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155833 Datasheet PDF : 20 Pages
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LTC1558-3.3/LTC1558-5
APPLICATIONS INFORMATION
Overview
The LTC1558 is a versatile backup battery control system
designed to provide all the functions necessary to imple-
ment a complete, highly integrated backup system within
a single chip. It allows the system to maintain its rated
supply voltage during backup, offering maximum system
design flexibility. The LTC1558 allows the use of a low cost
rechargeable NiCd cell for backup, eliminating the need for
expensive, replaceable 4.5V lithium backup cells.
The LTC1558 includes an onboard boost converter de-
signed to generate an adjustable voltage (3V to 10V) from
a single 1.2V NiCd cell. This voltage is connected to the
system’s DC/DC converter input, enabling the system to
continue operation when the main battery fails. A “smart”
recharging circuit uses an accumulating gas gauge to
measure the charge extracted from the backup battery
during a backup cycle. This measured charge is then
replaced in a fast recharge cycle, without wasting excess
power or overcharging the backup cell. An externally
adjustable trickle charge circuit maintains the cell charge
after the fast charge cycle has completed, minimizing
drain from the main battery during standby.
Included in the LTC1558 is a complete backup circuit that
monitors the main system power and automatically
switches in the backup circuit as the primary power supply
falls away (due to a weak or disconnected main battery).
The LTC1558 also performs VCC supervisory functions
during normal system operations. An LTC1558-3.3 moni-
tors a 3.3V supply voltage at its VCC pin whereas an
LTC1558-5 monitors a 5V supply at its VCC pin. In both
cases, the LTC1558 derives power for the majority of the
internal circuitry (except for the boost converter) from the
VCC pin. Table 1 shows the signal conditions for the
LTC1558’s various operating modes. Note that VCC in
Table 1 refers to the rated VCC voltage, 3.3V or 5V.
Boost Converter Operation
The LTC1558 uses an onboard synchronous boost con-
verter with a fixed peak current architecture that provides
a simple and flexible system solution while eliminating the
need for conventional frequency compensation. The boost
converter’s output, set by the external divider connected
to the FB pin, supports the main system regulator during
Table 1
OPERATING MODES
UVLO Reset
Pushbutton Reset
UVLO Reset Recovery
Backup Mode Activation
Backup Mode Exit
Boost Converter Activation
Boost Converter Deactivation
CONDITIONS
1V < VCC < VCC (Rated Value) – 9%
or VBAT < 0.9V
VCTL < 250mV
VCC > VCC (Rated Value) – 6%
VFB < (VREF – 7.5%)
VFB > (VREF – 6%)
VFB < (VREF – 7.5%)
VFB > (VREF – 7.5%)
backup. It can supply a minimum backup power of
100mW. The boost converter operates in a modified pulse
skipping mode; each switch cycle transfers a known
amount of charge from the backup cell to the regulated
output. This prevents uncontrolled discharge of the backup
cell and allows the LTC1558 to accurately measure the
charge removed from the backup cell by counting the
charge pulses.
The LTC1558 enters backup mode when the main battery
voltage drops. As shown in Figure 1, the main battery
voltage is scaled down by an external resistor divider and
fed to the LTC1558’s backup comparators. These com-
pare the scaled voltage with an internal trimmed VREF
(1.272V), switching the LTC1558 into backup mode
when VFB drops 7.5% below VREF. Upon entering backup
mode, the BACKUP pin is asserted and the internal boost
converter turns on. The BACKUP signal is used to turn off
the external P-channel MOSFET, isolating the main bat-
tery from the LTC1558 and the system regulator’s input.
The LTC1558’s boost converter will charge the input
capacitor CIN of the system regulator until VFB rises
above (VREF – 7.5%).
TO SYSTEM REGULATOR INPUT
LTC1588
MAIN
+
BATTERY
CIN R1
FB
R2
VREF
BOOST
CONVERTER
VBAK
BACKUP
LOGIC
BACKUP
1558 F01
Figure 1. Typical LTC1558 Connection
8
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