8
LTC1559-3.3/LTC1559-5
APPLICATIONS INFORMATION
WUU
U
Overview
The LTC1559 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 sys-
tem design flexibility. The LTC1559 allows the use of a
low cost rechargeable NiCd cell for backup, eliminating
the need for expensive, replaceable 4.5V lithium backup
cells.
The LTC1559 includes an onboard boost converter
designed to generate a fixed voltage (3.07V for 3.3V parts
and 4.625V for 5V parts) from a single 1.2V NiCd cell.
When connected to the system DC/DC converter’s output,
the LTC1559 enables the system connected to the V
CC
rail
to continue operation when the main power supply 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 exter-
nally adjustable trickle charge circuit maintains the cell
charge after the fast charge cycle has completed, minimiz-
ing drain from the main battery during standby.
Included in the LTC1559 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 LTC1559 also performs V
CC
supervisory functions
during normal system operations. An LTC1559-3.3
monitors a 3.3V supply voltage at its V
CC
pin while an
LTC1559-5 monitors a 5V supply at its V
CC
pin. In both
cases, the LTC1559 derives power for the majority of the
internal circuitry (except for the boost converter) from its
V
CC
pin. Table 1 shows the signal conditions for the
various operating modes of the LTC1559-3.3. Table 2
shows the signal conditions for the various operating
modes of the LTC1559-5.
Table 1. LTC1559-3.3 Operating Modes
OPERATING MODES CONDITIONS
UVLO Reset 1V < V
CC
< V
CC
(rated value) – 9% or
V
BAT
< 0.9V
Push-Button Reset V
CTL
< 250mV
UVLO Reset Recovery V
CC
> V
CC
(rated value) – 5.5%
Backup Mode Activation V
CC
< V
CC
(rated value) – 7%
Backup Mode Exit V
CC
> V
CC
(rated value) – 5.5%
or PS > V
CC
Boost Converter Activation V
CC
< V
CC
(rated value) – 7%
Boost Converter Deactivation V
CC
> V
CC
(rated value) – 7%
Table 2. LTC1559-5 Operating Modes
OPERATING MODES CONDITIONS
UVLO Reset 1V < V
CC
< V
CC
(rated value) – 9%
or V
BAT
< 0.9V
Push-Button Reset V
CTL
< 250mV
UVLO Reset Recovery V
CC
> V
CC
(rated value) – 6%
Backup Mode Activation V
CC
< V
CC
(rated value) – 7.5%
Backup Mode Exit V
CC
> V
CC
(rated value) – 6%
or PS > V
CC
Boost Converter Activation V
CC
< V
CC
(rated value) – 7.5%
Boost Converter Deactivation V
CC
> V
CC
(rated value) – 7.5%
Boost Converter Operation
The LTC1559 uses an onboard boost converter 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 to 93% (LTC1559-3.3) or 92.5%
(LTC1559-5) of the rated V
CC
, supports the system V
CC
during backup. It supplies 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 LTC1559 to accurately measure the
charge removed from the backup cell by counting the
charge pulses.
The LTC1559 enters backup mode when the main battery
voltage drops and causes V
CC
, the system regulator’s
output, to fall. As shown in Figure 1, V
CC
is scaled down
by an internal resistor divider and fed to the LTC1559’s
backup comparators. These compare the scaled voltage
