LTC3566 Datasheet, PDF (23/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566
APPLICATIONS INFORMATION
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 in-
dependently programmed by using an additional bias
resistor as shown in Figure 4b. 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 4b and results in an upper trip point of 45°C and
a lower trip point of 0°C.
USB Inrush Limiting
When a USB cable is plugged into a portable product,
the inductance of the cable and the high-Q ceramic input
capacitor form an L-C resonant circuit. If the cable does
not have adequate mutual coupling or if there is not much
impedance in the cable, it is possible for the voltage at
the input of the product to reach as high as twice the USB
voltage (~10V) before it settles out. To prevent excessive
voltage from damaging the LTC3566 during a hot insertion,
it is best to have a low voltage coefficient capacitor at the
V_BUS pin to the LTC3566. This is achievable by selecting an
MLCC capacitor that has a higher voltage rating than that
required for the application. For example, a 16V, X5R, 10μF
capacitor in a 1206 case would be a more conservative
choice than a 6.3V, X5R, 10μF capacitor in a smaller 0805
V_BUS
R_NOM
100k
NTC
3
V_BUS
LTC3566
NTC BLOCK
0.765 • V_{BUS –}
+
R_NTC
100k
–
0.349 • V_BUS
+
+
0.017 • V_BUS
–
(a)
TOO_COLD
V_BUS
R_NOM
105k
NTC
3
V_BUS
LTC3566
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_HOT
NTC_ENABLE
R1
12.7k
R_NTC
100k
–
0.349 • V_BUS
+
+
0.017 • V_BUS
–
3566 F04a
(b)
Figure 4. NTC Circuits
TOO_COLD
TOO_HOT
NTC_ENABLE
3566 F04b
3566fa
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