LTC3555 Datasheet, PDF (27/32 Pages) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

LTC3555/LTC3555-X
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC3555 family provides temperature qualified charg-
ing if a grounded thermistor and a bias resistor are con-
nected to NTC. By using a bias resistor whose value is equal
to the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are pre-programmed
to approximately 40°C and 0°C, respectively (assuming
a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be ad-
justed by either a modification of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modified but not both. The
other trip point will be determined by the characteristics
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-N1002F, used
in the following examples, has a nominal value of 10k
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_NTC|COLD = Value of thermistor at the cold trip point
R_NTC|HOT = Value of the thermistor at the hot trip
point
α_COLD = Ratio of R_NTC|COLD to R25
α_HOT = Ratio of R_NTC|HOT to R25
R_NOM = Primary thermistor bias resistor (see Figure 5a)
R1 = Optional temperature range adjustment resistor
(see Figure 5b)
The trip points for the LTC3555 family’s temperature quali-
fication are internally programmed at 0.349 • V_BUS for the
hot threshold and 0.765 • V_BUS for the cold threshold.
V_BUS
R_NOM
10k
NTC
3
V_BUS
LTC3555/LTC3555-X
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
T
R_NTC
10k
–
0.349 • V_BUS
+
TOO_HOT
+
0.1V
–
(5a)
NTC_ENABLE
3555 F05a
V_BUS
R_NOM
10.5k
NTC
3
V_BUS
LTC3555/LTC3555-X
NTC BLOCK
0.765 • V_{BUS –}
+
TOO_COLD
R1
1.27k
T
R_NTC
10k
–
0.349 • V_BUS
+
+
0.1V
–
TOO_HOT
NTC_ENABLE
(5b)
3555 F05b
Figure 5. NTC Circuits
Therefore, the hot trip point is set when:
R_NTC|HOT
R_NOM + R_NTC|HOT
• V_BUS
= 0.349 • V_BUS
and the cold trip point is set when:
R_NTC|COLD
R_NOM + R_NTC|COLD
•
V_BUS
= 0.765 •
V_BUS
Solving these equations for R_NTC|COLD and R_NTC|HOT
results in the following:
R_NTC|HOT = 0.536 • R_NOM
and
3555fd
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