LTC3558_15 Datasheet, PDF (18/32 Pages) Linear Technology – Linear USB Battery Charger with Buck and Buck-Boost Regulators

LTC3558
APPLICATIONS INFORMATION
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.
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.
20 ^VCC
R_NOM
100k
NTC BLOCK
0.765 • V_CC
(NTC RISING) _–
NTC
+
17
R_NTC
100k
–
0.349 • V_CC
(NTC FALLING) ₊
TOO_COLD
TOO_HOT
+
0.017 • V_CC
–
(NTC FALLING)
NTC_ENABLE
3558 F03
Figure 3. Typical NTC Thermistor Circuit
20 ^VCC
R_NOM
105k
0.765 • V_CC
(NTC RISING) _–
NTC
+
17
R1
12.7k
R_NTC
100k
–
0.349 • V_CC
(NTC FALLING) ₊
TOO_COLD
TOO_HOT
+
0.017 • V_CC
–
(NTC FALLING)
NTC_ENABLE
3558 F04
Figure 4. NTC Thermistor Circuit with Additional Bias Resistor
3558f
18