LTC4099EUDC View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4099EUDC' PDF : 36 Pages View PDF
LTC4099
Operation
Recall, however, that in many cases the actual battery
charge current, IBAT, will be lower than the programmed
current, ICHG, due to limited input power available and
prioritization of the system load drawn from VOUT.
Constant-Voltage
Once the battery terminal voltage reaches the preset float
voltage, the battery charger will hold the voltage steady
and the charge current will decrease naturally toward
zero. Two voltage settings, 4.100V and 4.200V, are avail-
able for final float voltage selection via the I2C port. For
applications that require as much run time as possible,
the 4.200V setting can be selected. For applications that
seek to extend battery life, the LTC4099’s default setting
of 4.100V should be used.
Full Capacity Charge Indication (C/x)
Since the PROG pin always represents the actual charge
current flowing, even in the constant-voltage phase of
charging, the PROG pin voltage represents the battery’s
state-of-charge during that phase. The LTC4099 has a full
capacity charge indication comparator on the PROG pin
which reports its results via the I2C port. Selection levels
for the C/x comparator of 50mV, 100mV, 200mV and 500mV
are available by I2C control. Recall that the PROG pin servo
voltage can be programmed from 500mV to 1.2V. If the 1V
servo setting represents the full charge rate of the battery
(1C), then the 100mV C/x setting would be equivalent to
C/10. Likewise the 200mV C/x setting would represent
C/5 and the 500mV setting C/2.
Charge Termination
The battery charger has a built-in termination safety
timer. When the voltage on the battery reaches the user-
programmed float voltage of 4.100V or 4.200V, the safety
timer is started. After the safety timer expires, charging of
the battery will discontinue and no more current will be
delivered. The safety timer’s default ending time of four
hours may be altered from one to eight hours in one-hour
increments by accessing the I2C port.
Automatic Recharge
After the battery charger terminates, it will remain off,
drawing only microamperes of current from the battery.
18
If the portable product remains in this state long enough,
the battery will eventually self discharge. To ensure that
the battery is always topped off, a new charge cycle will
automatically begin when the battery voltage falls below
VRECHRG (typically 4.100V for the 4.200V float voltage
setting and 4.000V for the 4.100V float voltage setting). In
the event that the safety timer is running when the battery
voltage falls below VRECHRG, it will reset back to zero. To
prevent brief excursions below VRECHRG from resetting the
safety timer, the battery voltage must be below VRECHRG for
more than 2.5ms. The charge cycle and safety timer will
also restart if the VBUS UVLO cycles LOW and then HIGH
(e.g., VBUS or WALL is removed and then replaced) or if
the charger is momentarily disabled using the I2C port.
The flow chart in Figure 4 represents the battery charger’s
algorithm.
Thermistor Measurement
The battery temperature is measured by placing a nega-
tive temperature coefficient (NTC) thermistor close to
the battery pack. The thermistor circuitry is shown in the
Block Diagram.
To use this feature, connect the thermistor between the NTC
pin and ground and a bias resistor from NTCBIAS to NTC.
The bias resistor should be a 1% resistor with a value equal
to the value of the chosen thermistor at 25°C (R25).
The LTC4099 will pause charging when the resistance of
the thermistor drops to 0.484 times the value of R25 or
4.84k for a 10k thermistor. For a Vishay curve 2 thermis-
tor, this corresponds to approximately 45°C. If the battery
charger is in constant-voltage (float) mode, the safety
timer also pauses until the thermistor indicates a return
to valid temperature. The LTC4099 is also designed to
pause charging when the value of the thermistor increases
to 2.816 times the value of R25. For a Vishay curve 2
10k thermistor, this resistance, 28.16k, corresponds to
approximately 0°C. The hot and cold comparators each
have approximately 4°C of hysteresis to prevent oscillation
about the trip point.
If the curve 2 thermistor’s temperature rises above 60°C,
its value will drop to 0.2954 times R25. When this happens,
the LTC4099 detects this critically high temperature and
indicates it via the I2C port (see Table 7). If this condition
4099fd
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