LTC4088 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4088' PDF : 24 Pages View PDF
LTC4088
Applications Information
From the Vishay Curve 1 R-T characteristics, rHOT is 0.2488
at 60°C. Using the above equation, RNOM should be set to
46.4k. With this value of RNOM, 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 4b. The following formulas can be used
to compute the values of RNOM and R1:
RNOM
=
rCOLD − rHOT
2.714
• R25
R1= 0.536 • RNOM − rHOT • R25
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
RNOM
=
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
The USB specification allows at most 10µF of downstream
capacitance to be hot-plugged into a USB hub. In most
LTC4088 applications, 10µF should be enough to provide
adequate filtering on VBUS. If more capacitance is required,
the following circuit can be used to soft-connect additional
capacitance.
In this circuit, capacitor C1 holds MP1 off when the cable
is first connected. Eventually the bottom plate of C1 dis-
charges to GND, applying increasing gate support to MP1.
The long time constant of R1 and C1 prevent the current
from building up in the cable too fast, thus dampening
out any resonant overshoot.
5V USB
INPUT USB CABLE
MP1
Si2333
C1
100nF
R1
40k
VBUS
C2 LTC4088
GND
4088 F05
Figure 5. USB Soft-Connect Circuit
VBUS
RNOM
100k
NTC
1
VBUS
LTC4088
NTC BLOCK
0.765 • VBUS –
+
T
RNTC
100k
–
0.349 • VBUS
+
+
0.1V
–
TOO_COLD
VBUS
RNOM
105k
NTC
1
VBUS
LTC4088
NTC BLOCK
0.765 • VBUS –
+
TOO_HOT
NTC_ENABLE
R1
12.7k
T
RNTC
100k
–
0.349 • VBUS
+
+
0.1V
–
4088 F04a
Figure 4. NTC Circuits
TOO_COLD
TOO_HOT
NTC_ENABLE
4088 F04b
4088fb
19
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