Datasheet

LTC4155
31
4155fc
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 LTC4155 has a
full capacity charge indication comparator on the PROG
pin which reports its results via the I
2
C port. Selection
levels for the C/x comparator of 24mV, 60mV, 120mV
and 240mV are available by I
2
C control bits CXSET[1:0]
(Table 13). Recall that the PROG pin servo voltage can be
programmed between 150mV to 1.2V. If the 1.2V servo
setting represents the full-charge rate of the battery (1C),
then the 120mV C/x setting would be equivalent to C/10.
Likewise, the 240mV C/x setting would represent C/5, the
24mV setting C/50 and the 60mV C/20. C/x indication is
masked unless the battery charger is in constant-voltage
mode to prevent false indication caused by limited input
power.
Charge Termination
The battery charge cycle is terminated either at the expira-
tion of a built-in programmable termination safety timer, or
optionally at the full capacity charge indication (C/x). When
the voltage on the battery reaches the user-programmed
float voltage, the safety timer is started and the C/x com-
parator is enabled. After the safety timer expires, charging
of the battery will discontinue and no more current will
be delivered. The safety timer’s default expiration time
of four hours may be altered from one to eight hours in
four settings using the I
2
C bit TIMER[1:0] (Table 10). The
eight hour timer termination setting will also terminate the
charge cycle before the expiration of the eight hour timer
when the battery charge current falls to the programmed
full capacity (C/x) charge indication threshold.
Automatic Recharge
After the battery charger terminates, it will remain off,
drawing only single microamperes of current from the
battery. If the portable product remains in this state long
enough, the battery will eventually self discharge. To en-
sure that the battery is always topped off, a new charge
cycle will automatically begin when the battery voltage
falls below V
RECHRG
(typically 97.6% of the programmed
V
FLOAT
). The termination safety timer will also reset back
to zero. To prevent brief excursions below V
RECHRG
from
resetting the safety timer, the battery voltage must be
below V
RECHRG
for more than 2.5ms. A new charge cycle
will also be initiated if input (USB or WALL) power is
cycled or if the charger is momentarily disabled using the
I
2
C port. The flow chart in Figure 7 represents the battery
charger’s algorithm.
NTC Thermistor Monitor
The LTC4155 includes a 7-bit expanded scale analog to
digital converter (ADC) to monitor the battery temperature
using an external negative temperature coefficient (NTC)
thermistor placed close to the battery pack. To use this
feature, connect the thermistor, R
NTC
, between the NTC pin
and ground, and a bias resistor, R
BIAS
, between NTCBIAS
and NTC, as shown in Figure 8. R
BIAS
should be a 1%
resistor with a value equal to the value of the chosen NTC
thermistor at 25°C (r
25
).
The thermistor measurement result is available via I
2
C port
status reporting, except when the ship-and-store feature
has been activated. When not in ship-and-store mode, the
thermistor is automatically measured approximately every
2.4 seconds. The thermistor measurement result avail-
able to the I
2
C port is updated at the end of each sample
period. The low duty cycle of thermistor bias reduces
OPERATION