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LTC4155

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 I2C port. Selection

levels for the C/x comparator of 24mV, 60mV, 120mV

and 240mV are available by I2C 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 I2C 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 VRECHRG (typically 97.6% of the programmed

VFLOAT). The termination safety timer will also 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. 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

I2C 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, RNTC, between the NTC pin

and ground, and a bias resistor, RBIAS, between NTCBIAS

and NTC, as shown in Figure 8. RBIAS should be a 1%

resistor with a value equal to the value of the chosen NTC

thermistor at 25°C (r25).

The thermistor measurement result is available via I2C 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 I2C port is updated at the end of each sample

period. The low duty cycle of thermistor bias reduces

OPERATION

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