LTC4090(RevA) 데이터 시트보기 (PDF) - Linear Technology
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LTC4090 Datasheet PDF : 28 Pages
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LTC4090/LTC4090-5
OPERATION
where VPROG is the PROG pin voltage and RPROG is the
total resistance from the PROG pin to ground. Keep in
mind that when the LTC4090/LTC4090-5 are powered
from the IN pin, the programmed input current limit takes
precedence over the charge current. In such a scenario,
the charge current cannot exceed the programmed input
current limit.
For example, if typical 500mA charge current is required,
calculate:
RPROG
=
1V
500mA
•
50,000
=
100k
For best stability over temperature and time, 1% metal
film resistors are recommended. Under trickle charge
conditions, this current is reduced to 10% of the full-
scale value.
The Charge Timer
The programmable charge timer is used to terminate the
charge cycle. The timer duration is programmed by an
external capacitor at the TIMER pin. The charge time is
typically:
tTIMER(hours)
=
CTIMER •RPROG • 3hours
0.1µF • 100k
The timer starts when an input voltage greater than the
undervoltage lockout threshold level is applied or when
leaving shutdown and the voltage on the battery is less than
the recharge threshold. At power-up or exiting shutdown
with the battery voltage less than the recharge threshold,
the charge time is a full cycle. If the battery is greater than
the recharge threshold the timer will not start and charging
is prevented. If after power-up the battery voltage drops
below the recharge threshold, or if after a charge cycle
the battery voltage is still below the recharge threshold,
the charge time is set to one-half of a full cycle.
The LTC4090/LTC4090-5 have a feature that extends charge
time automatically. Charge time is extended if the charge
current in constant current mode is reduced due to load
current or thermal regulation. This change in charge time
is inversely proportional to the change in charge current.
As the LTC4090/LTC4090-5 approach constant voltage
mode the charge current begins to drop. This change in
charge current is due to normal charging operation and
does not affect the timer duration.
Consider, for example, a USB charge condition where
RCLPROG = 2k, RPROG = 100k and CTIMER = 0.1μF. This
corresponds to a three hour charge cycle. However, if the
HPWR input is set to a logic low, then the input current
limit will be reduced from 500mA to 100mA. With no ad-
ditional system load, this means the charge current will
be reduced to 100mA. Therefore, the termination timer
will automatically slow down by a factor of five until the
charger reaches constant voltage mode (i.e. VBAT ap-
proaches 4.2V) or HPWR is returned to a logic high. The
charge cycle is automatically lengthened to account for
the reduced charge current. The exact time of the charge
cycle will depend on how long the charger remains in
constant current mode and/or how long the HPWR pin
remains logic low.
Once a time-out occurs and the voltage on the battery is
greater than the recharge threshold, the charge current
stops, and the ⎯C⎯H⎯R⎯G output assumes a high impedance
state if it has not already done so.
Connecting the TIMER pin to ground disables the battery
charger.
⎯C⎯H⎯R⎯G Status Output Pin
When the charge cycle starts, the ⎯C⎯H⎯R⎯G pin is pulled to
ground by an internal N-channel MOSFET capable of driv-
ing an LED. When the charge current drops below 10%
of the programmed full charge current while in constant
voltage mode, the pin assumes a high impedance state,
but charge current continues to flow until the charge
time elapses. If this state is not reached before the end
of the programmable charge time, the pin will assume a
high impedance state when a time-out occurs. The ⎯C⎯H⎯R⎯G
current detection threshold can be calculated by the fol-
lowing equation:
IDETECT
=
0.1V
RPROG
• 50,000
=
5000V
RPROG
4090fa
16
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