ltc3566 Linear Technology Corporation, ltc3566 Datasheet - Page 22

• Manufacturer Part Number: ltc3566
• Description: High Ef?ciency Usb Power Manager Plus 1a Buck-boost Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3566
and the V
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 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 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 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 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.
22
R25 = Value of the thermistor at 25°C
R
R
r
r
COLD
HOT
NTC|COLD
NTC|HOT
= Ratio of R
= Ratio of R
BUS
= Value of thermistor at the hot trip point
= Value of thermistor at the cold trip point
current limit is satisfi ed. Programming the
NTC|HOT
NTC|COLD
to R25
to R25
The trip points for the LTC3566’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
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
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
R1 = Optional temperature range adjustment resistor
(see Figure 4b)
R
R
R
R
R
R
HOT
NOM
NTC|HOT
NTC|COLD
NOM
NOM
NOM
NOM
= 0.536 and r
R
R
= Primary thermistor bias resistor (see Figure 4a)
NTC|HOT
NTC|COLD
=
=
+ R
+ R
r
0.536
3.25
NOM
COLD
r
= 0.536 • R
HOT
NTC|HOT
NTC|COLD
= 3.25 • R
equal to R25, the above equations result
• R25
• R25
COLD
• V
• V
NOM
NOM
BUS
BUS
BUS
= 3.25. Referencing these ratios
NOM
for the cold threshold.
NTC|COLD
= 0.349 • V
= 0.765 • V
, different in value from
and R
BUS
BUS
NTC|HOT
BUS
for the hot
results
3566p