LTC1558-3.3/LTC1558-5
PIN FUNCTIONS
Pin Numbers are Shown First for the SO-8 Package
Then the GN16 and S16 Packages
SW (Pins 1/1, 2):
Boost Converter Switching Node. Con-
nect a 22µH inductor from SW to the positive terminal of
the backup cell. In backup mode, this node is alternately
switched between ground and V
BAK
, generating the backup
output voltage. In fast or trickle charge mode, an internal
regulator outputs a constant DC current from this pin
through the 22µH inductor and into the NiCd battery.
During power-up or undervoltage lockout (UVLO), the SW
pin enters a high impedance state.
GND (Pins 2/4):
System Ground. The low power internal
circuitry returns to this pin in the 16-pin packages. GND
and PGND are bonded together to this pin in the SO-8.
CTL (Pins 3/5):
Control. This pin provides three functions.
In backup mode this pin enters a high impedance state and
monitors the backup battery cell voltage (V
BAT
). If V
BAT
drops below 0.9V, the LTC1558 enters into UVLO. During
trickle charge mode, an external resistor R
EXT
sets the
trickle charge current. In all modes, pulling the CTL pin
below 250mV will generate either a “soft” or “hard” reset
pulse. See the Applications Information section for more
information.
FB (Pins 4/7):
Output Voltage Feedback. This pin is fed to
the LTC1558’s internal comparators. The boost converter’s
output voltage is set with an external resistor divider
connected from V
BAK
to FB. The LTC1558 enters backup
mode when FB drops 7.5% below the internal reference
voltage (V
REF
). During backup, the boost converter runs
whenever FB drops below this (V
REF
– 7.5%) threshold.
The LTC1558 exits backup mode when FB rises above
(V
REF
– 6%).
RESET (Pins 5/11):
System Reset, Active Low. This is an
open-drain output. This pin provides a low going reset
signal to the system processor. A 200ms pulse is gener-
ated if the CTL pin is pulled low for more than two seconds
(“hard” reset) or when the LTC1558 comes out of UVLO.
The “hard reset” stops the internal boost converter if it is
running. This pin is held low whenever the LTC1558 is in
UVLO and is guaranteed to be valid when V
CC
is greater
than or equal to 1V.
RESET also provides a low going 100µs signal if the CTL
pin is pulled low for less than two seconds (“soft” reset).
Unlike hard reset, soft reset does not affect the LTC1558’s
current operating mode.
BACKUP (Pins 6/13):
Backup Active. This is an open-drain
output that pulls low unless the LTC1558 is in backup
mode. BACKUP signals the system controller that the
system is in backup mode so that it can reduce system
loading. BACKUP can also be used to drive the gate of a
P-channel MOSFET in series with the main system
regulator’s input. A 12V Zener diode is connected inter-
nally to this pin to act as a voltage clamp. See the
Applications Information section for more details.
V
CC
(Pins 7/14):
Power Supply Input. All internal circuits
except the boost converter are powered from this pin. A
0.1µF bypass capacitor is required from V
CC
to ground.
The UVLO detector inside the LTC1558 monitors V
CC
.If
V
CC
drops below the rated output voltage by greater than
9%, the LTC1558 enters UVLO mode and RESET is as-
serted. The LTC1558 will only exit from UVLO if V
CC
rises
to greater than – 6% of the rated output voltage. See the
Applications Information section for more details.
V
BAK
(Pins 8/15,16):
Backup Supply Output. The LTC1558’s
boost converter provides regulated output voltage to the
system through V
BAK
during backup mode.
16-Pin GN and SO Package
PGND (Pin 3):
Power Ground. The internal driver circuitry
returns to this pin. PGND should be connected to a low
impedance ground plane in close proximity to the NiCd
battery cell.
SHDN (Pin 6):
Chip Shutdown. A TTL-compatible active
low voltage at SHDN puts the LTC1558 into low power
shutdown mode. In shutdown, all internal circuits power
down and are held in a reset state. The SW, CTL and V
BAK
pins enter into high impedance states. In shutdown mode,
supply current drops to below 50µA and current drawn
from the backup cell drops to below 15µA.
U
U
U
5
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