Datasheet
LTC4098-3.6
25
409836f
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 70°C
hot trip point is desired.
From the Vishay curve 1 R-T characteristics,
a
HOT
is
0.1753 at 70°C. Using the previous equation, R
NOM
should
be set to 70.4k. With this value of R
NOM
, the cold trip point
is 7°C. Notice that the span is now 63°C rather than the
previous 60°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 9b. The following formulas can be used
to compute the values of R
NOM
and R1:
RR
RR R
NOM
COLD HOT
NOM HOT
=
=
αα
α
–
.
•
.• –•
301
25
1025 255
For example, to set the trip points to 0°C and 70°C with
a Vishay curve 1 thermistor choose:
Rk
k
NOM
==
32601753
301
100 102 5
.–.
.
•.
the nearest 1% value is 102k:
R1 = 0.25 • 102k – 0.1753 • 100k = 7.97k
the nearest 1% value is 8.06k. The final circuit is shown
in Figure 9b and results in an upper trip point of 70°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
LTC4098-3.6 applications, 10µF should be enough to
provide adequate filtering on V
BUS
.
If more capacitance is required, the OVP circuit will provide
adequate soft-connect time to prevent excessive inrush
currents. An additional 22µF on the V
BUS
pin will gener-
ally contribute less than 100mA to the hot-plug inrush
current.
Voltage overshoot on V
BUS
may sometimes be observed
when connecting the LTC4098-3.6 to a lab power supply.
This overshoot is caused by long leads from the power
supply to V
BUS
. Twisting the wires together from the sup-
ply to V
BUS
can greatly reduce the parasitic inductance
of these long leads, and keep the voltage at V
BUS
to safe
levels. USB cables are generally manufactured with the
power leads in close proximity, and thus fairly low parasitic
inductance.
APPLICATIONS INFORMATION