Datasheet LTC1558-5, LTC1558-3.3 Datasheet (Linear Technology)

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Page 1

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 VCC = 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, automatic backup switching and a microprocessor reset generator. The boost converter uses a synchronous switching architecture to achieve a typical efficiency of 70%, ensuring 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 programmable 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 VCC fault detector and reset generator eliminate the need for a separate microprocessor 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

15 20

VCC = 3.3V V

BAK

= 3.78V NiCd CELL CAPACITY = 110mAHrs

Backup Time

vs 3.3V Output Load Current

C1 1µF

1.2V NiCd

BACKUP

**BATTERY

MAIN

BATTERY

4.5V TO 10V

R1 14k

R2 100k

LOAD CURRENT 3A AT NORMAL MODE 30mA AT BACKUP MODE

LTC1558-3.3

L1

22µH*

R4 221k 1%

R3 220k

R5 100k 1%

RESET

Q1

Si4431DY

1558 TA01

BAK

BACKUP

RESET

GND

CTL

SUMIDA CD54-22µH SANYO CADNICA N-110AA CONSULT LTC1435 DATA SHEET FOR CIRCUIT APPLICATION INFORMATION

CIN 100µF 16V ×2

*

**

†

VCC

3.3V

SYSTEM

µP

C2

0.1µF

LTC1435†

SYNCHRONOUS

BUCK REGULATOR

Page 2

LTC1558-3.3/LTC1558-5

ABSOLUTE MAXIMUM RATINGS

(Note 1)

Terminal Voltages

..........................................................................................

BAK

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

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

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

BAT

= 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 VCC = 0V ● 0.1 15 µA

VCC(SHDN)

Supply Current During Shutdown (Note 3) ● 50 µA

BAK(ON)

Backup Request Trip Point Voltage at VFB Relative to V

REF

● –10.5 –7.5 –5.5 %

BST(ON)

Boost Converter Assertion Trip Point Voltage at VFB Relative to V

REF

● –10.5 –7.5 –5.5 %

BAK(OFF)

Backup Deassertion Trip Point Voltage at VFB Relative to V

REF

● –9 –6 –4 %

BST(OFF)

Boost Converter Deassertion Trip Point Voltage at VFB 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



1 2 3 4 5 6 7 8

16 15 14 13 12 11 10

SW SW

PGND

GND

CTL

SHDN

FB

BAK



BAK



 BACKUP RESET RESET NC LOBAT 

JMAX

= 125°C, θJA = 110°C/ W (GN)

JMAX

= 125°C, θJA = 110°C/ W (S)

ORDER PART

NUMBER

S8 PART MARKING

LTC1558CS8-3.3 LTC1558CS8-5

155833 15585

JMAX

= 125°C, θJA = 130°C/ W

1

2

3

8

7

6

TOP VIEW

S8 PACKAGE

8-LEAD PLASTIC SO

SW

GND

CTL

BAK



BACKUP

RESET

Page 3

LTC1558-3.3/LTC1558-5

ELECTRICAL CHARACTERISTICS

BAT

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

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

UVLO(ON)

VCC 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)VCC

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 VCC = 1V, I

SINK

= 10µA ● 5 200 mV

RST

RESET Output Voltage VCC = 4.25V, I

SINK

= 1.6mA ● 0.1 0.4 V

RESET Output Current Output Source Current, VCC = 3.3V 10 mA

Output Source Current, VCC = 5V 20 mA

Short-Circuit Current Output Sink Current, VCC = 3.3V 20 mA

Output Sink Current, V

= 5V 40 mA

Internal VCC Monitor Comparator

PLH

UVLO(ON)

Comparator Propagation ∆VCC = –(9% VCC + 300mV), VOD = 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 VCC = 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.

Page 4

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

VCC = 5V (LTC1558-5) V

BAK

= 6V

Output Power vs Battery Voltage

OUTPUT VOLTAGE, V

BAK

(V)

SWITCHING FREQUENCY (kHz)

1558 G03

125

100

BATT

= 1.2V

= 3.3V (LTC1558-3.3)

= 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, V

BAK

(V)

DUTY CYCLE (%)

1558 G04

100

BATT

= 1.2V

= 3.3V (LTC1558-3.3)

= 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)

234

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

234

1558 G09

2.5

3.0

3.10V

VCC = 3.3V (LTC1558-3.3)

Page 5

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 22µ H 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 (V

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 generated 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 VCC is greater than or equal to 1V.

RESET also provides a low going 100µ s 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 internally to this pin to act as a voltage clamp. See the Applications Information section for more details.

VCC (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 VCC to ground. The UVLO detector inside the LTC1558 monitors VCC.If VCC drops below the rated output voltage by greater than 9%, the LTC1558 enters UVLO mode and RESET is asserted. The LTC1558 will only exit from UVLO if VCC 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.

Page 6

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 TTLcompatible 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%

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

VCC/V

BAK

CTL

–

UVLO

DETECTOR

RESET

BACKUP

LOBAT

1558 BD

BLOCK DIAGRAM

1558 SW01

BAT

 MAIN BATTERY

MULTIPLE POWER OUTPUTS

1.2V NiCd

LTC1558

RESET

BAK

EXT

BACKUP

TO 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)

Page 7

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 VCC = 1V.

3. The LTC1558’s internal bandgap wakes up. The LTC1558’s internal boost converter does not turn on as RESET remains asserted. Once VFB is greater than (V

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 VFB 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 VFB doesn’t recover above (V

REF

–7.5%) (due to a heavy load), the boost converter increases peak charging current to 330mA. When VFB rises above (V

REF

– 7.5%), the boost converter stops but the

BACKUP pin remains asserted.

3. Recovery from Backup Mode. While the boost converter is running, the main battery is restored. This causes the external MOSFET’s body diode to conduct and VFB 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

Page 8

LTC1558-3.3/LTC1558-5

APPLICATIONS INFORMATION

WUU

Overview

The LTC1558 is a versatile backup battery control system designed to provide all the functions necessary to implement 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 designed 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 VCC supervisory functions during normal system operations. An LTC1558-3.3 monitors a 3.3V supply voltage at its VCC pin whereas an LTC1558-5 monitors a 5V supply at its VCC pin. In both cases, the LTC1558 derives power for the majority of the internal circuitry (except for the boost converter) from the VCC pin. Table 1 shows the signal conditions for the LTC1558’s various operating modes. Note that VCC in Table 1 refers to the rated VCC voltage, 3.3V or 5V.

Boost Converter Operation

The LTC1558 uses an onboard synchronous boost converter 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

1558 F01

BACKUP

LOGIC

REF

BAK

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 compare the scaled voltage with an internal trimmed V

REF

(1.272V), switching the LTC1558 into backup mode when VFB 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 battery from the LTC1558 and the system regulator’s input. The LTC1558’s boost converter will charge the input capacitor CIN of the system regulator until VFB rises above (V

REF

–7.5%).

Table 1

OPERATING MODES CONDITIONS

UVLO Reset 1V < VCC < V

(Rated Value) – 9%

or V

BAT

< 0.9V

Pushbutton Reset V

CTL

< 250mV

UVLO Reset Recovery VCC > V

(Rated Value) – 6%

Backup Mode Activation VFB < (V

REF

–7.5%)

Backup Mode Exit VFB > (V

REF

– 6%)

Boost Converter Activation VFB < (V

REF

–7.5%)

Boost Converter Deactivation VFB > (V

REF

–7.5%)

Page 9

LTC1558-3.3/LTC1558-5

Once VFB 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 VFB 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 transferred for the first two consecutive switch cycles. When VFB 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 V

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 externally to V

BAK

(CIN of the system regulator, Figure 1). The inductor current falls at a rate proportional to the difference 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 VFB has increased above the

REF

–7.5%)

threshold, the boost converter shuts off both switches and waits for VFB to drop below

REF

–7.5%)

again.

If VFB 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, VFB is still not above the boost threshold

REF

–7.5%)

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

APPLICATIONS INFORMATION

WUU

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

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 V

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 capacitor is located some distance away from the LTC1558, a local output capacitor may be necessary.

1

BOOST

CYCLE

BAK

ESR RIPPLE

DISCHARGE

PERIOD

DISCH

CHARGE

PERIOD

1588 F03

Figure 3. V

BAK

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

Page 10

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 VFB, the external

resistor ratio

= 6V/1.272V = 4.717

Therefore the noise amplitude seen by the FB comparators 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 t

DISCH

= 5µs

The internal VFB comparators are designed to have a slow response time to filter away this ripple. The (V

REF

–6%)

FB comparator has a 6µ s rising edge delay and 2µ s 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 VFB) to a

higher voltage. The LTC1558 detects the return of the main

APPLICATIONS INFORMATION

WUU

battery by watching for VFB 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 (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

Page 11

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 gracefully. In this case, the timer will shut off the N-channel switch transistor after a maximum charging time (14µ s).

The boost converter continues switching but delivers reduced output power, causing VCC to drop. The LTC1558

will enter UVLO either when VCC 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 VCC drops below (V

CC(RATED VOLTAGE)

–9%).

In addition, the LTC1558 is protected for safe area operation 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

WUU

The LTC1558’s boost converter is designed so that no current drains from the battery to the load during output short circuit or VCC = 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 VCC = 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 VCC 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 userprogrammed 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.

Page 12

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 conditions, even a partially charged large cell will still be capable of supporting several hours of backup. For example, 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 efficiency 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

WUU

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 50µ A 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 R

EXT

current into the backup battery. Since the LTC1558 trickle charges only after the completion of the fast recharge 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

47µF

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 gracefully if the backup cell is exhausted or overloaded. As described in the previous section, the LTC1558 will

Page 13

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 comparator 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 VCC supervisory circuit detects that VCC drops below – 9% of the rated VCC voltage due to overload or output short-circuit conditions. Once the UVLO circuit trips, the LTC1558 asserts the RESET pins until the VCC voltage drops below 1V. It will then remain off until VCC rises to within (VCC – 6%) of the rated output voltage. During power-up from UVLO, the LTC1558 asserts the RESET pins until the (VCC – 6%) threshold. Once VCC exceeds (VCC – 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 generates a pulse at the RESETpins after the trailing edge of

APPLICATIONS INFORMATION

WUU

the CTL signal. A short (less than 2s) low going signal at CTL will generate a “soft” reset (100µ s) 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-compatible 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 8µ A when pulled low at VCC = 5V. The chip consumes less than 50µ A during shutdown while VCC 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 22µ H (±20%) with < 0.2Ω DC resistance.

Using inductor values higher than 22µ H 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 22µ H will degrade the boost converter’s maximum output power and cause the gas gauge counter to overcharge the backup cell. Table 2 lists the recommended surface mount inductor part numbers.

CTL

RESET

A

20ms

DEBOUNCE

B

>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

100µs 100µs

CTL

1558 F05

RESET

CTL

RESET

200ms

CTL

> 2s

20ms

Figure 5. Pushbutton Resets

Page 14

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 44µ F

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 specifications 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 accommodate 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

WUU

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.

Page 15

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

C4

0.1

SENSE



0.033Ω

L1*

C3

4.7

F

16V

D1 CMDSH-3

1558 TA03

D2 MBRS140T3

R5 20k 1%

SUMIDA CDRH125-10 IRC LR2D1D-01-RQ33-F SUMIDA CD54-220

*

**

†

R1

35.7k 1%

C5 1000pF

Q2 N-CHANNEL Si4412DY

Q1 N-CHANNEL Si4412DY

SFB

EXTV

C2 1µF

510

C6

100pF

OSC



68pF

RC 10k



0.1

CC 330pF

C1 100pF

CC2

51pF

CIN 22µF 35V ×2

CIN 100µF 10V ×2

OUT



3.3V 3A

CTL

C11 47µF

6.3V

BACKUP BATTERY

1.2V NiCd

MAIN

BATTERY

4.5V TO 10V

Q11

P-CHANNEL

Si4431DY

R14 14k

LTC1558-3.3

L11

†



22µH

GND

R11 221k 1%

R12 100k 1%

5 6

RESET

PUSHBUTTON

2 4

BAK

RESET

BACKUP

R13 100k

C12 1µF

RESET

BACKUP

Typical “Low Voltage” Application

The maximum main battery voltage is less than the maximum 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 powering 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 VCC.

Page 16

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

C4

0.1

SENSE



0.033Ω

L1* 10

C3

4.7

F

16V

D1 CMDSH-3

1558 TA04

D2 MBRS140T3

R6 11k 1%

SUMIDA CDRH125-10 IRC LR2D1D-01-RQ33-F SUMIDA CD54-220

*

**

†

R1

35.7k 1%

C5 1000pF

Q2 N-CHANNEL Si4412DY

Q1 N-CHANNEL Si4412DY

SFB

EXTV

C2 1µF

510

C6

100pF

OSC



68pF

RC 10k



0.1

CC 330pF

C1 100pF

CC2

51pF

CIN 22µF 35V ×2

OUT



100µF 10V ×2

OUT

 5V 3A

CTL

C11 47µF

6.3V

BACKUP BATTERY

1.2V NiCd

MAIN

BATTERY

7V TO 18V

Q11

P-CHANNEL

Si4431DY

R14 14k

LTC1558-5

L11

†



22µH

GND

R11 422k 1%

R12 100k 1%

5 6

RESET

PUSHBUTTON

2 4

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 VCC.

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.

Page 17

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

1

2

3

4

5

6

7

16

15

14

13

12

11

10

9



LTC1149-5

C2

0.068µF

C1

0.047µF

D1 1N4148

R11 422k 1%

R12 100k 1%

R16

100k

Z11 12V

Q12

2N3906

D2

1N4148

CC 3300pF

R15 100k

MAIN BATTERY 48V

Q11

P-CHANNEL

MTD2955E

RC 1k

C4 470pF

C3

3.3µF

C5 1000pF

Q2 N-CHANNEL IRFZ34

D3 MBR380

C6

0.1µF

L1* 68µH

SENSE

**

0.04Ω

OUT



220µF 10V OS-CON

1558 TA05

OUT



5V

2.5A

*HURRICANE LAB HL-KI168M **IRC LR2512-01-RO40-5

†

SUMIDA CD54-220

Q1 P-CHANNEL MTD2955E

SHUTDOWN

R4

100Ω

R3

100Ω

CIN 100µF 100V

CTL

C11 47µF

6.3V

BACKUP BATTERY

1.2V NiCd

R14 14k

LTC1558-5

L11

†



22µH

5 6

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 VGS 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 VGS is clamped at approximately 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.

Page 18

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

678

0.229 – 0.244

(5.817 – 6.198)

0.150 – 0.157** (3.810 – 3.988)

0.189 – 0.196* (4.801 – 4.978)

12 11 10

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

Page 19

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)

0.150 – 0.157** (3.810 – 3.988)

0.386 – 0.394*

(9.804 – 10.008)

0.228 – 0.244

(5.791 – 6.197)

11 10

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)

0.150 – 0.157** (3.810 – 3.988)

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

Page 20

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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