LTC3555 Linear Technology, LTC3555 Datasheet - Page 27

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N011-N1003F, used
in the following examples, has a nominal value of 100k
and follows the Vishay “Curve 1” resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the Thermistor at 25°C
R
R
point
r
r
R
R1 = Optional temperature range adjustment resistor
(see Figure 5b)
V
COLD
HOT
BUS
NTC|COLD
NTC|HOT
NOM
T
R
100k
NOM
NTC
R
100k
= Ratio of R
3
NTC
= Primary thermistor bias resistor (see Figure 5a)
= Ratio of R
= Value of the thermistor at the hot trip
= Value of thermistor at the cold trip point
V
BUS
0.349 • V
0.765 • V
NTC|COLD
0.1V
NTC|COLD
BUS
BUS
(5a)
–
–
–
+
+
+
NTC BLOCK
to R25
LTC3555
to R25
TOO_COLD
TOO_HOT
NTC_ENABLE
3555 F05a
Figure 5. NTC Circuits
The trip points for the LTC3555’s temperature qualifi ca-
tion are internally programmed at 0.349 • V
threshold and 0.765 • V
Therefore, the hot trip point is set when:
and the cold trip point is set when:
Solving these equations for R
results in the following:
and
By setting R
in r
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.
R
R
R
R
HOT
V
NTC|HOT
NTC|COLD
BUS
NOM
NOM
T
R
105k
R1
12.7k
= 0.536 and r
R
R
NOM
NTC
R
100k
3
NTC
NTCHOT
NTC COLD
+
+
R
R
NOM
|
|
= 0.536 • R
NTCHOT
NTC COLD
= 3.25 • R
V
BUS
|
|
equal to R25, the above equations result
0.349 • V
0.765 • V
COLD
0.1V
•
•
V
BUS
NOM
BUS
NOM
BUS
V
BUS
BUS
= 3.25. Referencing these ratios
(5b)
for the cold threshold.
= 0 349
–
+
–
+
+
–
= 0 765
NTC BLOCK
LTC3555
.
NTC|COLD
.
•
•
V
LTC3555
V
BUS
BUS
and R
BUS
TOO_COLD
TOO_HOT
NTC_ENABLE
for the hot
NTC|HOT
27
3555 F05b
3555f