LTC1559-3.3/LTC1559-5
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APPLICATIONS INFORMATION
case, the NiCd cell is assumed to be damaged and the
LTC1559’s priority is shutting down the system grace-
fully. In this case, the timer will shut off the N-channel
switch transistor after a maximum charging time (14µs).
The boost converter continues switching but delivers
reduced output power, causing VCC to drop. The LTC1559
enters UVLO if VCC drops below VCC(rated value)
– 9% or if the LTC1559 detects that CTL is lower than
0.25V for two seconds, in which case “hard” reset occurs.
after approximately 2.2 hours, equivalent to about
512mAhr of charge.
Upon entering recharge mode (after the main battery is
restored) the LTC1559 connects a 16mA fast recharge
current source from VCC to the SW pin. At the same time,
an internal free running oscillator counts down the gas
gaugecounterataratedesignedtotypicallyreplace160%
of the charge previously removed from the backup cell.
When the gas gauge counter reaches zero, the LTC1559
reduces the charging current at the SW pin to the user-
programmed trickle charge current level.
The discharge time can also get abnormally long if a
serious overload condition occurs during switching. The
timer shuts off the P-channel pass transistor after 10µs,
protectingtheboostconverter. TheLTC1559entersUVLO
as VCC drops below VCC(rated value) – 9%.
Under some circumstances, the LTC1559 can exit backup
mode with invalid gas gauge contents. This occurs under
three possible conditions:
In addition, the LTC1559 has safe area operation protec-
tionwithaninternalthermalshutdowncircuit.Ifthedevice
is overloaded for a long period of time, the thermal
shutdown circuit forces the LTC1559 into UVLO. The
threshold temperature for thermal shutdown is typically
155°C.
1. The backup cell was completely exhausted during a
backup cycle and the LTC1559 entered UVLO.
2. Thebackupcellwasreplacedwhilethemainsupplywas
disabled.
3. A backup cycle was terminated prematurely by a “hard”
reset or an output overload.
The LTC1559’s boost converter is designed so that no
current drains from the battery to the load during output
short circuit or VCC = 0V conditions. This assures that the
system can be powered down for a long period of time.
Thiseliminatestheriskoffindinganonfunctioningbackup
system upon power-up.
Inthesecases,theLTC1559assumesthatthebackupcell
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 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 is
wasted. However, the LTC1559’s 16mA fast charge cur-
rent is not 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.
Backup Cell Fast Recharge
The LTC1559 includes an onboard gas gauge circuit,
consisting of a 23-bit divider and a 9-bit up/down counter.
The gas gauge logic assumes that the boost converter
uses a 22µH inductor, allowing it to accurately measure
battery charge by counting pulses. The gas gauge counts
up from zero as charge is removed from the backup cell in
backup mode. It takes 8.4 million 165mA boost pulses
(low current mode) to increment the up/down counter by
one count. In high current mode, the 330mA pulses skip
the first two bits of the divider because each 330mA pulse
carries four times as much energy as a 165mA pulse. At
maximum load and VCC = 4.625V (LTC1559-5), the gas
gauge counter will increment by one count every 7.5µs
while the boost converter is running. Full count is reached
Although the LTC1559 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
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Linear [ Linear ]
