LTC3566-2_15 Datasheet, PDF (22/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
APPLICATIONS INFORMATION
and the V_BUS current limit is satisfied. Programming the
battery charger for more current than is available will
not cause the average input current limit to be violated.
It will merely allow the battery charger to make use of
all available power to charge the battery as quickly as
possible, and with minimal power dissipation within the
battery charger.
Alternate NTC Thermistors and Biasing
The LTC3566 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 follow.
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 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 4a)
R1 = Optional temperature range adjustment resistor
(see Figure 4b)
The trip points for the LTC3566 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.
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
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 nonlinear 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
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