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LTC1558CS-3.3

Backup Battery Controller with Programmable Output

Linear Technology 

LTC1558-3.3/LTC1558-5

APPLICATIONS INFORMATION

proportional to the difference between VBAK (3V to 10V)

and the battery cell voltage, VBAT (1.2V).

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

= 6V/1.272V

= 4.717

Therefore the noise amplitude seen by the FB compara-

tors is:

= 66mV/4.717

= 14mV

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

The internal VFB comparators are designed to have a slow

response time to filter away this ripple. The (VREF – 6%)

FB comparator has a 6µs rising edge delay and 2µs falling

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.

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

higher voltage. The LTC1558 detects the return of the main

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

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 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

activates UVLO mode, shutting off the boost converter and

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.

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

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