LTC1558-3.3/LTC1558-5
SWITCHING WAVEFORMS
Cold Power Boot-Up Description
1. The V
BAT
voltage increases and turns on the Q
EXT
body
diode. V
BAK
follows V
BAT
by one body diode drop.
2. V
BAK
increases above the system regulator’s minimum
input voltage. The system regulator wakes up and
starts ramping up the system power supply. RESET
remains asserted from V
CC
= 1V.
Backup Mode (Main Battery Discharged)
1.2V
V
NICD
BOOST CONVERTER OUTPUT
– 7.5%
V
FB
–6%
– 7.5%
1V
0.9V
t
RISE
BACKUP
INDUCTOR
CURRENT
t
RISE
t
RISE
RESET
t
RISE
LOBAT
1
2
Backup Mode Description
1. Trigger into Backup Mode. The main battery fails and
V
FB
drops 7.5% below the LTC1558’s internal V
REF
. The
BACKUP pin is asserted after a t
RISE
delay time and the
LTC1558’s boost converter is turned on.
2. Backup Mode. The LTC1558’s boost converter charges
and discharges the inductor with 165mA peak current.
If V
FB
doesn’t recover above (V
REF
– 7.5%) (due to a
heavy load), the boost converter increases peak charg-
ing current to 330mA. When V
FB
rises above
(V
REF
– 7.5%), the boost converter stops but the
BACKUP pin remains asserted.
W
U
3. The LTC1558’s internal bandgap wakes up. The
LTC1558’s internal boost converter does not turn on as
RESET remains asserted. Once V
FB
is greater than
(V
REF
– 6%), BACKUP is deasserted after the t
PHL
delay
time. Q
EXT
turns on and V
BAK
= V
BAT
.
4. RESET is asserted for a further 200ms after V
CC
exceeds – 6% of its rated value.
– 7.5%
t
FALL
3
4
1558 SW03
3. Recovery from Backup Mode. While the boost con-
verter is running, the main battery is restored. This
causes the external MOSFET’s body diode to conduct
and V
FB
is pulled higher than (V
REF
– 6%). BACKUP
deasserts and the boost converter finishes its last cycle.
4. Trigger into UVLO. During backup, the 1.2V NiCd cell
grows weak and its terminal voltage falls. The LOBAT
pin is asserted to give an early warning when the cell
voltage drops below 1V. RESET is asserted when the
cell voltage drops below 0.9V and the LTC1558 enters
UVLO mode.
7
LINER [ LINEAR TECHNOLOGY ]
