LTC1558-3.3/LTC1558-5
U
W U U
APPLICATIONS INFORMATION
Inthesecases,theLTC1558assumesthatthebackupcell
is exhausted and presets the gas gauge counter to a
default capacity of 128mAhr. It then initiates a recharge
cycle.
Battery Backup Cell Trickle Charge
When the gas gauge counter reaches zero, the LTC1558
terminatesfastrechargeandreducestherechargecurrent
totheuser-programmedtricklecurrentlevel.TheLTC1558
provides a trickle current that the user can program from
50µA to 2mA. The trickle current is set by an external
resistor from the positive terminal of the backup cell to the
CTL pin. In trickle charge mode, CTL is regulated to 0.5V,
resulting in a CTL pin current of (VBAT – 0.5)/REXT. This
current is internally multiplied to feed back ten times the
REXT current into the backup battery. Since the LTC1558
trickle charges only after the completion of the fast re-
charge cycle, the backup cell voltage should be very close
to 1.2V. This simplifies the calculation of the REXT resistor
value. For example, a 47k resistor from VBAT to CTL sets
the trickle charge current to approximately 150µA.
Setting the gas gauge to this default value results in a fast
rechargecyclelongenoughtoreplenish1.6times128mAhr
of charge into the backup cell (13.9 hours). If the backup
cell is actually exhausted, it will be fully recharged. If the
battery is partially or fully charged, or is significantly
smaller than 128mAhr capacity, the extra charging time
will be wasted. However, the LTC1558’s 15mA fast charge
current should not be high enough to damage the cell.
Once the full-count recharge has been completed, the
backupcellisassumedtobefullychargedandsubsequent
backup/recharge cycles resume normally.
Although the LTC1558 will not fully recharge backup cells
larger than 128mAhr capacity upon power-up, it can still
beusedwithsuchcells.Suchacellwillbefullyreplenished
by the subsequent trickle charge cycle. Under most con-
ditions, even a partially charged large cell will still be
capable of supporting several hours of backup. For ex-
ample, a small 60mAhr button cell can back up the system
for 20 minutes at an output power of 100mW. Note that at
lower programmed VBAK values, the boost converter effi-
ciency improves and allows more backup time from the
same cell compared to a higher VBAK value.
V
CC
10I
SW
1.2V
NiCd
CELL
+
I
47µF
R
EXT
CTL
1×
11×
–
Once it reaches full recharge, a cell bigger than 512mAhr
islikelytooverrunthegasgaugecounterbeforeitrunsout
of charge during an extended backup cycle. The LTC1558
gas gauge counter will not roll over if this occurs; it will
stay at full count until the backup cycle ends and then
partially recharge the cell with a full count cycle as above.
+
+
0.5V
–
LTC1558
1558 F04
Very short backup cycles (<32s) may not extract enough
charge from the backup cell to increment the gas gauge
counter at all. To ensure that the backup cell is not slowly
“nibbled” away, the gas gauge counter is always
incrementedby1mAhreachtimethecontrollercomesout
of backup. This ensures that the backup cell is always
replenished with at least a 1mAhr charge every time the
LTC1558 enters backup mode.
Figure 4. Trickle Current Charger
Undervoltage Lockout
The LTC1558 includes an undervoltage lockout (UVLO)
system that ensures that the system will shutdown grace-
fully if the backup cell is exhausted or overloaded. As
described in the previous section, the LTC1558 will
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Linear [ Linear ]
