LTC4090(RevA) 查看數據表（PDF） - Linear Technology
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LTC4090 Datasheet PDF : 28 Pages
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LTC4090/LTC4090-5
OPERATION
For example, if the full charge current is programmed
to 500mA with a 100k PROG resistor the ⎯C⎯H⎯R⎯G pin will
change state at a battery charge current of 50mA.
Note: The end-of-charge (EOC) comparator that moni-
tors the charge current latches its decision. Therefore,
the first time the charge current drops below 10% of the
programmed full charge current while in constant volt-
age mode, it will toggle ⎯C⎯H⎯R⎯G to a high impedance state.
If, for some reason the charge current rises back above
the threshold, the ⎯C⎯H⎯R⎯G pin will not resume the strong
pull-down state. The EOC latch can be reset by a recharge
cycle (i.e., VBAT drops below the recharge threshold) or
toggling the input power to the part.
Automatic Recharge
After the battery charger terminates, it will remain off
drawing only microamperes of current from the battery. If
the product remains in this state long enough, the battery
will eventually self discharge. To ensure that the battery is
always topped off, a charge cycle will automatically begin
when the battery voltage falls below VRECHRG (typically
4.1V). To prevent brief excursions below VRECHRG from
resetting the safety timer, the battery voltage must be
below VRECHRG for more than a few milliseconds. The
charge cycle and safety timer will also restart if the IN
UVLO cycles low and then high (e.g. IN, is removed and
then replaced).
Thermal Regulation
To prevent thermal damage to the IC or surrounding
components, an internal thermal feedback loop will
automatically decrease the programmed charge current
if the die temperature rises to approximately 105°C.
Thermal regulation protects the LTC4090/LTC4090-5
from excessive temperature due to high power operation
or high ambient thermal conditions and allows the user
to push the limits of the power handling capability with a
given circuit board design without risk of damaging the
LTC4090/LTC4090-5 or external components. The benefit
of the LTC4090/LTC4090-5 thermal regulation loop is that
charge current can be set according to actual conditions
rather than worst-case conditions with the assurance that
the battery charger will automatically reduce the current
in worst-case conditions.
Undervoltage Lockout
An internal undervoltage lockout circuit monitors the input
voltage (IN) and the output voltage (OUT) and disables
either the input current limit or the battery charger circuits
or both. The input current limit circuitry is disabled until
VIN rises above the undervoltage lockout threshold and VIN
exceeds VOUT by 50mV. The battery charger circuits are dis-
abled until VOUT exceeds VBAT by 50mV. Both undervoltage
lockout comparators have built-in hysteresis.
NTC Thermistor
The battery temperature is measured by placing a nega-
tive temperature coefficient (NTC) thermistor close to
the battery pack. To use this feature connect the NTC
thermistor, RNTC, between the NTC pin and ground and a
bias resistor, RNOM, from VNTC to NTC. RNOM should be
a 1% resistor with a value equal to the value of the chosen
NTC thermistor at 25°C (denoted R25C).
The LTC4090/LTC4090-5 will pause charging when the
resistance of the NTC thermistor drops to 0.41 times the
value of R25C or approximately 4.1k (for a Vishay “Curve
2” thermistor, this corresponds to approximately 50°C).
The safety timer also pauses until the thermistor indicates
a return to a valid temperature. As the temperature drops,
the resistance of the NTC thermistor rises. The LTC4090/
LTC4090-5 are also designed to pause charging (and timer)
when the value of the NTC thermistor increases to 2.82
times the value of R25C. For a Vishay “Curve 2” thermistor
this resistance, 28.2k, corresponds to approximately 0°C.
The hot and cold comparators each have approximately
3°C of hysteresis to prevent oscillation about the trip point.
Grounding the NTC pin disables all NTC functionality.
4090fa
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