LTC3557-1 Linear Technology, LTC3557-1 Datasheet - Page 23

• Manufacturer Part Number: LTC3557-1
• Description: USB Power Manager
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC3557/LTC3557-1 provides temperature qualifi ed
charging if a grounded thermistor and a bias resistor are
connected to NTC. By using a bias resistor whose value is
equal to the room temperature resistance of the thermis-
tor (R25) the upper and lower temperatures are pre-pro-
grammed 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 modifi cation 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 modifi ed 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-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.
The trip points for the LTC3557/LTC3557-1’s temperature
qualifi cation are internally programmed at 0.349 • V
R25 = Value of the Thermistor at 25°C
R
R
point
r
r
R
ure 8)
R1 = Optional temperature range adjustment resistor
(see Figure 9)
COLD
HOT
NTC|COLD
NTC|HOT
NOM
= Ratio of R
= Ratio of R
= Primary thermistor bias resistor (see Fig-
= Value of the thermistor at the hot trip
= Value of thermistor at the cold trip point
NTC|HOT
NTC|COLD
to R25
to R25
VNTC
for the hot threshold and 0.765 • V
threshold.
Therefore, the hot trip point is set when:
and the cold trip point is set when:
Solving these equations for R
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.
By using a bias resistor, R
the hot and cold trip points can be moved in either direc-
tion. The temperature span will change somewhat due to
the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
where r
desired hot and cold trip points. Note that these equations
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 60°C
hot trip point is desired.
R
R
R
R
R
R
HOT
NTC|HOT
NTC|COLD
NOM
NOM
NOM
NOM
= 0.536 and r
R
R
HOT
NTCHOT
NTC COLD
=
=
+
+
R
R
r
0 536
NOM
|
|
3 25
= 0.536 • R
COLD
r
and r
NTCHOT
NTC COLD
HOT
.
= 3.25 • R
.
LTC3557/LTC3557-1
|
|
equal to R25, the above equations result
•
COLD
•
R
R
COLD
25
•
25
•
V
NOM
NOM
V
are the resistance ratios at the
VNTC
NOM
= 3.25. Referencing these ratios
VNTC
NTC|COLD
, different in value from R25,
= 0 349
= 0 765
.
.
and R
VNTC
•
•
V
V
VNTC
VNTC
NTC|HOT
for the cold
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results
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