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PDF LTC1559-5 Data sheet ( Hoja de datos )
| Número de pieza | LTC1559-5 | |
| Descripción | Backup Battery Controller with Fixed Output | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
s Complete Battery Backup System in an SO-8,
16-Pin GN or SO Package
s Generates Fixed Backup Voltage (3.07V/4.63V) 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 Onboard Power-Up and Push-Button Reset
Generator
s Performs VCC Supervisory Functions
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
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTC1559-3.3/LTC1559-5
Backup Battery Controller
with Fixed Output
DESCRIPTION
The LTC®1559 is a backup battery controller that provides
all the functions necessary to implement a backup 3.3V or
5V power supply using a single NiCd cell. It includes a 1.2V
to 3.07V/4.63V boost converter, an intelligent 2-stage
battery charger, automatic backup switching and a micro-
processor reset generator. The boost converter uses a
synchronous switching architecture to achieve a typical
efficiency of 70%, ensuring 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, thereby improving system efficiency and
extending the life of the backup cell. The LTC1559 also
provides a user programmable trickle charge current to
compensate for self-discharge losses in the backup cell.
The LTC1559’s automatic backup switching scheme
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 LTC1559 is available in an SO-8, 16-pin GN or SO
package.
TYPICAL APPLICATION
**BACKUP
BATTERY
1.2V
NiCd
MAIN
BATTERY
4.5V TO 28V
+ C1
1µF
S1
RESET
*L1
22µH
R1 1 8
68k
2
SW
GND
VBAK
VCC
7
3
CTL
LTC1559-3.3
6
BACKUP
4 PS
RESET 5
+
C2
1µF
R2
100k
SYSTEM
µP
†LTC1435
SYNCHRONOUS
BUCK
REGULATOR
Q1
P-MOSFET
VOUT
Si9424DY
3.3V AT NORMAL MODE
C3
100µF
10V
+
* SUMIDA CD54-22µH 3A
** PANASONIC P-11AAH
VBAK
33mA (3.07V) AT BACKUP MODE
† CONSULT LTC1435 DATA SHEET FOR
>33mA (3.3V) AT NORMAL MODE
APPLICATION CIRCUIT INFORMATION
1559 TA01
Backup Time vs
VBAK Output Load Current
800
VBAK = 3.07V
700 NiCd CELL = P-11AAH
(110mA Hrs)
600
500
400
300
200
100
0
05
10 15 20 25 30 35
VBAK LOAD CURRENT (mA)
1559 TA02
1
1 page  
LTC1559-3.3/LTC1559-5
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 8-pin
package.
CTL (Pins 3/5): Control. This pin provides three functions.
In backup mode this pin is a high impedance input and
monitors the backup battery cell voltage (VBAT). If VBAT
drops below 0.9V, the LTC1559 enters UVLO. During
trickle charge mode, an external resistor REXT sets the
trickle charge current. In all modes, pulling the CTL pin
below 250mV generates either a “soft” or “hard” reset
pulse. See the Applications Information section for more
information.
PS (Pins 4/7): Power Supply Sense. This pin senses the
presence of the main supply and triggers the LTC1559 to
terminate backup mode. During backup, VCC is driven
externally by the LTC1559’s boost convert’s output (VBAK).
When PS > VCC during backup, the LTC1559 pulls down
the BACKUP pin, reconnecting the system regulator out-
put to the system VCC. The PS pin is needed in applications
that use a P-channel MOSFET (driven by the BACKUP
signal) to isolate the system regulator during backup. If
not needed, PS can be disabled by tying it to ground.
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 if the LTC1559 comes out of UVLO. This
“hard reset” stops the internal boost converter if it is
running. This pin is held low if the LTC1559 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 whenever
the CTL pin is pulled low for less than two seconds (“soft”
reset). Unlike hard reset, soft reset does not affect the
LTC1559’s current operating mode.
BACKUP (Pins 6/13): System Backup Signal. This is a
TTL-compatible output driver that pulls low unless the
LTC1559 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 output. See the Applications Informa-
tion 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 LTC1559 monitors VCC. If
VCC drops below the rated output voltage by 9%, the
LTC1559 enters UVLO mode and RESET is asserted. The
LTC1559-3.3 exits UVLO if VCC rises to greater than
– 5.5% of the rated output voltage. The LTC1559-5 exits
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.TheLTC1559’s
boost converter provides the regulated output voltage to
the system through VBAK during backup mode.
16-Pin GN and SO Packages
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 LTC1559 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 
LTC1559-3.3/LTC1559-5
APPLICATIONS INFORMATION
high when inactive (typically a boost regulator with an
output catch diode), the LTC1559 detects the return of the
main supply by watching for VCC to exceed VCC (rated
value) – 5.5% (LTC1559-3.3). The LTC1559 then shuts
down its internal boost converter and begins to recharge
the NiCd cell. In such applications, the PS pin is not used
and can be tied to ground. No external P-channel MOSFET
is required to isolate the main supply from the system VCC
during backup.
In systems where the main supply’s output impedance is
low when inactive (typically buck regulators), the main
supply must be disconnected from the system VCC during
backup to prevent the inactive supply from loading the
LTC1559. This is typically accomplished using an external
P-channel MOSFET as shown in Figure 1. When the main
supply is restored, the P-channel MOSFET’s body diode
forward-biases. This allows current to flow into the sys-
tem VCC, but the forward drop across this diode may
prevent VCC from reaching the VCC (rated value)
– 5.5% (LTC1559-3.3) threshold that deactivates the
LTC1559’s backup mode. In such systems, the PS pin
should connect directly to the output of the main system
supply. When the system regulator’s voltage rises about
2.5% above the backup VCC, the PS comparator triggers
and causes the LTC1559 to deassert the BACKUP pin
signal. This signals the system controller to restore sys-
tem loading and resume normal operation. At the same
time, the external P-MOSFET is driven by the BACKUP
signal. The P-channel MOSFET turns on and allows the
main regulator to bypass its body diode and drive the
system VCC directly.
Since the user can replace the main battery anytime during
the LTC1559’s backup operation, the BACKUP signal may
be deasserted while the boost converter is switching. To
prevent the potential problem of residual energy in the
inductor, the LTC1559 will only stop the boost converter
after it completes the current boost converter cycle.
UVLO Under Excessive Backup Load
Very heavy loads (above the LTC1559’s maximum power
output) will pull the boost converter’s output below the
boost threshold. Under these conditions, the LTC1559’s
boost converter continues to supply 330mA current pulses
to the load while charge on the VCC capacitor drains away.
If VCC drops below VCC (rated voltage) – 9% for more than
7.5µs, the LTC1559’s VCC supervisory circuit activates
UVLO mode, shutting off the boost converter and assert-
ing the Reset pins. The 7.5µs delay prevents the LTC1559
from being fooled by brief transients or noise spikes on its
VCC pin. Upon receipt of the reset signal, the host system
should shut down in an orderly manner. The LTC1559’s
VCC supervisory circuit will remain alive until VCC is less
than 1V to ensure a valid RESET pin signal.
Backup Cell Voltage Monitoring
As the boost converter removes charge from the backup
NiCd cell, the cell’s terminal voltage falls. Permanent
damage to the NiCd cell can occur if it is discharged to
below 0.9V. To prevent this, the LTC1559 monitors the
cell’s terminal voltage through the CTL pin during backup.
If the CTL pin drops below 0.9V for more than 20µs, the
UVLO circuit shuts down the boost converter and asserts
the RESET and RESET pins. Since the CTL pin can also be
connected to an external push-button reset, the LTC1559
includes 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 push-button 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 LTC1559 will treat this as a “hard” reset
after two seconds.
An optional LOBAT output, available in the 16-pin GN or SO
package, can be used to signal the system if the cell
voltage falls below 1V, giving an early warning that the
backup cell is heavily discharged. The LOBAT pin is
disabled if the LTC1559 is in trickle charge mode,
because the CTL pin is regulated to 0.5V by the LTC1559.
Fault Protection and Thermal Limit
The LTC1559’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
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LTC1559-5.PDF ] | |
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