12

LTC1558-3.3/LTC1558-5

In these cases, the LTC1558 assumes that the backup cell

is exhausted and presets the gas gauge counter to a

default capacity of 128mAhr. It then initiates a recharge

cycle.

Setting the gas gauge to this default value results in a fast

recharge cycle long enough to replenish 1.6 times 128mAhr

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

backup cell is assumed to be fully charged and subsequent

backup/recharge cycles resume normally.

Although the LTC1558 will not fully recharge backup cells

larger than 128mAhr capacity upon power-up, it can still

be used with such cells. Such a cell will be fully replenished

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

ciency improves and allows more backup time from the

Once it reaches full recharge, a cell bigger than 512mAhr

is likely to overrun the gas gauge counter before it runs out

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.

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

incremented by 1mAhr each time the controller comes out

of backup. This ensures that the backup cell is always

replenished with at least a 1mAhr charge every time the

LTC1558 enters backup mode.

Battery Backup Cell Trickle Charge

When the gas gauge counter reaches zero, the LTC1558

terminates fast recharge and reduces the recharge current

to the user-programmed trickle current level. The LTC1558

provides a trickle current that the user can program from

50

µA 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,

current is internally multiplied to feed back ten times the

trickle charges only after the completion of the fast re-

charge cycle, the backup cell voltage should be very close

the trickle charge current to approximately 150

µA.

47

µF

I

10I

1.2V

NiCd

CELL

SW

CTL

0.5V

1558 F04

11

×

1

×

LTC1558

+

–

+

+

–

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