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LTC1558-5

Backup Battery Controller with Programmable Output

Linear Technology 

LTC1558-3.3/LTC1558-5

APPLICATIONS INFORMATION

Once VFB rises above (VREF – 7.5%), the LTC1558’s boost

converter deactivates and the freshly charged input

capacitor supplies power to the system regulator. The

cycle repeats again when the input capacitor’s charge is

drained away and VFB again drops below (VREF – 7.5%).

The BACKUP pin remains asserted until the main battery

is restored. This ensures that the LTC1558 does not switch

in and out of backup mode unnecessarily.

The LTC1558’s boost converter minimizes output ripple

under light load conditions by reducing the charge trans-

ferred for the first two consecutive switch cycles. When

VFB falls below (VREF – 7.5%), the boost operation starts

by connecting the SW pin to ground through an internal

0.5Ω N-channel MOSFET (N1 in the Block Diagram). The

current through the external 22µH inductor rises linearly

through this switch.

165mA

(PEAK)

330mA

(PEAK)

LIGHT CURRENT MODE

HEAVY CURRENT MODE 1558 F02

Figure 2. Inductor Current During Switching

When the switch current reaches an internally preset level

of 165mA, the boost converter connects the SW pin to the

VBAK pin through an internal 2Ω P-channel MOSFET (P1 in

the Block Diagram). The inductor current discharges

through P1, charging up the capacitor connected exter-

nally to VBAK (CIN of the system regulator, Figure 1). The

inductor current falls at a rate proportional to the differ-

ence between the backup cell voltage and the output

voltage VBAK. When the inductor current reaches zero,

indicating all of its energy has been transferred to the

output capacitor, the LTC1558 looks at the FB pin voltage.

If VFB has increased above the (VREF – 7.5%) threshold,

the boost converter shuts off both switches and waits for

VFB to drop below (VREF – 7.5%) again.

If VFB is still less than (VREF – 7.5%) after the first boost

cycle, the LTC1558 immediately reconnects SW to ground,

repeating the boost cycle. If after two consecutive pulses,

VFB is still not above the boost threshold (VREF – 7.5%),

the LTC1558 decides that the load is not so light after all,

and doubles the internal inductor charging current limit to

330mA for subsequent cycles. This is high current mode.

By doubling the peak inductor current, each boost cycle

effectively carries four times more energy compared to

low current mode (E = 1/2 • LI2), doubling the available

output power. When VFB exceeds the (VREF – 7.5%) boost

threshold, the LTC1558 stops the boost converter and

resets the internal two pulse counter. The next time VFB

falls below (VREF – 7.5%), the boost converter restarts in

low current mode for at least two boost cycles. Moderate

or changing loads will cause the LTC1558 to shift between

the two peak inductor current limits, keeping the output in

tight regulation. Near its maximum load capability, the

LTC1558 will stay in 330mA high current mode and the

output voltage VBAK will hover around the user pro-

grammed value.

VBAK Capacitor ESR

The type of output capacitor and the user programmed

VBAK value will affect the LTC1558’s output ripple and

efficiency. In most applications, the main VBAK capacitor

is primarily determined by the requirements of the main

power supply. Such a capacitor will generally meet the

requirements of the LTC1558. In unusual circumstances

or circuits where the main system regulator’s input ca-

pacitor is located some distance away from the LTC1558,

a local output capacitor may be necessary.

1

BOOST

CYCLE

VBAK

ESR RIPPLE

DISCHARGE

PERIOD

tDISCH

CHARGE

PERIOD

tCH

1588 F03

Figure 3. VBAK Ripple

The maximum ripple on the VBAK pin is equal to capacitor

ESR voltage drop due to the boost converter’s output

current pulses. The ripple frequency and output duty cycle

is proportional to the inductor discharge time. Given a

fixed inductor value (22µH) and a known peak current

limit, the booster’s discharge time in each boost cycle is

9

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