Datasheet

LTC3566/LTC3566-2
23
3566fb
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:
Figure 4. NTC Circuits
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
0.017 • V
BUS
NTC_ENABLE
3566 F04a
LTC3566/LTC3566-2
NTC BLOCK
(a)
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
3
V
BUS
V
BUS
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
V
BUS
V
BUS
NTC_ENABLE
3566 F04b
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
3
LTC3566/LTC3566-2
NTC BLOCK
(b)
0.017 • V
BUS
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 fi nal 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 family during a hot
insertion, it is best to have a low voltage coeffi cient capacitor
at the V
BUS
pin to the LTC3566 family. 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