LTC1558-5 Linear Technology, LTC1558-5 Datasheet - Page 8

• Manufacturer Part Number: LTC1558-5
• Description: Backup Battery Controller with Programmable Output
• Manufacturer: Linear Technology

APPLICATIONS
LTC1558-3.3/LTC1558-5
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 V
during normal system operations. An LTC1558-3.3 moni-
tors a 3.3V supply voltage at its V
LTC1558-5 monitors a 5V supply at its V
cases, the LTC1558 derives power for the majority of the
internal circuitry (except for the boost converter) from the
V
LTC1558’s various operating modes. Note that V
Table 1 refers to the rated V
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
8
CC
pin. Table 1 shows the signal conditions for the
U
INFORMATION
U
CC
voltage, 3.3V or 5V.
CC
supervisory functions
W
CC
pin whereas an
CC
pin. In both
U
CC
in
Table 1
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 V
(1.272V), switching the LTC1558 into backup mode
when V
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 C
above (V
Boost Converter Deactivation
Boost Converter Activation
Backup Mode Activation
UVLO Reset Recovery
MAIN
BATTERY
OPERATING MODES
Pushbutton Reset
Backup Mode Exit
FB
UVLO Reset
REF
drops 7.5% below V
IN
Figure 1. Typical LTC1558 Connection
+
– 7.5%).
of the system regulator until V
C
IN
R1
R2
FB
V
LTC1588
REF
TO SYSTEM REGULATOR INPUT
1V < V
V
REF
CC
CC
> V
. Upon entering backup
V
V
V
V
FB
FB
FB
< V
FB
CC
or V
V
CONVERTER
CONDITIONS
CTL
< (V
< (V
> (V
BACKUP
> (V
CC
BOOST
(Rated Value) – 6%
LOGIC
BAT
(Rated Value) – 9%
< 250mV
REF
REF
REF
REF
< 0.9V
– 7.5%)
– 7.5%)
– 7.5%)
– 6%)
BACKUP
1558 F01
V
BAK
FB
rises
REF