LTC1558-3.3/LTC1558-5
U
W U U
APPLICATIONS INFORMATION
proportional to the difference between VBAK (3V to 10V)
and the battery cell voltage, VBAT (1.2V).
battery by watching for VFB to exceed (VREF –6%). The
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
mainbatterytobypassitsbodydiodeanddrivethesystem
regulator directly.
Assuming ESR = 0.2Ω, IIND(PEAK) = 330mA, VBAK = 6V,
VRIPPLE(P-P) = (IIND(PEAK))(RESR(CAP)
)
= (330mA)(0.2Ω)
= 66mV
Since VBAK must be scaled down to VFB, the external
resistor ratio
Sincetheusercanreplacethemainbatteryanytimeduring
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.
= 6V/1.272V
= 4.717
Therefore the noise amplitude seen by the FB compara-
tors is:
= 66mV/4.717
= 14mV
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 VBAK
capacitor (CIN) drains away. The system regulator will not
maintain its output regulation and the system VCC will
drop. When VCC drops below –9% of the rated voltage for
more than 9µs, the LTC1558’s VCC supervisory circuit
activatesUVLOmode,shuttingofftheboostconverterand
asserting the RESET pins. The 9µs delay prevents the
LTC1558 from being fooled by brief transients or noise
spikes on its VCC pin. Upon receipt of the reset signals, the
host system should shut down in a orderly manner. The
LTC1558’s VCC supervisory circuit will remain alive until
VCC is less than 1V to ensure valid reset pin signals.
The discharge time period,
tDISCH = (L • IIND(PEAK))/(VBAK – VBAT
)
= (22µH • 330mA)/(6V – 1.2V)
= 1.5µs
For lowest VBAK = 3V and maximum IIND(PEAK) = 445mA,
VRIPPLE(P-P) = 89mV
RB resistor ratio = 2.358
Noise amplitude = 37.7mV
tDISCH = 5µs
TheinternalVFB comparatorsaredesignedtohaveaslow
response time to filter away this ripple. The (VREF –6%)
FBcomparatorhasa6µsrisingedgedelayand2µsfalling
edge delay. The (VREF – 7.5%) FB comparator has a
similar 6µs rising time delay but a much longer falling
time delay of 20µs. 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.
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 20µs, 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
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 VBAK (and VFB) to a
highervoltage.TheLTC1558detectsthereturnofthemain
10
Linear [ Linear ]
