
tion. 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
0.538
• R25
R_NOM
=
r_COLD
3.17
• R25
where r_HOT and r_COLD are the resistance ratios at the
desired 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 4. 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 4 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
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