LTC3557 Datasheet, PDF (23/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC3557/LTC3557-1 provides temperature qualified
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 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
adjusted 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
temperature 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_NTC|COLD = Value of thermistor at the cold trip point
R_NTC|HOT = Value of the thermistor at the hot trip
point
r_COLD = Ratio of R_NTC|COLD to R25
r_HOT= Ratio of R_NTC|HOT to R25
R_NOM = Primary thermistor bias resistor (see Figure 8)
R1 = Optional temperature range adjustment resistor
(see Figure 9)
The trip points for the LTC3557/LTC3557-1’s temperature
qualification are internally programmed at 0.349 • V_VNTC
for the hot threshold and 0.765 • V_VNTC for the cold
threshold.
Therefore, the hot trip point is set when:
R_NTC|HOT
R_NOM + R_NTC|HOT
• V_VNTC
= 0.349 • V_VNTC
and the cold trip point is set when:
R_NTC|COLD
R_NOM + R_NTC|COLD
•
V_VNTC
=
0.765 •
V_VNTC
Solving these equations for R_NTC|COLD and R_NTC|HOT results
in the following:
R_NTC|HOT = 0.536 • R_NOM
and
R_NTC|COLD = 3.25 • R_NOM
By setting R_NOM equal to R25, the above equations result
in r_HOT = 0.536 and r_COLD = 3.25. Referencing these ratios
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_NOM, different in value from
R25, the hot and cold trip points can be moved in either
direction. 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:
R_NOM
=
r_HOT
0.536
• R25
R_NOM
=
r_COLD
3.25
•
R25
where r_HOT and r_COLD are the resistance ratios at the
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.
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