LTC1558CS-3.3 - Linear Technology Corporation

LTC1558-3.3/LTC1558-5

Backup Battery Controller

with Programmable Output

■

Complete Battery Backup System in an

SO-8, 16-Pin GN or SO Package

■

Generates Adjustable Backup Voltage from a

Single 1.2V NiCd Button Cell

■

Automatic Main Supply to Backup Switching

■

Minimum 100mW Output Power

■

Automatic Fast Recharge of NiCd Battery

■

Programmable NiCd Trickle Charge Current

■

Smart NiCd Charger Minimizes Recharge Time and

Maximizes System Efficiency After Backup

■

On-Chip Power-On Reset

■

Pushbutton Reset Input

■

Reset Assertion Guaranteed at V

= 1V

■

Short-Circuit Protection

■

Thermal Limiting

FEATURES

DESCRIPTION

The LTC

1558 is a backup battery controller that provides

all the functions necessary to implement a backup power

supply using a single NiCd cell. It includes a 1.2V boost

converter, an intelligent 2-stage battery charger, auto-

matic backup switching and a microprocessor reset gen-

erator. The boost converter uses a synchronous switching

architecture to achieve a typical efficiency of 70%, ensur-

ing maximum backup lifetime from a small NiCd cell.

The on-chip NiCd charger uses an internal gas gauge to

minimize fast recharge time and prevent overcharging of

the backup cell. The LTC1558 also provides a user pro-

grammable trickle charge current to compensate for self

discharge losses in the backup cell.

The LTC1558’s automatic backup switching architecture

requires minimum intervention from the host system and

provides feedback to the host to minimize system loading

in the backup state. Its internal V

fault detector and reset

generator eliminate the need for a separate microproces-

sor supervisory chip in most applications.

The LTC1558 is available in an SO-8, 16-lead GN or SO

package.

APPLICATIONS

■

Notebook Computers

■

Palmtop Computers/PDAs

■

Portable Instruments

■

Battery-Powered Systems

TYPICAL APPLICATION

, LTC and LT are registered trademarks of Linear Technology Corporation.

LOAD CURRENT (mA)

BACKUP TIME (MINUTES)

150

200

250

350

400

450

1558 TA02

100

300

10 25 30

515 20

= 3.3V

BAK

= 3.78V

NiCd CELL

CAPACITY = 110mAHrs

Backup Time

vs 3.3V Output Load Current

1µF

1.2V

NiCd

BACKUP

**BATTERY

MAIN

BATTERY

4.5V TO 10V

14k R2

100k

LOAD CURRENT

3A AT NORMAL MODE

30mA AT BACKUP MODE

LTC1558-3.3

22µH*

221k

220k

100k

RESET

Si4431DY

1558 TA01

BAK

BACKUP

RESET

GND

CTL

SUMIDA CD54-22µH

SANYO CADNICA N-110AA

CONSULT LTC1435 DATA SHEET FOR

CIRCUIT APPLICATION INFORMATION

100µF

16V

×2

†

3.3V

SYSTEM

µP

0.1µF

LTC1435

†

SYNCHRONOUS

BUCK REGULATOR

LTC1558-3.3/LTC1558-5

ABSOLUTE MAXIMUM RATINGS

(Note 1)

Terminal Voltages

CC..........................................................................................

BAK

, BACKUP ..................................................... 12V

SW ...................................................................... 14V

All Other Pins .................................. –0.3V to V

+ 0.3V

Input Currents (SW)........................................... 500mA

BAK

Output Current................... Short-Circuit Protected

Operating Ambient Temperature Range ....... 0°C to 70°C

Junction Temperature .......................................... 125°C

Storage Temperature Range .................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)...................300°C

PACKAGE/ORDER INFORMATION

ELECTRICAL CHARACTERISTICS

VBAT = 1.2V, TA = 0°C to 70°C unless otherwise noted.

Consult factory for Industrial and Military grade parts.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Battery Backup Switching

Operating Voltage Range LTC1558-3.3 ●2.90 3.465 V

LTC1558-5 ●4.40 5.250 V

BAT

Backup Battery Cell Voltage ●1.0 1.2 1.5 V

REF

Internal Reference Voltage ●1.247 1.272 1.297 V

VCC

Quiescent Supply Current (Note 2) ●155 250 µA

BAT

Peak Inductor Current (Backup Mode) Boost Converter in Low Current Mode (Note 7) ●80 165 225 mA

Boost Converter in High Current Mode (Note 7) ●225 330 445 mA

BAT(SHDN)

Battery Standby Current V

= 0V ●0.1 15 µA

VCC(SHDN)

Supply Current During Shutdown (Note 3) ●50 µA

BAK(ON)

Backup Request Trip Point Voltage at V

Relative to V

REF

●–10.5 –7.5 –5.5 %

BST(ON)

Boost Converter Assertion Trip Point Voltage at V

Relative to V

REF

●–10.5 –7.5 –5.5 %

BAK(OFF)

Backup Deassertion Trip Point Voltage at V

Relative to V

REF

●–9 –6 –4 %

BST(OFF)

Boost Converter Deassertion Trip Point Voltage at V

Relative to V

REF

●–10.5 –7.5 –5.5 %

LOBAT

Low V

BAT

Detect (Note 3) ●0.95 1 1.05 V

ORDER PART

NUMBER

LTC1558CGN-3.3

LTC1558CGN-5

LTC1558CS-3.3

LTC1558CS-5

TOP VIEW

S PACKAGE

16-LEAD PLASTIC SO

GN PACKAGE

16-LEAD PLASTIC SSOP

PGND

GND

CTL

SHDN

BAK

BACKUP

RESET

LOBAT

JMAX

= 125°C, θ

= 110°C/ W (GN)

JMAX

= 125°C, θ

= 110°C/ W (S)

ORDER PART

NUMBER

S8 PART MARKING

LTC1558CS8-3.3

LTC1558CS8-5

155833

15585

JMAX

= 125°C, θ

= 130°C/W

TOP VIEW

S8 PACKAGE

8-LEAD PLASTIC SO

GND

CTL

BAK

BACKUP

RESET

LTC1558-3.3/LTC1558-5

ELECTRICAL CHARACTERISTICS

VBAT = 1.2V, TA = 0°C to 70°C unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

UVLO(ON)

UVLO Trip Voltage (Note 4) LTC1558-3.3 ●2.90 3.00 3.10 V

LTC1558-5 ●4.40 4.55 4.70 V

UVLO(OFF)

UVLO Recovery Trip Voltage (Note 4) LTC1558-3.3 ●3.00 3.10 3.20 V

LTC1558-5 ●4.55 4.70 4.85 V

LOBAT

BAT

UVLO Trip Voltage (Note 5) ●0.85 0.9 0.95 V

Backup Battery Charger

CHGF

Battery Charge Current Fast Recharge ●11 16 21 mA

CHGT

Programmable Trickle Charge Current Range 0.05 2 mA

RECH

Fast Recharge Factor (Note 6) 1.35 1.6 1.85 C/C

TRK

Nominal Trickle Charge Multiplier Factor I

CHGT

= 1mA 8 10 12 A/A

CTL(CLAMP)

Trickle Charge Clamp Voltage I

CHGT

= 1mA ●0.45 0.5 0.55 V

Pushbutton Reset

CTL

CTL Input Threshold 250 mV

CTL

CTL Input Low Time (Debounce Time) 20 ms

Reset Timer

HRESET

Pushbutton Duration for Hard Reset 1.10 1.8 3.4 s

RST

RESET Pulse Width V

CTL

Low for <t

HRESET

(Soft Reset) ●50 80 150 µs

CTL

Low for >t

HRESET

(Hard Reset) ●115 185 345 ms

RST1

RESET Output Voltage V

= 1V, I

SINK

= 10µA●5 200 mV

RST

RESET Output Voltage V

= 4.25V, I

SINK

= 1.6mA ●0.1 0.4 V

RESET Output Current Output Source Current, V

= 3.3V 10 mA

Output Source Current, V

= 5V 20 mA

Short-Circuit Current Output Sink Current, V

= 3.3V 20 mA

Output Sink Current, V

= 5V 40 mA

Internal V

Monitor Comparator

PLH

UVLO(ON)

Comparator Propagation ∆V

= –(9% V

+ 300mV), V

= 300mV 9 µs

Delay (Rising)

Shutdown Pin (Note 3)

SHDN

SHDN Input Threshold Logic Low, V

●0.8 V

Logic High, V

●2V

SHDN

SHDN Pin Bias Current V

= 5V, V

SHDN

= 0V ●815 µA

Feedback Pin

FB Pin Bias Current 110 nA

The ● denotes specifications which apply over the full operating

temperature range.

Note 1: Absolute Maximum Ratings are those values beyond which the life of

a device may be impaired.

Note 2: Quiescent current is measured during pushbutton reset.

Note 3: Only applies to 16-pin version.

Note 4: Thresholds will track each other and are guaranteed not to overlap.

Note 5: Low cell voltage reset is only triggered when 0.25V < V

CTL

< 0.9V

for at least 20µs while in backup mode.

Note 6: Fast recharge factor is defined as the ratio of charge replenished to

the NiCd battery during fast recharge to the charge drawn from the NiCd

battery during backup.

Note 7: The LTC1558 switches automatically between the low and high

operating current levels. See Applications Information for more details.

LTC1558-3.3/LTC1558-5

TYPICAL PERFORMANCE CHARACTERISTICS

NiCd TERMINAL VOLTAGE (V)

1.0

OUTPUT POWER (mW)

100

125

150

1.1 1.2 1.3 1.4

1558 G01

= 5V (LTC1558-5)

BAK

= 6V

Output Power vs Battery Voltage

OUTPUT VOLTAGE, VBAK (V)

SWITCHING FREQUENCY (kHz)

1558 G03

0468

125

100

VBATT = 1.2V

VCC = 3.3V (LTC1558-3.3)

IPK = 330mA

Boost Converter Switching

Frequency

NiCd CELL CAPACITY (mAHr)

BACKUP TIME (HOURS)

0.5

1.0

1.5

2.0

2.5

3.0

125 250 375 500

1558 G02

BATT

= 1.2V

BAK

= 6V

OUT

= 100mW

Backup Time vs Battery Capacity

Normalized Fast Recharge

Current vs Temperature

Trickle Charge Multiplier Factor

Boost Converter Switching

Duty Cycle

OUTPUT VOLTAGE, VBAK (V)

DUTY CYCLE (%)

1558 G04

0468

100

VBATT = 1.2V

VCC = 3.3V (LTC1558-3.3)

IPK = 330mA

NiCd BATTERY TRICKLE CURRENT (mA)

0.05

7.5

TRICKLE CHARGE FACTOR (mA/mA)

9.5

10.0

10.5

0.1 1 2

1558 G05

9.0

8.5

8.0

BATT

= 1.2V

TEMPERATURE (°C)

0.950

CURRENT RATIO (mA/mA)

0.960

0.970

0.980

0.990

25 50

1558 G06

1.000

0.955

0.965

0.975

0.985

0.995

1.005

Fast Recharge Time (Assume

Fully Exhausted NiCd Battery)

BATTERY CAPACITY (mAHr)

TIME (HOURS)

128 256

1558 G07

512

RESET Output Voltage

vs Supply Voltage

SUPPLY VOLTAGE (V)

RESET VOLTAGE (V)

1234

4.55V

1558 G08

4.7V

VCC = 5V (LTC1558-5)

RESET Output Voltage

vs Supply Voltage

SUPPLY VOLTAGE (V)

RESET VOLTAGE (V)

0.5

1.0

1.5

2.0

3.5

1234

1558 G09

2.5

3.0

3.10V

VCC = 3.3V (LTC1558-3.3)

LTC1558-3.3/LTC1558-5

PIN FUNCTIONS

UUU

Pin Numbers are Shown First for the SO-8 Package

Then the GN16 and S16 Packages

SW (Pins 1/1, 2): Boost Converter Switching Node. Con-

nect a 22µH inductor from SW to the positive terminal of

the backup cell. In backup mode, this node is alternately

switched between ground and V

BAK

, generating the backup

output voltage. In fast or trickle charge mode, an internal

regulator outputs a constant DC current from this pin

through the 22µH inductor and into the NiCd battery.

During power-up or undervoltage lockout (UVLO), the SW

pin enters a high impedance state.

GND (Pins 2/4): System Ground. The low power internal

circuitry returns to this pin in the 16-pin packages. GND

and PGND are bonded together to this pin in the SO-8.

CTL (Pins 3/5): Control. This pin provides three functions.

In backup mode this pin enters a high impedance state and

monitors the backup battery cell voltage (V

BAT

). If V

BAT

drops below 0.9V, the LTC1558 enters into UVLO. During

trickle charge mode, an external resistor R

EXT

sets the

trickle charge current. In all modes, pulling the CTL pin

below 250mV will generate either a “soft” or “hard” reset

pulse. See the Applications Information section for more

information.

FB (Pins 4/7): Output Voltage Feedback. This pin is fed to

the LTC1558’s internal comparators. The boost converter’s

output voltage is set with an external resistor divider

connected from V

BAK

to FB. The LTC1558 enters backup

mode when FB drops 7.5% below the internal reference

voltage (V

REF

). During backup, the boost converter runs

whenever FB drops below this (V

REF

– 7.5%) threshold.

The LTC1558 exits backup mode when FB rises above

REF

– 6%).

RESET (Pins 5/11): System Reset, Active Low. This is an

open-drain output. This pin provides a low going reset

signal to the system processor. A 200ms pulse is gener-

ated if the CTL pin is pulled low for more than two seconds

(“hard” reset) or when the LTC1558 comes out of UVLO.

The “hard reset” stops the internal boost converter if it is

running. This pin is held low whenever the LTC1558 is in

UVLO and is guaranteed to be valid when V

is greater

than or equal to 1V.

RESET also provides a low going 100µs signal if the CTL

pin is pulled low for less than two seconds (“soft” reset).

Unlike hard reset, soft reset does not affect the LTC1558’s

current operating mode.

BACKUP (Pins 6/13): Backup Active. This is an open-drain

output that pulls low unless the LTC1558 is in backup

mode. BACKUP signals the system controller that the

system is in backup mode so that it can reduce system

loading. BACKUP can also be used to drive the gate of a

P-channel MOSFET in series with the main system

regulator’s input. A 12V Zener diode is connected inter-

nally to this pin to act as a voltage clamp. See the

Applications Information section for more details.

(Pins 7/14): Power Supply Input. All internal circuits

except the boost converter are powered from this pin. A

0.1µF bypass capacitor is required from V

to ground.

The UVLO detector inside the LTC1558 monitors V

.If

drops below the rated output voltage by greater than

9%, the LTC1558 enters UVLO mode and RESET is as-

serted. The LTC1558 will only exit from UVLO if V

rises

to greater than –6% of the rated output voltage. See the

Applications Information section for more details.

BAK

(Pins 8/15, 16): Backup Supply Output. The LTC1558’s

boost converter provides regulated output voltage to the

system through V

BAK

during backup mode.

16-Pin GN and SO Package

PGND (Pin 3): Power Ground. The internal driver circuitry

returns to this pin. PGND should be connected to a low

impedance ground plane in close proximity to the NiCd

battery cell.

SHDN (Pin 6): Chip Shutdown. A TTL-compatible active

low voltage at SHDN puts the LTC1558 into low power

shutdown mode. In shutdown, all internal circuits power

down and are held in a reset state. The SW, CTL and V

BAK

pins enter into high impedance states. In shutdown mode,

supply current drops to below 50µA and current drawn

from the backup cell drops to below 15µA.

LTC1558-3.3/LTC1558-5

PIN FUNCTIONS

UUU

LOBAT (Pin 9): Low-Battery Detector Output. This is an

open-drain output that pulls low when the backup cell

drops below 1V. It gives early warning to the system

controller that the backup cell is getting weak. This pin is

disabled when the LTC1558 is in trickle charge mode.

RESET (Pin 12): System Reset, Active High. This is a TTL-

compatible output driver. It can be used to connect to

systems that require active high logic. The RESET output

will go high whenever RESET is pulled low. If RESET is

externally pulled low, RESET will go high.

BAT

BACKUP

RESET

BAK

1558 SW02

200ms

RATED BATTERY TERMINAL VOLTAGE

RATED V

VOLTAGE

–6%

1234

SWITCHING WAVEFORMS

–

LEVEL SENSE

AND DEBOUNCE

THERMAL

LIMIT

SHUTDOWN

LOGIC BANDGAP

REF

= 1.272V

CHARGER

RESET

GENERATOR

BOOST/BACKUP

LOGIC

GAS GAUGE

REF

SHDN

RESET

BAK

CTL

–

UVLO

DETECTOR

RESET

BACKUP

LOBAT

1558 BD

BLOCK DIAGRAM

1558 SW01

BAT

MAIN

BATTERY

MULTIPLE

POWER

OUTPUTS

1.2V

NiCd SW

LTC1558

RESET

BAK

EXT

BACKUP

SYSTEM

CONTROL

FOR MORE DETAILED APPLICATION SCHEMATICS

PLEASE REFER TO THE TYPICAL APPLICATIONS SECTION

MAIN SYSTEM

REGULATOR

Simplified LTC1558 Connections in a

Battery Backup System Cold Power Boot-Up (Main Battery Replaced/Turned On)

LTC1558-3.3/LTC1558-5

SWITCHING WAVEFORMS

Cold Power Boot-Up Description

1. The V

BAT

voltage increases and turns on the Q

EXT

body

diode. V

BAK

follows V

BAT

by one body diode drop.

2. V

BAK

increases above the system regulator’s minimum

input voltage. The system regulator wakes up and

starts ramping up the system power supply. RESET

remains asserted from V

= 1V.

3. The LTC1558’s internal bandgap wakes up. The

LTC1558’s internal boost converter does not turn on as

RESET remains asserted. Once V

is greater than

REF

– 6%), BACKUP is deasserted after the t

PHL

delay

time. Q

EXT

turns on and V

BAK

= V

BAT

4. RESET is asserted for a further 200ms after V

exceeds –6% of its rated value.

Backup Mode Description

1. Trigger into Backup Mode. The main battery fails and

drops 7.5% below the LTC1558’s internal V

REF

. The

BACKUP pin is asserted after a t

RISE

delay time and the

LTC1558’s boost converter is turned on.

2. Backup Mode. The LTC1558’s boost converter charges

and discharges the inductor with 165mA peak current.

If V

doesn’t recover above (V

REF

–7.5%) (due to a

heavy load), the boost converter increases peak charg-

ing current to 330mA. When V

rises above

REF

– 7.5%), the boost converter stops but the

BACKUP pin remains asserted.

3. Recovery from Backup Mode. While the boost con-

verter is running, the main battery is restored. This

causes the external MOSFET’s body diode to conduct

and V

is pulled higher than (V

REF

– 6%). BACKUP

deasserts and the boost converter finishes its last cycle.

4. Trigger into UVLO. During backup, the 1.2V NiCd cell

grows weak and its terminal voltage falls. The LOBAT

pin is asserted to give an early warning when the cell

voltage drops below 1V. RESET is asserted when the

cell voltage drops below 0.9V and the LTC1558 enters

UVLO mode.

Backup Mode (Main Battery Discharged)

INDUCTOR

CURRENT

RESET

BACKUP

NICD

LOBAT

1558 SW03

RISE

–7.5%

1.2V

–7.5% –7.5%

0.9V

–6%

BOOST CONVERTER OUTPUT

FALL

RISE

4321

LTC1558-3.3/LTC1558-5

APPLICATIONS INFORMATION

WUUU

Overview

The LTC1558 is a versatile backup battery control system

designed to provide all the functions necessary to imple-

ment a complete, highly integrated backup system within

a single chip. It allows the system to maintain its rated

supply voltage during backup, offering maximum system

design flexibility. The LTC1558 allows the use of a low cost

rechargeable NiCd cell for backup, eliminating the need for

expensive, replaceable 4.5V lithium backup cells.

The LTC1558 includes an onboard boost converter de-

signed to generate an adjustable voltage (3V to 10V) from

a single 1.2V NiCd cell. This voltage is connected to the

system’s DC/DC converter input, enabling the system to

continue operation when the main battery fails. A “smart”

recharging circuit uses an accumulating gas gauge to

measure the charge extracted from the backup battery

during a backup cycle. This measured charge is then

replaced in a fast recharge cycle, without wasting excess

power or overcharging the backup cell. An externally

adjustable trickle charge circuit maintains the cell charge

after the fast charge cycle has completed, minimizing

drain from the main battery during standby.

Included in the LTC1558 is a complete backup circuit that

monitors the main system power and automatically

switches in the backup circuit as the primary power supply

falls away (due to a weak or disconnected main battery).

The LTC1558 also performs V

supervisory functions

during normal system operations. An LTC1558-3.3 moni-

tors a 3.3V supply voltage at its V

pin whereas an

LTC1558-5 monitors a 5V supply at its V

pin. In both

cases, the LTC1558 derives power for the majority of the

internal circuitry (except for the boost converter) from the

pin. Table 1 shows the signal conditions for the

LTC1558’s various operating modes. Note that V

Table 1 refers to the rated V

voltage, 3.3V or 5V.

Boost Converter Operation

The LTC1558 uses an onboard synchronous boost con-

verter with a fixed peak current architecture that provides

a simple and flexible system solution while eliminating the

need for conventional frequency compensation. The boost

converter’s output, set by the external divider connected

to the FB pin, supports the main system regulator during

Figure 1. Typical LTC1558 Connection

+BOOST

CONVERTER

TO SYSTEM REGULATOR INPUT

LTC1588

MAIN

BATTERY CIN

1558 F01

BACKUP

LOGIC

VREF

VBAK

BACKUP

backup. It can supply a minimum backup power of

100mW. The boost converter operates in a modified pulse

skipping mode; each switch cycle transfers a known

amount of charge from the backup cell to the regulated

output. This prevents uncontrolled discharge of the backup

cell and allows the LTC1558 to accurately measure the

charge removed from the backup cell by counting the

charge pulses.

The LTC1558 enters backup mode when the main battery

voltage drops. As shown in Figure 1, the main battery

voltage is scaled down by an external resistor divider and

fed to the LTC1558’s backup comparators. These com-

pare the scaled voltage with an internal trimmed V

REF

(1.272V), switching the LTC1558 into backup mode

when V

drops 7.5% below V

REF

. Upon entering backup

mode, the BACKUP pin is asserted and the internal boost

converter turns on. The BACKUP signal is used to turn off

the external P-channel MOSFET, isolating the main bat-

tery from the LTC1558 and the system regulator’s input.

The LTC1558’s boost converter will charge the input

capacitor C

of the system regulator until V

rises

above (V

REF

–7.5%).

Table 1

OPERATING MODES CONDITIONS

UVLO Reset 1V < V

< V

(Rated Value) – 9%

or V

BAT

< 0.9V

Pushbutton Reset V

CTL

< 250mV

UVLO Reset Recovery V

> V

(Rated Value) – 6%

Backup Mode Activation V

< (V

REF

–7.5%)

Backup Mode Exit V

> (V

REF

– 6%)

Boost Converter Activation V

< (V

REF

–7.5%)

Boost Converter Deactivation V

> (V

REF

–7.5%)

LTC1558-3.3/LTC1558-5

Once V

rises above

REF

–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 V

again drops below

REF

– 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

falls below

REF

–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.

LIGHT CURRENT MODE

165mA

(PEAK)

330mA

(PEAK)

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

BAK

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 V

BAK

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 V

BAK

. 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 V

has increased above the

REF

–7.5%)

threshold,

the boost converter shuts off both switches and waits for

to drop below

REF

–7.5%)

again.

If V

is still less than

REF

–7.5%)

after the first boost

cycle, the LTC1558 immediately reconnects SW to ground,

repeating the boost cycle. If after two consecutive pulses,

is still not above the boost threshold

REF

–7.5%)

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

APPLICATIONS INFORMATION

WUUU

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 • LI

), doubling the available

output power. When V

exceeds the

REF

–7.5%)

boost

threshold, the LTC1558 stops the boost converter and

resets the internal two pulse counter. The next time V

falls below

REF

–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 V

BAK

will hover around the user pro-

grammed value.

BAK

Capacitor ESR

The type of output capacitor and the user programmed

BAK

value will affect the LTC1558’s output ripple and

efficiency. In most applications, the main V

BAK

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.

BOOST

CYCLE

BAK

ESR RIPPLE

DISCHARGE

PERIOD

DISCH

CHARGE

PERIOD

1588 F03

Figure 3. VBAK Ripple

The maximum ripple on the V

BAK

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

LTC1558-3.3/LTC1558-5

proportional to the difference between V

BAK

(3V to 10V)

and the battery cell voltage, V

BAT

(1.2V).

Assuming ESR = 0.2Ω, I

IND(PEAK)

= 330mA, V

BAK

= 6V,

RIPPLE(P-P)

= (I

IND(PEAK)

)(R

ESR(CAP)

)

= (330mA)(0.2Ω)

= 66mV

Since V

BAK

must be scaled down to V

, 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,

DISCH

= (L • I

IND(PEAK)

)/(V

BAK

– V

BAT

)

= (22µH • 330mA)/(6V – 1.2V)

= 1.5µs

For lowest V

BAK

= 3V and maximum I

IND(PEAK)

= 445mA,

RIPPLE(P-P)

= 89mV

RB resistor ratio = 2.358

Noise amplitude = 37.7mV

DISCH

= 5µs

The internal V

comparators are designed to have a slow

response time to filter away this ripple. The (V

REF

–6%)

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

edge delay. The (V

REF

– 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 V

BAK

(and V

) to a

higher voltage. The LTC1558 detects the return of the main

APPLICATIONS INFORMATION

WUUU

battery by watching for V

to exceed

REF

–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 V

BAK

capacitor (C

) drains away. The system regulator will not

maintain its output regulation and the system V

will

drop. When V

drops below –9% of the rated voltage for

more than 9µs, the LTC1558’s V

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 V

pin. Upon receipt of the reset signals, the

host system should shut down in a orderly manner. The

LTC1558’s V

supervisory circuit will remain alive until

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

LTC1558-3.3/LTC1558-5

internal logic to ensure that the low cell voltage reset is

triggered only if the CTL pin is between 0.9V and 0.25V.

This will prevent a pushbutton reset (which pulls CTL

below 250mV) from being mistaken as a low cell voltage

condition. Unusual situations where the NiCd cell voltage

drops drastically below 0.25V will also trigger UVLO, since

the LTC1558 will treat this as a “hard” reset after 2

seconds.

An optional LOBAT output, available in the 16-pin GN or SO

package, can be used to signal the system when the cell

voltage falls below 1V, giving an early warning that the

backup cell is heavily discharged. The LOBAT output is

disabled when the LTC1558 is in trickle charge mode

because the CTL pin is pulled to 0.5V by the LTC1558.

Fault Protection and Thermal Limit

The LTC1558’s boost converter incorporates two internal

timers that turn off the switch transistors if the inductor

charge or discharge time gets abnormally long.

The inductor charge time may get abnormally long if the

NiCd cell voltage drops below 0.25V without triggering the

0.25V < V

BAT

< 0.9V low cell voltage comparator. In this

case, the NiCd cell is assumed to be damaged and the

LTC1558’s priority is shutting down the system grace-

fully. In this case, the timer will shut off the N-channel

switch transistor after a maximum charging time (14µs).

The boost converter continues switching but delivers

reduced output power, causing V

to drop. The LTC1558

will enter UVLO either when V

drops below

CC(RATED VOLTAGE)

–9%) or after the LTC1558 detects

CTL lower than 0.25V for 2 seconds, in which case “hard”

reset occurs.

The discharging time can also get abnormally long if a

serious overload condition occurs during switching. The

timer will shut off the P-channel pass transistor after 10µs,

protecting the boost converter. The LTC1558 will end up

in UVLO as V

drops below (V

CC(RATED VOLTAGE)

–9%).

In addition, the LTC1558 is protected for safe area opera-

tion with an internal thermal shutdown circuit. If the device

is overloaded for a long period of time, the thermal

shutdown circuit forces the LTC1558 into UVLO. The

threshold temperature for thermal shutdown is typically

155°C.

APPLICATIONS INFORMATION

WUUU

The LTC1558’s boost converter is designed so that no

current drains from the battery to the load during output

short circuit or V

= 0V conditions. This assures that the

system can be powered down for a long period of time.

This eliminates the risk of finding a nonfunctioning backup

system upon power-up.

Backup Cell Fast Recharge

The LTC1558 includes an onboard gas gauge circuit,

consisting of a 23-bit divider and a 9-bit up/down counter.

The gas gauge logic assumes that the boost converter

uses a 22µH inductor, allowing it to estimate battery

charge by counting switch pulses. The gas gauge counts

up from zero as charge is removed from the backup cell in

backup mode. It takes roughly 8.4 million 165mA boost

pulses (low current mode) to increment the up/down

counter by one count. In high current mode, the 330mA

pulses skip the first two bits of the divider because each

330mA pulse carries four times as much energy as a

165mA pulse. At maximum load and V

= 5V, the gas

gauge divider will increment by one count every 7.5µs

while the boost converter is running. Full count is reached

after approximately 2.2 hours, equivalent to about 512mAhr

of charge.

Upon entering recharge mode (after the main battery is

restored) the LTC1558 connects a 16mA fast recharge

current source from V

to the SW pin. At the same time,

an internal free running oscillator counts down the gas

gauge counter at a rate designed to replace about 160% of

the charge previously removed from the backup cell.

When the gas gauge counter reaches zero, the LTC1558

reduces the charging current at the SW pin to the user-

programmed trickle charge current level.

Under some circumstances, the LTC1558 can exit the

backup mode with invalid gas gauge contents. This can

occur under three possible conditions:

a) The backup cell was completely exhausted during a

backup cycle and the LTC1558 entered UVLO.

b) The backup cell was replaced while the main supply was

disabled.

c) A backup cycle was terminated prematurely by a “hard”

reset or an output overload.

LTC1558-3.3/LTC1558-5

In these cases, the LTC1558 assumes that the backup cell

is exhausted and presets the gas gauge counter to a

default capacity of 128mAhr. It then initiates a recharge

cycle.

Setting the gas gauge to this default value results in a fast

recharge cycle long enough to replenish 1.6 times 128mAhr

of charge into the backup cell (13.9 hours). If the backup

cell is actually exhausted, it will be fully recharged. If the

battery is partially or fully charged, or is significantly

smaller than 128mAhr capacity, the extra charging time

will be wasted. However, the LTC1558’s 15mA fast charge

current should not be high enough to damage the cell.

Once the full-count recharge has been completed, the

backup cell is assumed to be fully charged and subsequent

backup/recharge cycles resume normally.

Although the LTC1558 will not fully recharge backup cells

larger than 128mAhr capacity upon power-up, it can still

be used with such cells. Such a cell will be fully replenished

by the subsequent trickle charge cycle. Under most con-

ditions, even a partially charged large cell will still be

capable of supporting several hours of backup. For ex-

ample, a small 60mAhr button cell can back up the system

for 20 minutes at an output power of 100mW. Note that at

lower programmed V

BAK

values, the boost converter effi-

ciency improves and allows more backup time from the

same cell compared to a higher V

BAK

value.

Once it reaches full recharge, a cell bigger than 512mAhr

is likely to overrun the gas gauge counter before it runs out

of charge during an extended backup cycle. The LTC1558

gas gauge counter will not roll over if this occurs; it will

stay at full count until the backup cycle ends and then

partially recharge the cell with a full count cycle as above.

Very short backup cycles (<32s) may not extract enough

charge from the backup cell to increment the gas gauge

counter at all. To ensure that the backup cell is not slowly

“nibbled” away, the gas gauge counter is always

incremented by 1mAhr each time the controller comes out

of backup. This ensures that the backup cell is always

replenished with at least a 1mAhr charge every time the

LTC1558 enters backup mode.

APPLICATIONS INFORMATION

WUUU

Battery Backup Cell Trickle Charge

When the gas gauge counter reaches zero, the LTC1558

terminates fast recharge and reduces the recharge current

to the user-programmed trickle current level. The LTC1558

provides a trickle current that the user can program from

50µA to 2mA. The trickle current is set by an external

resistor from the positive terminal of the backup cell to the

CTL pin. In trickle charge mode, CTL is regulated to 0.5V,

resulting in a CTL pin current of (V

BAT

– 0.5)/R

EXT

. This

current is internally multiplied to feed back ten times the

EXT

current into the backup battery. Since the LTC1558

trickle charges only after the completion of the fast re-

charge cycle, the backup cell voltage should be very close

to 1.2V. This simplifies the calculation of the R

EXT

resistor

value. For example, a 47k resistor from V

BAT

to CTL sets

the trickle charge current to approximately 150µA.

EXT

47µFI

10I

1.2V

NiCd

CELL

CTL

0.5V

1558 F04

11×1×

LTC1558

–

Figure 4. Trickle Current Charger

Undervoltage Lockout

The LTC1558 includes an undervoltage lockout (UVLO)

system that ensures that the system will shutdown grace-

fully if the backup cell is exhausted or overloaded. As

described in the previous section, the LTC1558 will

LTC1558-3.3/LTC1558-5

terminate backup operation and remain off until the main

power supply returns. It then runs a fast recharge cycle to

recharge the backup cell. An onboard low-battery com-

parator in the 16-pin GN or SO package provides an early

warning signal when the backup cell drops below 1V.

The UVLO circuit also trips if the LTC1558’s V

supervi-

sory circuit detects that V

drops below –9% of the rated

voltage due to overload or output short-circuit condi-

tions. Once the UVLO circuit trips, the LTC1558 asserts

the RESET pins until the V

voltage drops below 1V. It will

then remain off until V

rises to within (V

– 6%) of the

rated output voltage. During power-up from UVLO, the

LTC1558 asserts the RESET pins until the (V

– 6%)

threshold. Once V

exceeds (V

– 6%), the RESET pins

remain asserted for another 200ms (“hard” reset) before

being released to inform the system to start operating.

Reset Operation

The LTC1558 includes an onboard pushbutton reset switch

controller. If the CTL pin is pulled to ground (<250mV) by

a pushbutton or an open-drain output, the LTC1558 gen-

erates a pulse at the RESETpins after the trailing edge of

APPLICATIONS INFORMATION

WUUU

the CTL signal. A short (less than 2s) low going signal at

CTL will generate a “soft” reset (100µs) pulse at the RESET

pins. A low CTL signal for more than 2s will generate a

“hard” reset pulse at its RESETpins. During “hard” reset,

the LTC1558 will disable the boost converter if it is in

backup mode. All signals at the CTL pin are debounced for

20ms to prevent multiple resets, allowing the CTL pin to be

connected directly to a pushbutton to ground.

The RESET pin is an open-drain output that requires an

external pull-up resistor. The RESET pin is a TTL-compat-

ible CMOS output.

Shutdown

The 16-pin LTC1558 has a TTL-compatible input, SHDN,

that shuts down the whole chip, asserts the RESET pins

and places the CTL, V

BAK

and SW pins into high impedance

states. The SHDN pin has an internal pull-up that ensures

the chip will not shut down if the pin is left floating. The

SHDN pin typically draws 8µA when pulled low at V

= 5V.

The chip consumes less than 50µA during shutdown while

is still alive.

Although there is no SHDN pin for the SO-8 package, the

user can shut down the part by pulling CTL to ground. The

chip enters “hard” reset leaving only the bandgap and

comparators alive. The charger and the boost converter

shut off completely. Note that the backup cell slowly

discharges through R

EXT

in this mode.

Inductor Selection

The LTC1558 is designed to operate with a recommended

inductor value of 22µH (±20%) with < 0.2Ω DC resistance.

Using inductor values higher than 22µH will deliver more

output power but will cause the gas gauge counter to

count inaccurately and under recharge the backup cell. At

the same time, the N-channel transistor timer will limit the

peak current if the charging time becomes overextended

due to the higher inductor value. Using inductor values

lower than 22µH will degrade the boost converter’s maxi-

mum output power and cause the gas gauge counter to

overcharge the backup cell. Table 2 lists the recom-

mended surface mount inductor part numbers.

CTL

RESET

20ms

DEBOUNCE

>0.25V

CTL

< 20ms

20ms < t

CTL

< 2s

>0.25V

“SOFT” PUSHBUTTON RESET AT CTL

A. CTL < 0.25V FOR LESS THAN 20ms

B. CTL > 0.25V FOR MORE THAN 20ms

“HARD” PUSHBUTTON RESET AT CTL

CTL < 0.25V FOR MORE THAN 2s

20ms DEBOUNCE AT FALLING AND RISING RESET EDGE

100µs100µs

CTL

1558 F05

RESET

CTL

RESET

200ms

CTL

> 2s

20ms

Figure 5. Pushbutton Resets

LTC1558-3.3/LTC1558-5

Table 2. Recommended Inductors

PART TYP INDUCTOR

MANUFACTURER NUMBER VALUE DCR (Ω)

Sumida CD54-220 22µH ±20% 0.18

Sumida CDRH73/74 22µH ±20% 0.2/0.11

Capacitor Selection

The LTC1558 requires a minimum V

BAK

capacitor of 44µF

to ensure that the boost converter can regulate the output

at 20mA load. The capacitor’s ESR should be small (< 0.2Ω)

to minimize voltage spikes that might incorrectly trigger

the LTC1558’s internal FB comparators. Note that the

LTC1558 can usually share the output capacitor with the

system regulator. Thus its ratings like V

MAX

, I

RIPPLE(RMS)

etc., will all have to meet the system regulator’s specifica-

tions as well.

Battery Selection

A primary application for the LTC1558 is a “bridging”

supply, only providing backup current while the main

system battery is being replaced. In these applications,

the LTC1558 works well with NiCd button cells or small

cylindrical cells, reducing system costs and board space.

It is optimized for use with up to 512mAhr battery

capacities.

The LTC1558 can work with standard or memory backup

specific NiCd cells. Memory backup cells can operate at

higher temperatures and have lower self discharge rates.

The LTC1558’s trickle charger is designed to accommo-

date both memory backup cells (with low self discharge)

and standard cells (with higher self discharge). Some

recommended manufacturers and part numbers are listed

in Table 3.

APPLICATIONS INFORMATION

WUUU

Table 3. Button/Cylindrical NiCd 1.2V Cells

PART CAPACITY

MANUFACTURER NUMBER (mAhr) R

SERIES

(Ω)

SAFT GB60 60 1.1

(Memory Backup) GB170 170 0.4

GB280 280 0.4

SAFT VB10E 100 0.038

(Standard) VB22E 220 0.022

VB30E 300 0.017

VB60E 600 0.014

Sanyo N-50AAA 55 0.055

(Standard) N-110AA 120 0.03

N-120TA 130 0.034

N-150N 170 0.027

N-200AAA 220 0.021

N-270AA 305 0.015

N-500A 500 0.09

Panasonic P-11AA 110 0.08

(Standard)

The internal resistance of the backup cell increases power

dissipation as the boost converter draws current from it

during switching, degrading efficiency. Due to the fixed

inductor peak current architecture, the LTC1558’s boost

converter output power drops significantly when the NiCd

cell’s internal resistance increases at the end of its charge.

This is because the inductor charging time will increase

due to a larger R/L time constant, decreasing the switching

frequency. It is advisable, especially for batteries with high

internal resistance, to include a 47µF bypass capacitor

across the battery to ensure that the boost converter can

deliver the maximum output power regardless of the NiCd

internal resistance.

LTC1558-3.3/LTC1558-5

TYPICAL APPLICATIONS

LTC1558-3.3 Low Main Battery Voltage (4.5V to 10V) Application

SENSE

–

SENSE

INTV

BOOST

OSC

SGND PGND

RUN/SS

OSENSE

LTC1435

0.1

F R

SENSE

0.033Ω

L1*

4.7

16V

D1 CMDSH-3

1558 TA03

MBRS140T3 R5

20k

SUMIDA CDRH125-10

IRC LR2D1D-01-RQ33-F

SUMIDA CD54-220

†

35.7k

1000pF

N-CHANNEL

Si4412DY

N-CHANNEL

Si4412DY

SFB

EXTV

1µF

510

100pF

OSC

68pF

10k

0.1

330pF

100pF

51pF

22µF

35V

×2

100µF

10V

×2

OUT

3.3V

CTL

C11

47µF

6.3V

BACKUP

BATTERY

1.2V

NiCd

MAIN

BATTERY

4.5V TO 10V

Q11

P-CHANNEL

Si4431DY

R14

14k SW

LTC1558-3.3

L11

†

22µH

GND

R11

221k

R12

100k

RESET

PUSHBUTTON 2

BAK

RESET

BACKUP

R13

100k

C12

1µF

RESET

BACKUP

Typical “Low Voltage” Application

The maximum main battery voltage is less than the maxi-

mum V

BAK

pin voltage (12V). This configuration has the

lowest number of external components.

The LTC1435’s minimum input voltage is 3.5V. The V

BAK

voltage, set by R11 and R12, is programmed to 3.8V.

Therefore, the main battery’s lowest voltage should be

3.8V + 1 body diode drop = 4.5V. This will enable a fresh

main battery to turn on the external P-channel MOSFET

and power up the system out of UVLO during cold power

boot or out of backup mode when the LTC1558 is power-

ing up the system.

A 100k pull-up resistor enables the open-drain BACKUP

pin to turn the external P-channel MOSFET off when V

BAK

is higher than V

LTC1558-3.3/LTC1558-5

TYPICAL APPLICATIONS

LTC1558-5 Medium Main Battery Voltage (7V to 18V) Application

SENSE

–

SENSE

INTV

BOOST

OSC

SGND PGND

RUN/SS

OSENSE

LTC1435

0.1

F R

SENSE

0.033Ω

L1*

4.7

16V

D1 CMDSH-3

1558 TA04

MBRS140T3 R6

11k

SUMIDA CDRH125-10

IRC LR2D1D-01-RQ33-F

SUMIDA CD54-220

†

35.7k

1000pF

N-CHANNEL

Si4412DY

N-CHANNEL

Si4412DY

SFB

EXTV

1µF

510

100pF

OSC

68pF

10k

0.1

330pF

100pF

51pF

22µF

35V

×2

OUT

100µF

10V

×2

OUT

CTL

C11

47µF

6.3V

BACKUP

BATTERY

1.2V

NiCd

MAIN

BATTERY

7V TO 18V

Q11

P-CHANNEL

Si4431DY

R14

14k SW

LTC1558-5

L11

†

22µH

GND

R11

422k

R12

100k

RESET

PUSHBUTTON 2

BAK

RESET

BACKUP

R13

100k

C12

1µF

D11

1N5818

RESET

BACKUP

Typical “Medium Voltage” Application

The maximum main battery voltage is more than the

maximum V

BAK

pin voltage (12V). This configuration is

needed for most notebook computers that have 3-cell or

4-cell series connected lithium battery packs.

The Schottky diode D11 (1N5818) prevents the main

battery’s high terminal voltage from overstressing the

LTC1558’s V

BAK

pin during nonbackup conditions. An

internal Zener inside the LTC1558 will clamp V

BAK

to 12V

when the 1N5818’s reverse bias leakage current

increases at high temperature.

A 100k pull-up resistor enables the LTC1558’s open-drain

BACKUP pin to turn the external P-channel MOSFET off

during backup mode, even when V

BAK

is higher than V

The main battery pack should have an internal control to

shut itself down once its energy is used up. This prevents

the lithium cells from deep discharge damage. Once the

main battery shuts down, the FB voltage drops and the

LTC1558 switches to backup mode.

LTC1558-3.3/LTC1558-5

TYPICAL APPLICATIONS

LTC1558-5 High Main Battery Voltage (48V) Application

PGATE

P-DRIVE

SENSE

–

CAP

SHUTDOWN 2

RGND

NGATE

PGND

SGND

SHUTDOWN 1

SENSE

LTC1149-5

0.068µF

0.047µF

1N4148

R11

422k

R12

100k

R16

100k

Z11

12V

Q12

2N3906

1N4148

3300pF

R15

100k

MAIN

BATTERY

48V

Q11

P-CHANNEL

MTD2955E

470pF

3.3µF

1000pF

N-CHANNEL

IRFZ34

MBR380

0.1µF

L1*

68µH R

SENSE

0.04Ω

OUT

220µF

10V

OS-CON

1558 TA05

OUT

2.5A

*HURRICANE LAB HL-KI168M

**IRC LR2512-01-RO40-5

†

SUMIDA CD54-220

P-CHANNEL

MTD2955E

SHUTDOWN

100Ω

100µF

100V

CTL

C11

47µF

6.3V

BACKUP

BATTERY

1.2V

NiCd

R14

14k SW

LTC1558-5

L11

†

22µH

RESET

PUSHBUTTON

D11

MBR170

BAK

RESET

BACKUP

GND

C12

1µF

RESET

BACKUP

Typical “High Voltage” Application

The maximum main battery voltage is 48V.

The Schottky diode D11 (MBR170) prevents the main

battery’s high terminal voltage from overstressing the

LTC1558’s V

BAK

pin during nonbackup conditions. An

internal Zener inside the LTC1558 will clamp V

BAK

to 12V

when the MBR170’s reverse bias leakage current

increases at high temperature.

As shown above, the design must ensure that V

BAT

does

not force the external P-channel MOSFET’s V

above its

maximum rating (15V for the MTD2955E) shown during

nonbackup mode.

During nonbackup mode, the LTC1558’s open-drain

BACKUP pin is low. The external 12V Zener and 2N3906

conduct and the MTD2955E’s V

is clamped at approxi-

mately 12V. During backup, the BACKUP pin floats and the

2N3906’s base voltage is pushed nearer to V

BAK

. The

MTD2955E is effectively turned off, isolating the main

battery from V

BAK

during backup.

The main battery pack should have an internal control to

shut itself down once its energy is used up. This prevents

it from deep discharge damage.

LTC1558-3.3/LTC1558-5

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

GN Package

16-Lead Plastic SSOP (Narrow 0.150)

(LTC DWG # 05-08-1641)

GN16 (SSOP) 1197

* DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

345678

0.229 – 0.244

(5.817 – 6.198)

0.150 – 0.157**

(3.810 – 3.988)

16 15 14 13

0.189 – 0.196*

(4.801 – 4.978)

12 11 10 9

0.016 – 0.050

(0.406 – 1.270)

0.015 ± 0.004

(0.38 ± 0.10) × 45°

0° – 8° TYP

0.007 – 0.0098

(0.178 – 0.249)

0.053 – 0.068

(1.351 – 1.727)

0.008 – 0.012

(0.203 – 0.305)

0.004 – 0.0098

(0.102 – 0.249)

0.025

(0.635)

BSC

LTC1558-3.3/LTC1558-5

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

0.016 – 0.050

0.406 – 1.270

0.010 – 0.020

(0.254 – 0.508)× 45°

0° – 8° TYP

0.008 – 0.010

(0.203 – 0.254)

12345678

0.150 – 0.157**

(3.810 – 3.988)

16 15 14 13

0.386 – 0.394*

(9.804 – 10.008)

0.228 – 0.244

(5.791 – 6.197)

12 11 10 9

S16 0695

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYP

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

1234

0.150 – 0.157**

(3.810 – 3.988)

8765

0.189 – 0.197*

(4.801 – 5.004)

0.228 – 0.244

(5.791 – 6.197)

0.016 – 0.050

0.406 – 1.270

0.010 – 0.020

(0.254 – 0.508)× 45°

0°– 8° TYP

0.008 – 0.010

(0.203 – 0.254)

SO8 0996

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYP

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

S Package

16-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LTC1558-3.3/LTC1558-5

1558f LT/TP 0298 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1998

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

(408) 432-1900

FAX: (408) 434-0507

●

TELEX: 499-3977

●

www.linear-tech.com

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