LTC3586-1_15 Datasheet, PDF (26/36 Pages) Linear Technology – High Efficiency USB Power Manager with Boost, Buck-Boost and Dual Bucks

LTC3586/LTC3586-1
Applications Information
Over-Programming the Battery Charger
The USB high power specification allows for up to 2.5W to
be drawn from the USB port (5V • 500mA). The PowerPath
switching regulator transforms the voltage at V_BUS to just
above the voltage at BAT with high efficiency, while limiting
power to less than the amount programmed at CLPROG.
In some cases the battery charger may be programmed
(with the PROG pin) to deliver the maximum safe charging
current without regard to the USB specifications. If there
is insufficient current available to charge the battery at the
programmed rate, the PowerPath regulator will reduce
charge current until the system load on V_OUT is satisfied
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 LTC3586/LTC3586-1 provide 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, respec-
tively (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-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|COLD to R25
R_NOM = Primary thermistor bias resistor
(see Figure 6a)
R1 = Optional temperature range adjustment resistor
(see Figure 6b)
The trip points for the LTC3586/LTC3586-1’s temperature
qualification 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.
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