Datasheet
LTC4088
19
4088fb
applicaTions inForMaTion
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 “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 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
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 V
BUS
. 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.
Figure 5. USB Soft-Connect Circuit
Figure 4. NTC Circuits
–
+
–
+
R
NOM
100k
R
NTC
100k
NTC
0.1V
NTC_ENABLE
4088 F04a
LTC4088
NTC BLOCK
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
1
V
BUS
V
BUS
T
–
+
–
+
R
NOM
105k
R
NTC
100k
R1
12.7k
NTC
V
BUS
V
BUS
0.1V
NTC_ENABLE
4088 F04b
TOO_COLD
TOO_HOT
0.765 • V
BUS
0.349 • V
BUS
–
+
1
LTC4088
NTC BLOCK
T
R1
40k
5V USB
INPUT
4088 F05
C1
100nF
C2
MP1
Si2333
USB CABLE
V
BUS
GND
LTC4088