LTC3553-2_15 Datasheet, PDF (19/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
20 ^VBUS
R_NOM
105k

t
7_BUS
(NTC RISING)
–
NTC
15
+
R1
12.7k
R_NTC
100k
–

t
7_BUS
(NTC FALLING) ₊
TOO_COLD
TOO_HOT
+

t
7_BUS
(NTC FALLING)
–
NTC_ENABLE
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Figure 2. NTC Thermistor Circuit With Additional Bias Resistor
The trip points for the LTC3553-2’s temperature qualifica-
tion are internally programmed at 0.35 • V_BUS for the hot
threshold and 0.76 • V_BUS for the cold threshold.
Therefore, the hot trip point is set when:
R _NTC|HOT
R_NOM +R_NTC|HOT
• V_BUS
=
0.35 • V_BUS
and the cold trip point is set when:
R _NTC|COLD
R_NOM +R_NTC|COLD
• V _BUS
=
0.76 • V _BUS
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
direction. The temperature span will change somewhat due
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
0.538
• R25
R _NOM
=
r _COLD
3.17
• 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 independently set, the other is determined by the de-
fault 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 temperature increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 2. 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 2 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
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