LTC1558-5 Linear Technology, LTC1558-5 Datasheet - Page 10

• Manufacturer Part Number: LTC1558-5
• Description: Backup Battery Controller with Programmable Output
• Manufacturer: Linear Technology

APPLICATIONS
LTC1558-3.3/LTC1558-5
proportional to the difference between V
and the battery cell voltage, V
Assuming ESR = 0.2 , I
V
Since V
resistor ratio
Therefore the noise amplitude seen by the FB compara-
tors is:
The discharge time period,
For lowest V
The internal V
response time to filter away this ripple. The (V
FB comparator has a 6 s rising edge delay and 2 s falling
edge delay. The (V
similar 6 s rising time delay but a much longer falling
time delay of 20 s. This enables the comparator to
control the booster properly, and avoids turning off the
boost converter prematurely due to false triggering by
the ESR ripple.
Exit from Backup
When a new battery is inserted into the system, the higher
main battery voltage turns on the external P-channel
MOSFET’s body diode and raises V
higher voltage. The LTC1558 detects the return of the main
10
RIPPLE(P-P)
V
RB resistor ratio = 2.358
Noise amplitude = 37.7mV
t
DISCH
RIPPLE(P-P)
t
DISCH
BAK
= 5 s
BAK
must be scaled down to V
= (I
= (330mA)(0.2 )
= 66mV
= 6V/1.272V
= 4.717
= 66mV/4.717
= 14mV
= (L • I
= (22 H • 330mA)/(6V – 1.2V)
= 1.5 s
FB
= 89mV
IND(PEAK)
comparators are designed to have a slow
= 3V and maximum I
U
IND(PEAK)
REF
– 7.5%) FB comparator has a
INFORMATION
IND(PEAK)
)(R
U
ESR(CAP)
)/(V
BAT
BAK
(1.2V).
= 330mA, V
W
– V
)
BAK
IND(PEAK)
BAT
BAK
FB
(and V
, the external
)
(3V to 10V)
BAK
U
= 445mA,
REF
FB
= 6V,
– 6%)
) to a
battery by watching for V
LTC1558 then stops its internal boost converter and
begins to recharge the NiCd cell. BACKUP is deasserted to
signal to the system controller to restore system loading
and resume normal operations. At the same time, the
external P-channel MOSFET is driven by the BACKUP
signal. The P-channel MOSFET turns on and allows the
main battery to bypass its body diode and drive the system
regulator directly.
Since the user can replace the main battery anytime during
the LTC1558’s backup operations, the BACKUP signal
may be deasserted while the boost converter is switching.
To prevent the potential problem of residual energy in the
inductor, the LTC1558 will only stop the boost converter
after it has completed the current boost cycle.
UVLO Lockout Under Excessive Backup Load
Very heavy loads (above the LTC1558’s maximum power
output) will pull the boost converter’s output below the
boost threshold. Under these conditions, the LTC1558’s
boost converter will continue to supply 330mA current
pulses to the system regulator while charge on the V
capacitor (C
maintain its output regulation and the system V
drop. When V
more than 9 s, the LTC1558’s V
activates UVLO mode, shutting off the boost converter and
asserting the RESET pins. The 9 s delay prevents the
LTC1558 from being fooled by brief transients or noise
spikes on its V
host system should shut down in a orderly manner. The
LTC1558’s V
V
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 LTC1558 monitors the
cell’s terminal voltage through the CTL pin during backup.
If the CTL pin drops below 0.9V for more than 20 s, the
UVLO circuit shuts down the boost converter and asserts
the RESET pins. Since the CTL pin can also be connected
to an external pushbutton reset, the LTC1558 includes
CC
is less than 1V to ensure valid reset pin signals.
IN
CC
CC
) drains away. The system regulator will not
CC
supervisory circuit will remain alive until
drops below – 9% of the rated voltage for
pin. Upon receipt of the reset signals, the
FB
to exceed (V
CC
supervisory circuit
REF
– 6%). The
CC
BAK
will