LTC3577-4_15 Datasheet, PDF (26/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
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 the thermistor at the hot trip point
r_COLD = Ratio of R_NTC|COLD to R25
r_HOT = Ratio of R_NTC|HOT to R25
R_NOM = Primary thermistor bias resistor (see Figure 3)
R1 = Optional temperature range adjustment resistor
(see Figure 4)
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.
NTCBIAS
34
R_NOM
100k
NTC
35
NTC BLOCK
0.76 • NTCBIAS
–
+
R_NTC
100k
–
0.35 • NTCBIAS
+
LTC3577-3/
LTC3577-4
TOO_COLD
TOO_HOT
0.26 • NTCBIAS
+
–
BATTERY
OVERTEMP
357734 F03
Figure 3. Typical NTC Thermistor Circuit
NTCBIAS
34
R_NOM
105k
NTC
35
R1
12.7k
R_NTC
100k
NTC BLOCK
0.76 • NTCBIAS
–
+
–
0.35 • NTCBIAS
+
LTC3577-3/
LTC3577-4
TOO_COLD
TOO_HOT
0.26 • NTCBIAS
+
–
BATTERY
OVERTEMP
357734 F04
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
26
357734fb