LTC3567 Datasheet, PDF (23/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter with I2C Control

LTC3567
APPLICATIONS INFORMATION
R_NTC|HOT = Value of 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 4a)
R1 = Optional temperature range adjustment resistor
(see Figure 4b)
The trip points for the LTC3567’s temperature qualifica-
tion are internally programmed at 0.349 • V_BUS for the hot
threshold and 0.765 • V_BUS for the cold threshold.
Therefore, the hot trip point is set when:
R_NTC|HOT
R_NOM + R_NTC|HOT
• V_BUS
= 0.349 • V_BUS
and the cold trip point is set when:
R_NTC|COLD
R_NOM + R_NTC|COLD
• V_BUS
= 0.765 • V_BUS
Solving these equations for R_NTC|COLD and R_NTC|HOT results
in the following:
R_NTC|HOT = 0.536•R_NOM
and
R_NTC|COLD = 3.25•R_NOM
By setting R_NOM equal to R25, the above equations result
in r_HOT = 0.536 and r_COLD = 3.25. 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
0.536
•
R25
R_NOM
=
r_COLD
3.25
• 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 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 LTC3567 during a hot insertion,
3567f
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