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PDF LTC1558CS8-3.3 Data sheet ( Hoja de datos )
| Número de pieza | LTC1558CS8-3.3 | |
| Descripción | Backup Battery Controller with Programmable Output | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LTC1558-3.3/LTC1558-5
Backup Battery Controller
with Programmable Output
FEATURES
s Complete Battery Backup System in an
SO-8, 16-Pin GN or SO Package
s Generates Adjustable Backup Voltage from a
Single 1.2V NiCd Button Cell
s Automatic Main Supply to Backup Switching
s Minimum 100mW Output Power
s Automatic Fast Recharge of NiCd Battery
s Programmable NiCd Trickle Charge Current
s Smart NiCd Charger Minimizes Recharge Time and
Maximizes System Efficiency After Backup
s On-Chip Power-On Reset
s Pushbutton Reset Input
s Reset Assertion Guaranteed at VCC = 1V
s Short-Circuit Protection
s Thermal Limiting
U
APPLICATIONS
s Notebook Computers
s Palmtop Computers/PDAs
s Portable Instruments
s Battery-Powered Systems
DESCRIPTION
The LTC®1558 is a backup battery controller that provides
all the functions necessary to implement a backup power
supply using a single NiCd cell. It includes a 1.2V boost
converter, an intelligent 2-stage battery charger, auto-
matic backup switching and a microprocessor reset gen-
erator. The boost converter uses a synchronous switching
architecture to achieve a typical efficiency of 70%, ensur-
ing maximum backup lifetime from a small NiCd cell.
The on-chip NiCd charger uses an internal gas gauge to
minimize fast recharge time and prevent overcharging of
the backup cell. The LTC1558 also provides a user pro-
grammable trickle charge current to compensate for self
discharge losses in the backup cell.
The LTC1558’s automatic backup switching architecture
requires minimum intervention from the host system and
provides feedback to the host to minimize system loading
in the backup state. Its internal VCC fault detector and reset
generator eliminate the need for a separate microproces-
sor supervisory chip in most applications.
The LTC1558 is available in an SO-8, 16-lead GN or SO
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATION
1.2V
NiCd
BACKUP
**BATTERY
+
C1
1µF
RESET
S1
L1
22µH*
LTC1558-3.3
R1
14k
1 SW
VBAK 8
2
GND
VCC 7
36
CTL BACKUP
4
FB
5
RESET
+ C2
0.1µF
R2
100k
SYSTEM
µP
MAIN
BATTERY
4.5V TO 10V
Q1
Si4431DY
R3
220k
R4 +
221k
1%
R5
100k
1%
CIN
100µF
16V
×2
LTC1435†
SYNCHRONOUS
BUCK REGULATOR
VCC
3.3V
*SUMIDA CD54-22µH
**SANYO CADNICA N-110AA
†CONSULT LTC1435 DATA SHEET FOR
CIRCUIT APPLICATION INFORMATION
LOAD CURRENT
3A AT NORMAL MODE
30mA AT BACKUP MODE
1558 TA01
Backup Time
vs 3.3V Output Load Current
450
VCC = 3.3V
400 VBAK = 3.78V
NiCd CELL
350 CAPACITY = 110mAHrs
300
250
200
150
100
50
0
0 5 10 15 20 25 30
LOAD CURRENT (mA)
1558 TA02
1
1 page  
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 VBAK, 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 (VBAT). If VBAT
drops below 0.9V, the LTC1558 enters into UVLO. During
trickle charge mode, an external resistor REXT 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 VBAK to FB. The LTC1558 enters backup
mode when FB drops 7.5% below the internal reference
voltage (VREF). During backup, the boost converter runs
whenever FB drops below this (VREF – 7.5%) threshold.
The LTC1558 exits backup mode when FB rises above
(VREF – 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 VCC 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.
VCC (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 VCC to ground.
The UVLO detector inside the LTC1558 monitors VCC.If
VCC 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 VCC rises
to greater than – 6% of the rated output voltage. See the
Applications Information section for more details.
VBAK (Pins8/15,16):BackupSupplyOutput.TheLTC1558’s
boost converter provides regulated output voltage to the
system through VBAK 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 VBAK
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.
5
5 Page 
LTC1558-3.3/LTC1558-5
APPLICATIONS INFORMATION
internal logic to ensure that the low cell voltage reset is
triggered only if the CTL pin is between 0.9V and 0.25V.
This will prevent a pushbutton reset (which pulls CTL
below 250mV) from being mistaken as a low cell voltage
condition. Unusual situations where the NiCd cell voltage
drops drastically below 0.25V will also trigger UVLO, since
the LTC1558 will treat this as a “hard” reset after 2
seconds.
An optional LOBAT output, available in the 16-pin GN or SO
package, can be used to signal the system when the cell
voltage falls below 1V, giving an early warning that the
backup cell is heavily discharged. The LOBAT output is
disabled when the LTC1558 is in trickle charge mode
because the CTL pin is pulled to 0.5V by the LTC1558.
Fault Protection and Thermal Limit
The LTC1558’s boost converter incorporates two internal
timers that turn off the switch transistors if the inductor
charge or discharge time gets abnormally long.
The inductor charge time may get abnormally long if the
NiCd cell voltage drops below 0.25V without triggering the
0.25V < VBAT < 0.9V low cell voltage comparator. In this
case, the NiCd cell is assumed to be damaged and the
LTC1558’s priority is shutting down the system grace-
fully. In this case, the timer will shut off the N-channel
switch transistor after a maximum charging time (14µs).
The boost converter continues switching but delivers
reduced output power, causing VCC to drop. The LTC1558
will enter UVLO either when VCC drops below
(VCC(RATED VOLTAGE) – 9%) or after the LTC1558 detects
CTL lower than 0.25V for 2 seconds, in which case “hard”
reset occurs.
The discharging time can also get abnormally long if a
serious overload condition occurs during switching. The
timer will shut off the P-channel pass transistor after 10µs,
protecting the boost converter. The LTC1558 will end up
in UVLO as VCC drops below (VCC(RATED VOLTAGE) – 9%).
In addition, the LTC1558 is protected for safe area opera-
tion with an internal thermal shutdown circuit. If the device
is overloaded for a long period of time, the thermal
shutdown circuit forces the LTC1558 into UVLO. The
threshold temperature for thermal shutdown is typically
155°C.
The LTC1558’s boost converter is designed so that no
current drains from the battery to the load during output
short circuit or VCC = 0V conditions. This assures that the
system can be powered down for a long period of time.
This eliminates the risk of finding a nonfunctioning backup
system upon power-up.
Backup Cell Fast Recharge
The LTC1558 includes an onboard gas gauge circuit,
consisting of a 23-bit divider and a 9-bit up/down counter.
The gas gauge logic assumes that the boost converter
uses a 22µH inductor, allowing it to estimate battery
charge by counting switch pulses. The gas gauge counts
up from zero as charge is removed from the backup cell in
backup mode. It takes roughly 8.4 million 165mA boost
pulses (low current mode) to increment the up/down
counter by one count. In high current mode, the 330mA
pulses skip the first two bits of the divider because each
330mA pulse carries four times as much energy as a
165mA pulse. At maximum load and VCC = 5V, the gas
gauge divider will increment by one count every 7.5µs
while the boost converter is running. Full count is reached
after approximately 2.2 hours, equivalent to about 512mAhr
of charge.
Upon entering recharge mode (after the main battery is
restored) the LTC1558 connects a 16mA fast recharge
current source from VCC to the SW pin. At the same time,
an internal free running oscillator counts down the gas
gauge counter at a rate designed to replace about 160% of
the charge previously removed from the backup cell.
When the gas gauge counter reaches zero, the LTC1558
reduces the charging current at the SW pin to the user-
programmed trickle charge current level.
Under some circumstances, the LTC1558 can exit the
backup mode with invalid gas gauge contents. This can
occur under three possible conditions:
a) The backup cell was completely exhausted during a
backup cycle and the LTC1558 entered UVLO.
b) The backup cell was replaced while the main supply was
disabled.
c) A backup cycle was terminated prematurely by a “hard”
reset or an output overload.
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC1558CS8-3.3.PDF ] | |
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