LTC1559CGN-3.3 查看數據表（PDF） - Linear Technology

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LTC1559CGN-3.3

Backup Battery Controller with Fixed Output

Linear Technology 

LTC1559-3.3/LTC1559-5

APPLICATIONS INFORMATION

SYSTEM

REGULATOR

VCC

LTC1559

R1

R2

VREF

BOOST

VBAK

CONVERTER

BACKUP

LOGIC

SYSTEM

+

VCC

COUT

BACKUP

PS

1559 F01

Figure 1. Typical LTC1559 Connection

with an internal trimmed VREF (1.272V), switching the

LTC1559 into backup mode if VCC drops 7%

(LTC1559-3.3) or 7.5% (LTC1559-5) below its rated

value. Upon entering backup mode, the BACKUP pin is

asserted and the internal boost converter turns on. The

BACKUP signal turns off the external P-channel MOSFET

(if used), isolating the system regulator from the LTC1559.

The boost converter charges the VCC capacitor, COUT, of

the system regulator until VCC rises above VCC (rated

value) – 7% (LTC1559-3.3) or VCC (rated value) – 7.5%

(LTC1559-5).

Once VCC rises above VCC (rated value) – 7% (LTC1559-

3.3), the boost converter deactivates and the freshly

charged VCC capacitor COUT supplies power to the system.

The cycle repeats again when the VCC capacitor’s charge

is drained away and VCC again drops below VCC (rated

value) – 7% (LTC1559-3.3). The BACKUP pin remains

asserted until the main battery is restored. This ensures

that the LTC1559 does not switch in and out of backup

mode unnecessarily.

The LTC1559’s boost converter minimizes output ripple

under light load conditions by reducing the charge trans-

165mA

(PEAK)

330mA

(PEAK)

ferred for the first two consecutive switch cycles. If VCC

falls below VCC (rated value) – 7% (LTC1559-3.3), the

boost operation starts by connecting the SW pin internally

to ground through an internal 0.5Ω N-channel MOSFET

(N1 in the Block Diagram). The current through the exter-

nal 22µH inductor rises linearly through this switch.

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. The

inductor current discharges through the P-channel (P1 in

the Block Diagram) and charges up the system’s VCC

capacitor (COUT 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 LTC1559 monitors the VCC voltage.

If VCC has increased above the VCC (rated value) – 7%

(LTC1559-3.3) threshold, the boost converter shuts off

both switches and waits for VCC to drop below VCC (rated

value) – 7% (LTC1559-3.3) again.

1

BOOST

CYCLE

VBAK

ESR RIPPLE

LIGHT CURRENT MODE

HEAVY CURRENT MODE 1559 F02

Figure 2. Inductor Current During Switching

DISCHARGE

PERIOD

tDISCH

CHARGE

PERIOD

tCH

1559 F03

Figure 3. VBAK Ripple

9

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