LTC1558-3.3/LTC1558-5
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APPLICATIONS INFORMATION
internal logic to ensure that the low cell voltage reset is
triggered only if the CTL pin is between 0.9V and 0.25V.
This will prevent a pushbutton reset (which pulls CTL
below 250mV) from being mistaken as a low cell voltage
condition. Unusual situations where the NiCd cell voltage
dropsdrasticallybelow0.25VwillalsotriggerUVLO,since
the LTC1558 will treat this as a “hard” reset after 2
seconds.
The LTC1558’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.
Backup Cell Fast Recharge
The LTC1558 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 estimate battery
charge by counting switch pulses. The gas gauge counts
up from zero as charge is removed from the backup cell in
backup mode. It takes roughly 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 = 5V, the gas
gauge divider will increment by one count every 7.5µs
while the boost converter is running. Full count is reached
afterapproximately2.2hours,equivalenttoabout512mAhr
of charge.
AnoptionalLOBAToutput,availableinthe16-pinGNorSO
package, can be used to signal the system when the cell
voltage falls below 1V, giving an early warning that the
backup cell is heavily discharged. The LOBAT output is
disabled when the LTC1558 is in trickle charge mode
because the CTL pin is pulled to 0.5V by the LTC1558.
Fault Protection and Thermal Limit
The LTC1558’s boost converter incorporates two internal
timers that turn off the switch transistors if the inductor
charge or discharge time gets abnormally long.
The inductor charge time may get abnormally long if the
NiCdcellvoltagedropsbelow0.25Vwithouttriggeringthe
0.25V < VBAT < 0.9V low cell voltage comparator. In this
case, the NiCd cell is assumed to be damaged and the
LTC1558’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 LTC1558
will enter UVLO either when VCC drops below
(VCC(RATED VOLTAGE) –9%) or after the LTC1558 detects
CTL lower than 0.25V for 2 seconds, in which case “hard”
reset occurs.
Upon entering recharge mode (after the main battery is
restored) the LTC1558 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
gauge counter at a rate designed to replace about 160% of
the charge previously removed from the backup cell.
When the gas gauge counter reaches zero, the LTC1558
reduces the charging current at the SW pin to the user-
programmed trickle charge current level.
The discharging time can also get abnormally long if a
serious overload condition occurs during switching. The
timerwillshutofftheP-channelpasstransistorafter10µs,
protecting the boost converter. The LTC1558 will end up
in UVLO as VCC drops below (VCC(RATED VOLTAGE) –9%).
Under some circumstances, the LTC1558 can exit the
backup mode with invalid gas gauge contents. This can
occur under three possible conditions:
a) The backup cell was completely exhausted during a
backup cycle and the LTC1558 entered UVLO.
In addition, the LTC1558 is protected for safe area opera-
tionwithaninternalthermalshutdowncircuit.Ifthedevice
is overloaded for a long period of time, the thermal
shutdown circuit forces the LTC1558 into UVLO. The
threshold temperature for thermal shutdown is typically
155°C.
b) Thebackupcellwasreplacedwhilethemainsupplywas
disabled.
c) A backup cycle was terminated prematurely by a “hard”
reset or an output overload.
11
Linear [ Linear ]
