LTC3576 Datasheet, PDF (38/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
APPLICATIONS INFORMATION
The trip points for the LTC3576/LTC3576-1’s temperature
qualification are internally programmed at 0.349 • NTCBIAS
for the hot threshold and 0.765 • NTCBIAS for the cold
threshold.
Therefore, the hot trip point is set when:
R_{NTC HOT}
• NTCBIAS = 0.349 • NTCBIAS
R_NOM + R_NTCHOT
And the cold trip point is set when:
R_{NTCCOLD • NTCBIAS = 0.765 • NTCBIAS}
R_NOM + R_{NTC COLD}
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 fol-
lowing equations can be used to 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.
38
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, r_COLD is 1.436
and 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 inde-
pendently programmed by using an additional bias resistor
as shown in Figure 10. 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 10 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
Hot Plugging and USB Inrush Current Limiting
The overvoltage protection circuit provides inrush current
limiting due to the long time it takes for OVGATE to fully
enhance the N-channel MOSFET. This prevents the current
from building up in the cable too quickly thus dampen-
ing out any resonant overshoot on V_BUS. It is possible to
observe voltage overshoot on V_BUS when connecting the
LTC3576/LTC3576-1 to a lab power supply if the overvoltage
protection circuit is not used. This overshoot is caused by
the inductance of the long leads from the power supply to
V_BUS. Twisting the wires together from the supply to V_BUS
can greatly reduce the parasitic inductance of these long
leads keeping the voltage at V_BUS to safe levels. USB cables
are generally manufactured with the power leads in close
proximity, and thus have fairly low parasitic inductance.
3576f