LT1239CS - Linear Technology

LT1239

Backup Battery

Management Circuit

■

Micropower Operation (I

= 20

■

Adjustable Regulator for Battery Charging

■

4.85V Regulator for Battery Regulation

■

Cell Voltage Equalization in 2-Cell Systems

■

Low-Battery Detector Protects Lithium Cells

■

Comparator for Automatic Power Switching

■

Shutdown

■

Output Current Sensing

■

Current and Thermal Limiting

■

Reverse Output Protection

■

16-Pin SO Package

■

Operates on 7V to 30V Input

The LT

1239 is a micropower backup battery manage-

ment system for portable computers and instrumenta-

tion. It contains two regulators for regulating the battery

voltage and memory voltage and a comparator for switch-

ing between main power and backup power. The first

regulator provides a constant voltage charge for the

backup batteries and is adjustable from 3.75V up to 20V.

An equalization amplifier combined with the first regulator

provides precision charge equalization for a 2-cell

lithium-ion system. A second regulator with 4.85V output

provides a regulated backup battery voltage to the memory

when main power is lost. The second regulator also

isolates the backup battery from the main 5V supply

during normal operation when the memory is being pow-

ered by the 5V supply.

A comparator is included which provides automatic

switchover from main 5V power to backup power ensur-

ing uninterrupted power for memory and power monitor-

FEATURES

DESCRIPTION

■

Backup Battery Management Systems for Portable

Computers

■

Lithium-Ion Backup Systems

■

NiCd Backup Systems

APPLICATIONS

TYPICAL APPLICATION

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

Lithium-Ion Backup System

604k

69.8k

681k

22µF

3.4V

Li-Ion

CELL

3.4V

Li-Ion

CELL

E/A (IN)

E/A (OUT)

OUT 1

ADJ

MON1

300pF

INPUT 2

120Ω* 120Ω*

INPUT 2

OUT 2

MON2

SHDN1

MAIN

BATTERY

PACK

7V TO 24V

INPUT 1

22µF

INPUT 1

SHDN2

OUT

GND

* REQUIRED BY SOME SAFETY AGENCIES

SEE APPLICATIONS INFORMATION

FOR INFORMATION ON

SELECTING VALUES.

LTC1239 • TA01

–

5V SYSTEM

POWER

MEMORY POWER

MANAGEMENT

LOW-BATTERY

DETECT

REGULATOR

FOR INFORMATION PURPOSES ONLY

OBSOLETE:

Contact Linear Technology for Potential Replacement

LT1239

DESCRIPTION

ing circuitry. A low-battery detector with a 5V threshold

powers down the second regulator and the error amplifier

to limit the discharge voltage of the backup cells. This

prevents deep discharge damage to the lithium cells. Both

regulators have independent shutdown and current moni-

tor functions.

LT1239CS

ORDER PART

NUMBER

Consult factory for Industrial and Military grade parts.

JMAX

= 100°C, θ

= 120°C/ W

PARAMETER CONDITIONS MIN TYP MAX UNITS

Regulator 1 (Notes 7, 8)

Regulated Output Voltage (V

ADJ

= V

OUT1

IN1

= 4.3V, I

OUT

= 1mA, T

= 25°C 3.700 3.750 3.800 V

IN1

= 4.8V to 24V, I

OUT

= 1mA to 30mA ●3.650 3.750 3.825 V

Line Regulation I

LOAD

= 1mA, V

IN1

= 4.3V to 30V ●210 mV

Load Regulation V

IN1

= 5V, I

LOAD

= 1mA to 30mA, T

= 25°C–12–25mV

IN1

= 5V, I

LOAD

= 1mA to 30mA ●–20 –50 mV

IN1

= 5V, I

LOAD

= 1mA to 50mA, T

= 25°C–20mV

IN1

= 5V, I

LOAD

= 1mA to 50mA –30 mV

Dropout Voltage (Note 9) I

LOAD

= 1mA, T

= 25°C 0.15 0.20 V

LOAD

= 30mA, T

= 25°C 0.25 0.40 V

LOAD

= 50mA, T

= 25°C 0.30 V

Ground Pin Current (Notes 10, 11) I

LOAD

= 0mA, V

IN1

= 3.75V ●20 30 µA

LOAD

= 30mA, V

IN1

= 3.75V ●0.80 1.2 mA

LOAD

= 50mA, V

IN1

= 3.75V 1.35 mA

Adjust Pin Bias Current (Note 12) T

= 25°C 40 120 nA

ELECTRICAL CHARACTERISTICS

PACKAGE/ORDER INFORMATION

(Note 1)

Input 1 Voltage ...................................................... ±30V

Input 2 Voltage .............................................30V, –0.6V

Output 1 Voltage...........................................30V, –0.6V

Output 2 Voltage.............................................6V, –0.6V

Adjust Pin Current ................................................ 10mA

SHDN1, SHDN2 (Note 2)

Input Voltage ..............................................6V, –0.6V

Input Current ...................................................... 5mA

MON1

Voltage

(Note 3) .......................... (V

IN1

– 30V) < I

MON1

< V

IN1

MON2

Voltage

(Note 4) .......................... (V

IN2

– 30V) < I

MON2

< V

IN2

E/A Output Voltage (Note 5) .... –0.6V < V

E/A(OUT)

< V

IN2

E/A Input Voltage (Note 5) ..........–0.6V < V

E/A(IN)

< V

IN2

5V Input Voltage .............................................6V, –0.6V

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

Junction Temperature Range.............................(Note 6)

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

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

ABSOLUTE MAXIMUM RATINGS

TOP VIEW

S PACKAGE

16-LEAD PLASTIC SO

ADJ

GND

SHDN1

GND

SHDN2

E/A (IN)

E/A (OUT)

OUT 1

IMON1

INPUT 1

5VIN

OUT 2

IMON2

INPUT 2

LT1239

ELECTRICAL CHARACTERISTICS

PARAMETER CONDITIONS MIN TYP MAX UNITS

Regulator 1 (Notes 7, 8)

Shutdown Threshold V

OUT1

= Off to On ●1.20 2.8 V

OUT1

= On to Off ●0.25 0.75 V

Shutdown Pin Current (Note 13) V

SHDN1

= 0V ●24 µA

Quiescent Current in Shutdown (Note 10) V

IN1

= 24V, V

SHDN1

= 0V ●10 16 µA

Ripple Rejection V

IN1

= 5V (Avg), V

RIPPLE

= 0.5V

P-P

RIPPLE

= 120Hz, I

LOAD

= 20mA, T

= 25°C5059 dB

Current Limit V

IN1

= 7V, V

OUT1

= 0V, T

= 25°C3050mA

OUT1

= V

OUT1(NOM)

– 100mV, T

= 25°C4070mA

Reverse Output Current V

OUT1

= 3.75V, V

IN1

< 3.75V ●612 µA

OUT1

= 3.75V, V

IN1

= Open Circuit ●612 µA

Current Monitor Pin Output Current V

OUT1

= 3.75V, V

IMON1

= 0V, I

OUT1

= 1mA 4.6 µA

OUT1

= 3.75V, V

IMON1

= 0V, I

OUT1

= 10mA ●38 44 50 µA

OUT1

= 3.75V, V

IMON1

= 0V, I

OUT1

= 50mA 215 µA

Comparator

Output Saturation Voltage (V

5VIN

– V

OUT2

IN1

= 7V, V

IN2

= 0V, V

5VIN

= 5V, I

OUT2

= 1mA ●12 40 mV

IN1

= 7V, V

IN2

= 0V, V

5VIN

= 5V, I

OUT2

= 30mA ●110 150 mV

IN1

= 7V, V

IN2

= 0V, V

5VIN

= 5V, I

OUT2

= 50mA ●135 220 mV

Low-Battery Detector

Turn-Off Threshold T

= 25°C 4.85 5.00 5.15 V

Turn-On Threshold T

= 25°C 5.3 V

Hysteresis T

= 25°C 0.2 0.3 V

Regulator 2

Regulated Output Voltage V

IN2

= 6.8V, I

OUT

= 1mA, T

= 25°C 4.775 4.850 4.925 V

Output Voltage Temperature Coefficient –0.5 mV/°C

Line Regulation I

OUT2

= 1mA, V

IN2

= 5.4V to 10V ●25 mV

Load Regulation V

IN2

= 6.8V, I

LOAD

= 1mA to 30mA, T

= 25°C–12–25mV

IN2

= 6.8V, I

LOAD

= 1mA to 30mA ●–20 –50 mV

IN2

= 6.8V, I

LOAD

= 1mA to 50mA, T

= 25°C–20mV

IN2

= 6.8V, I

LOAD

= 1mA to 50mA –30 mV

Ground Pin Current I

LOAD

= 0mA, V

IN2

= 5.4V ●16 25 µA

LOAD

= 30mA, V

IN2

= 5.4V ●0.80 1.2 mA

LOAD

= 50mA, V

IN2

= 5.4V ●1.35 mA

Shutdown Threshold V

OUT2

= Off to On ●1.20 2.8 V

OUT2

= On to Off ●0.25 0.75 V

Shutdown Pin Current V

SHDN2

= 0V ●1.7 4 µA

Ripple Rejection V

IN2

= 6.4V (Avg), V

RIPPLE

= 0.5V

P-P

RIPPLE

= 120Hz, I

LOAD

= 20mA, T

= 25°C5058 dB

Current Limit V

IN2

= 6.8V, V

OUT2

= 0V, T

25°C3050mA

OUT2

= V

OUT2(NOM)

– 100mV, T

= 25°C4070mA

Reverse Output Current V

OUT2

= 4.85V, V

IN2

< 4.85V ●612 µA

OUT2

= 4.85V, V

IN2

= Open Circuit ●612 µA

Current Monitor Pin Output Current V

OUT2

= 6.8V, V

IMON2

= 0V, I

OUT2

= 1mA 4.7 µA

OUT2

= 6.8V, V

IMON2

= 0V, I

OUT2

= 10mA ●35 41 47 µA

OUT2

= 6.8V, V

IMON2

= 0V, I

OUT2

= 50mA 210 µA

Error Amplifier

Bias Current V

E/A(IN)

= 3.4V, V

IN2

= 6.8V ●320 nA

LT1239

ELECTRICAL CHARACTERISTICS

PARAMETER CONDITIONS MIN TYP MAX UNITS

Offset Voltage ●015 mV

Output Current Sourcing V

IN2

= 6.8V, V

E/A(IN)

= 3.4V, T

= 25°C35mA

Sinking V

IN2

= 6.8V, V

E/A(IN)

= 3.4V, T

= 25°C35mA

Regulator 2, Low Battery Detector and Error Amplifier

Quiescent Current V

IN2

= 6.8V, 5V

= 0V, V

E/A(IN)

= 3.4V ●20 30 µA

IN2

= 6.8V, 5V

= 0V, V

E/A(IN)

= 3.4V, V

PIN6

= 0V ●812 µA

IN2

= 4.8V, 5V

= 0V, V

E/A(IN)

= 2.4V ●36 µA

The ● denotes specifications which apply over the full operating

temperature range.

Note 1: All voltages are with respect to the ground pins of the device

(pins 2, 4, 5) unless otherwise specified.

Note 2: The shutdown pin input voltage rating is required for a low

impedance source. Internal protection devices connected to the shutdown

pin will turn on and clamp the pin to approximately 7V or –0.6V. This

range allows the use of 5V logic devices to drive the pin directly. For high

impedance sources or logic running on supply voltages greater than 5.5V,

the maximum current driven into the shutdown pin must be limited to

5mA.

Note 3: The current monitor pin for regulator 1 (pin 15) can be pulled 30V

below the input pin (pin 14). The current monitor pin must not be pulled

above the input pin.

Note 4: The current monitor pin for regulator 2 (pin 11) can be pulled 30V

below the input pin (pin 10). The current monitor pin must not be pulled

above the input pin.

Note 5: E/A (OUT) pin should not be pulled below ground or above

the voltage at Input 2.

Note 6: The device is specified to an operating temperature range of 0°C to

70°C. The device is guaranteed to be functional up to the thermal

shutdown temperature. The thermal shutdown temperature for this device

is approximately 100°C.

Note 7: Operating conditions are limited by maximum junction

temperature. The regulated output specification will not apply for all

possible combinations of input voltage and output current. When

operating at maximum output current, the input voltage range must be

limited. When operating at maximum input voltage, the output current

range must be limited.

Note 8: Regulator 1 of the LT1239 is tested and specified with the adjust

pin (pin 1) tied to the output pin (pin 16). See Applications Information.

Note 9: Dropout voltage is the minimum input/output voltage required to

maintain regulation at the specified output current. In dropout, the output

voltage measured at the package pins will be equal to (V

– V

DROPOUT

Note 10: The quiescent current of the comparator is included in the

ground pin current and quiescent current specifications for regulator 1.

The comparator output is turned off (pin 13 = 0V, pin 12 = 5V) during

these tests.

Note 11: Ground pin current for regulator 1 is tested with V

= V

OUT

(nominal) and a current source load. This means that the device is tested

in it’s dropout region. Ground pin current will decrease slightly at higher

input voltages.

Note 12: Adjust pin current flows into the adjust pin.

Note 13: Shutdown pin current at V

SHDN

= 0V flows out of

the shutdown pin.

Note 14: 6.8V is the nominal voltage of two lithium-ion cells.

TYPICAL PERFORMANCE CHARACTERISTICS

Low-Battery Detector Thresholds

vs Temperature Regulator 2 Output Voltage vs

Temperature Regulator 1 Adjust Pin Voltage vs

Temperature

TEMPERATURE (°C)

–50 25 75

LT1239 • TPC01

–25 0 50 100

LOW-BATTERY DETECTOR THRESHOLD (V)

5.60

5.50

5.40

5.30

5.20

5.10

5.00

4.90

START-UP THRESHOLD

SHUTDOWN THRESHOLD

TEMPERATURE (°C)

–50

REGULATOR 2 OUTPUT VOLTAGE (V)

–25 025 50

LT1239 • TPC02

4.975

4.950

4.925

4.900

4.875

4.850

4.825

4.800

4.775

4.750

4.725 100

TEMPERATURE (°C)

–50

ADJUST PIN VOLTAGE (V)

–25 025 50

LT1239 • TPC03

3.80

3.79

3.78

3.77

3.76

3.75

3.74

3.73

3.72

3.71

3.70 100

LT1239

TYPICAL PERFORMANCE CHARACTERISTICS

Regulator 1, Comparator Quiescent

Current vs Input Voltage, Pin 14

Regulator 1 IMON Current vs

Output Current

Regulator 2 IMON2 Current vs

Output Current

OUTPUT CURRENT (mA)

MON2

CURRENT (µA)

250

200

150

100

040

LT1239 • TPC04

10 20 30 50

IN2

= 6.8V

IMON2

= 0V

OUTPUT CURRENT (mA)

MON1

CURRENT (µA)

250

200

150

100

040

LT1239 • TPC05

10 20 30 50

IMON1

= 0V

IN1

= 24V

OUT1

= 6.8V

IN1

= 5V

OUT1

= 3.75V

IN1

= 5V

OUT1

= 3.75V

INPUT VOLTAGE, PIN 14 (V)

QUIESCENT CURRENT (µA)

LT1239 • TPC06

VADJ (PIN 1) = VOUT (PIN 16)

VPIN3 = 0V

(REGULATOR 1 IN SHUTDOWN)

Regulator 2 Reverse Output

Current vs Output Voltage

Regulator 1 Reverse Output

Current vs Output Voltage

Comparator Output Saturation

Voltage vs Output Current

TEMPERATURE (°C)

–50

SHUTDOWN PIN THRESHOLD (V)

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0050 75

LT1239 • TPC10

–25 25 100 125

(ON-TO-OFF)

LOAD

= 1mA

(OFF-TO-ON)

LOAD

= 1mA

(OFF-TO-ON)

LOAD

= 30mA

Regulator 2, Error Amp, Low-

Battery Detector Quiescent Current

INPUT 2 VOLTAGE, PIN 10 (V)

QUIESCENT CURRENT (µA)

02468

LT1239 • TPC11

10103579

SHDN2

= OPEN CIRCUIT

SHDN2

= 0V

(REGULATOR 2

IN SHUTDOWN)

Shutdown Pin Threshold

OUTPUT VOLTAGE (V)

REVERSE OUTPUT CURRENT (µA)

LT1239 • TPC09

213579

4610

OUTPUT VOLTAGE (V)

REVERSE OUTPUT CURRENT (µA)

LT1239 • TPC08

213579

4610

IN1

= 0V

ADJ (PIN 1) = V

OUT

(PIN 16)

OUTPUT CURRENT (mA)

OUTPUT SATURATION VOLTAGE (mV)

400

350

300

250

200

150

100

080

LT1239 • TPC07

20 40 60 100

10 30 50 90

LT1239

PIN FUNCTIONS

UUU

ADJ (Pin 1): Adjust Pin of Regulator 1. The regulator will

servo the adjust pin to 3.75V referred to ground. Bias

current will be approximately 50nA and will flow into the

adjust pin.

GND (Pin 2): Ground Pin for Regulator 1. Note that the

three ground pins (pins 2, 4, 5) are connected together

internally and should all be grounded externally.

SHDN1 (Pin 3): Shutdown Pin for Regulator 1. Regulator

1 output will be on if the shutdown pin is either: 1) Left

floating (open circuit) or 2) pulled up to the 5V rail. If the

shutdown function is not used, the shutdown pin is nor-

mally left open circuit. Regulator 1 output will be off if the

shutdown pin is pulled to ground. The shutdown pin

current with the pin pulled to ground will be in the range of

2µA flowing out of the pin. The shutdown pin current with

the pin pulled up to 5V will be zero.

GND (Pin 4): Ground. This ground pin is tied to the

substrate of the die, between regulator 1 and the rest of the

circuit. It is used as an isolation barrier between regulator

1 and the rest of the circuitry.

GND (Pin 5): Ground Pin for Regulator 2.

SHDN2 (Pin 6): Shutdown Pin for Regulator 2. Regulator

2 output will be on if the shutdown pin is either: 1) Left

floating (open circuit) or 2) pulled up to the 5V rail. If the

shutdown function is not used, the shutdown pin is nor-

mally left open circuit. Regulator 2 output will be off if the

shutdown pin is pulled to ground. The shutdown pin

current with the pin pulled to ground will be in the range of

2µA flowing out of the pin. The shutdown pin current with

the pin pulled up to 5V will be zero.

E/A (IN) (Pin 7): Noninverting Input of the Error Amplifier.

This pin should be tied to the center tap point in the output

divider for regulator 1. The bias current for this pin will be

in the range of 3nA and it will flow out of the pin.

E/A (OUT) (Pin 8): Output of the Error Amplifier. This is

normally connected to the center tap of the backup cells.

NC (Pin 9): Not Connected.

INPUT 2 (Pin 10): Input Pin (V

) for Regulator 2, the Error

Amplifier, and the Low-Battery Detection Circuit.

MON

2 (Pin 11): Current Monitor Pin for Regulator 2. If the

current monitor function is not used, this pin should be

tied to the output pin of regulator 2.

OUT 2 (Pin 12): Output of Regulator 2. It is also the

inverting input and output of the comparator. If the main

5V system supply is up and running then the comparator

output will pull the output of regulator 2 up to 5V.

(Pin 13): Noninverting Input of the comparator and

the collector of the output driver. The collector of the

output driver is normally connected to the main 5V system

supply.

INPUT 1 (Pin 14): Input Pin (V

) of Regulator 1.

MON

1 (Pin 15): Current Monitor Pin for Regulator 1. The

current flowing out of this pin will be approximately 1/200

of the output current of regulator 1. If the current monitor

function is not used, this pin should be tied to the output

pin of regulator 1.

OUT 1 (Pin 16): Output of Regulator 1.

Regulator 1: Regulator 1 is used to supply the charging

current to the backup batteries. It converts the voltage on

the main battery to a fixed output voltage to charge the

backup cells. The output voltage is set with a voltage

divider connected between the output and ground with a

tap point of the divider connected to the adjust pin. The

regulator servos its output in order to maintain the adjust

pin at 3.75V referred to ground. The resistor divider

should be chosen such that the divider current is approxi-

mately 5µA. This means the impedance from the adjust pin

to ground should be approximately 750kΩ. For safety

requirements a resistor can be placed between the output

pin and the top of the divider that sets the regulated output

voltage. The regulator will regulate the voltage at the top of

the divider. Quiescent current will be 10µA to 15µA. Output

short-circuit current will be approximately 70mA.

FUNCTIONAL DESCRIPTIO

UUU

LT1239

Comparator: The output of the comparator is connected to

the output of regulator 2. This point provides power to

memory and power management circuitry. The compara-

tor looks at the main 5V power line and the output voltage

of regulator 2. If the main 5V line is up and regulating the

comparator output will pull up to 5V and supply power to

the memory from the main 5V regulator. If the main 5V

power line drops below 4.85V the comparator switches off

and regulator 2 will supply power to the memory from the

backup batteries. The comparator is powered from the raw

battery voltage at the input of regulator 1.

Error Amplifier: The Error Amplifier is used to equalize the

cell voltages of two lithium-ion cells connected in series.

The error amplifier is designed to source or sink 5mA.

Low-Battery Detector: The low-battery detector circuit

acts as an undervoltage lockout. This circuit turns regula-

tor 2 and the error amplifier off if the backup battery

voltage drops below 5V. The low-battery detector circuit

will turn regulator 2 and the error amplifier back on when

the backup battery voltage rises above 5.3V. This circuit

has a quiescent current of approximately 3µA in the

undervoltage condition.

Regulator 2: Regulator 2 is used to regulate the voltage of

the backup batteries and isolate the backup batteries from

the main 5V line. This regulator will prevent reverse

current flow from the main 5V supply back into the backup

cells.

BLOCK DIAGRAM

APPLICATIONS INFORMATION

WUUU

Device Overview

The LT1239 provides several functions needed for backup

battery management. It provides:

1. Battery Charging: The LT1239 can be set up to charge

lithium-ion or nickel cadmium batteries in either con-

stant voltage or constant current mode.

2. Memory Power Control: The LT1239 provides power

for the memory and includes automatic switchover

between the backup battery and the main 5V system

power. When the 5V system supply is up and running it

is used to power the memory, the regulator prevents

reverse current flow back into the backup battery.

Automatic switchover occurs when the 5V system

supply drops below 4.85V and the regulator then pro-

vides power to the memory from the backup cells.

Memory power is uniterruptable.

E/A (IN)

E/A (OUT)

OUT 1

MON1

SHDN1

INPUT 2

ADJUST

INPUT 1

IN IN

24 15

14 16

13 5V

GROUND PINS 2, 4, 5 ARE TIED TO SUBSTRATE

1239 BD

GND

OUT 2

REGULATOR

MON2

SHDN2

5 11

GND

–

E/A

REGULATOR

INPUT 1

COMP POWER

SWITCH

LOW-BATTERY

DETECTOR

FUNCTIONAL DESCRIPTIO

UUU

LT1239

APPLICATIONS INFORMATION

WUUU

Equalizing Lithium-Ion Cells

The error amplifier on the LT1239 is used to equalize the

cell voltages in a 2-cell lithium-ion backup system. The

error amplifier is internally connected as a unity-gain

follower and is designed to sink or source about 3mA. The

bias current for the error amplifier will be approximately

3nA and will flow out of the pin. The output voltage of the

error amplifier can be set by connecting the input to a tap

point on the resistor divider used to set the output voltage

for regulator 1 as shown in Figure 2. The error amplifier

will then equalize the cell voltages by charging the cell with

the lowest output voltage. The output voltage of regulator

1 controls the total cell voltage and the error amplifier

forces the cell voltages to be equal. The error amplifier

output current will go to zero when the cell voltages are

equal and the total cell voltage is equal to the output

voltage of regulator 1.

3. Protection: Regulator 1 allows the use of current limit-

ing resistors to prevent overcharging lithium-ion cells.

A low-battery detector shuts down regulator 2 and the

error amplifier to prevent over discharging the lithium

cells. An error amplifier is included to provide voltage

equalization for two series connected lithium-ion cells.

Adjusting Output Voltage

Regulator 1 is an adjustable regulator. This allows the

output voltage to be set for various battery types and

voltages. The output voltage is adjustable from 3.75V up

to 20V. The regulator will servo its output voltage in order

to maintain the adjust pin at 3.75V with respect to ground.

The output voltage is set with a resistor divider from output

to ground as shown in Figure 1. The resistor values should

be chosen so that the current in the divider is approxi-

mately 5µA. This means that the impedance from the

adjust pin to ground should be approximately 750kΩ. The

bias current at the adjust pin is 50nA (typical) and will flow

into the adjust pin. The error in the output voltage, due to

the adjust pin bias current will be equal to the bias current

multiplied by the value of R2 ( I

ADJ

× R2). This error is small

and is compensated for in the formulas shown in Figure 1.

Figure 1. Adjusting Output Voltage

R1 ≈ 750k

LT1239 • F01

50nA

5µA

OUT

= 3.75 1 + + I

ADJ

(R2)

)

OUT

– 3.75V)

(3.75V/R1) + I

ADJ

R2 =

CHOOSE: R1 = 750k

ADJ

= 50nA

REGULATOR 1

IN 1 OUT 1

ADJ

Example: To set the output voltage to 6.8V for a 2-cell

lithium-ion system, use R1 = 750k and I

ADJ

= 50nA.

Then:

6.8V – 3.75V

(3.75V/750k) + 50nA

R2 = = 604k

Figure 2. Equalizing Lithium-Ion Cells

REGULATOR 1 R2 = 604k

69.8k

3.4V

681k

LT1239 • F02

3.75V

6.8V

E/A (IN)

–

E/A

E/A (OUT)

10µF

REGULATOR 1

IN 1 OUT 1

ADJ

For battery voltages greater than the low-battery detection

threshold the error amplifier is active. For battery voltages

lower than the low-battery detection threshold the output

of the error amplifier is inactive. When the error amplifier

is active it can source or sink approximately 3mA. When

the error amplifier is inactive its output is a high imped-

ance, as long as it is not forced above V

IN2

or below

ground.

The error amplifier is powered from the same supply pin

as regulator 2. In most applications the backup batteries

and the output of regulator 1 will provide power to this

point. This means that the protection resistors (R4 in

Figure 5) in series with the output of regulator 2 will limit

the output current capability of the error amplifier in a

fault condition.

LT1239

APPLICATIONS INFORMATION

WUUU

Using the Current Monitor Function

The current monitor pin outputs a current proportional to

the output current of the regulator. Both regulator 1 and

regulator 2 have independent current monitor pins. The

current monitor function can be used to monitor charge in

the backup cells, to set up a constant current output or to

adjust the current limit of the regulator. The current

monitor pin should be tied to the output pin if the current

monitor function is not used. This will minimize quiescent

current.

The current output of the current monitor pin can be

converted to a voltage by feeding the current monitor pin

output current through a resistor. The voltage across the

resistor will be proportional to output current. This signal

can be used to monitor the output current for either

regulator. Regulator 1 output current is equal to the charge

current for the backup batteries plus the load current of

regulator 2. If regulator 1 output current is greater than

regulator 2 output current, the difference between the

currents is the charge current for the backup cells. If

regulator 2 output current is greater than regulator 1

output current, the difference between the currents is the

discharge current for the backup cells. By integrating the

difference between regulator 1 output current and regula-

tor 2 output current the total charge in the backup cells can

be determined.

Constant Current Charging

NiCd backup batteries are normally charged with a con-

stant current trickle charge. This can be accomplished

using regulator 1 and the circuit shown in Figure 3.

In this circuit the voltage at the adjust pin is proportional

to the output current. Regulator 1 will servo its output to

force 3.75V at the adjust pin. The output current will be

scaled from the current monitor pin current by a ratio of

220:1. Output current is equal to 220 × current monitor pin

current. The output current is set by choosing resistor R1,

in the formula shown in Figure 3. Regulator 1 will source

a constant current as long as the voltage at its input is

greater than the battery voltage plus the dropout voltage of

regulator 1. External power monitoring circuitry can be

used to shutdown regulator 1 to terminate charge when a

low current sleep mode is desired.

Setting Current Limit Using the Current Monitor Pin

With the addition of some simple external circuitry the

current monitor pin can be used to control the output

short-circuit current of the regulator. As shown in Figure

4, the current monitor pin can be tied to ground through

a resistor to generate a voltage proportional to output

current. When the voltage across R3 is equal to approxi-

mately 0.6V (one V

) Q1 will turn on and pull down on the

shutdown pin of the regulator. Q1 effectively steals drive

current from the regulator to limit the output current. C1

is needed to roll off the gain of Q1. Current limit can be set

using the formula shown in Figure 4. This circuit can be

used with either regulator. The shutdown function can

also be used. An open-collector gate connected in parallel

with Q1 can shut down the regulator.

LT1239

IN 1

SHDN1

OUT 1

MON1

ADJ

LT1239 • F03

(3.75V)

10µF

2µF

GND

7-24V

NiCd

BACKUP

BATTERY

SHDN1

CHARGE

< 0.25V OFF

> 2.8V ON

NC ON

R1 = × 220

3.75V

CHARGE

Figure 3. Constant Current Charging

Figure 4. Reducing Current Limit

Using the Comparator

The comparator in the LT1239 is intended to be used as an

automatic switchover circuit between the main 5V

IN 1

SHDN2

OUT 1

ADJ

MON1

LT1239 • F04

10µF

2µF

GND

NiCd

BACKUP

BATTERY

MAIN

BATTERIES

R3 = × 220

0.6V

LIM

Q1 2N3904

LT1239

APPLICATIONS INFORMATION

WUUU

system power and the backup batteries. The comparator

output will be driven high if the output of the 5V system

supply is greater than the 4.85V output of regulator 2.

Regulator 2 will act as a diode to prevent current flow from

the 5V system supply back into the backup battery. Cur-

rent flow into the output of regulator 2, with the output

pulled up to 5V, will be limited to approximately 6µA and

will flow to ground. If the main 5V system supply drops

below the 4.85V output of regulator 2 the comparator will

switch off and regulator 2 will provide power to the

memory. The comparator combined with regulator 2 and

the batteries provide an uninterruptable power source to

the memory and power monitoring circuitry.

Choosing Current Limiting Resistors

Due to UL safety considerations, circuits used to charge

lithium-ion batteries must have external resistors (passive

components) to limit the available charge current in the

event of a failure in the charging circuit. The LT1239 allows

these resistors to be placed in series with the output

transistor of the regulator 1 as shown in Figure 5. The

current limiting resistor (R4) will be in series with the main

charge current path but will be inside the feedback loop of

regulator 1. Because the resistors are inside the feedback

loop they will not affect output voltage regulation in

normal operating conditions. The resistors should be

selected so that they limit the charge current below the

maximum level specified by the battery manufacturer. For

a typical 3.4V, 50mA rechargeable backup cell (Panasonic

VL2330) the maximum charge current is specified at

300mA. Most users will choose to limit the current well

below the maximum charge current. It is important to note

that these resistors can also limit the charge current

during normal operation. Since the charge current for a

typical lithium-ion button cell is normally less than 20mA,

limited by the internal impedance of the cells during a

constant voltage charge, the current limiting resistors do

not significantly affect the charge times for the backup

cells. The worst case would occur if the regulator failed as

a short and the main battery is at its maximum charge

voltage. The current limiting resistor (R4) must be chosen

to limit the current to less than the manufacturers maxi-

mum charging current with the difference between the

main battery voltage and the backup battery voltage dropped

across it.

For example with a main battery voltage of 24V max, a

backup battery voltage of 6.8V and a maximum charge

current of 300mA, R4 must be greater than (24V – 6.8V)/

300mA, R4 > 57Ω.

R4 can also be used to limit the power dissipated by

regulator 1 as shown in the following section. C1 is needed

for stability in circuits with protection resistors (R4).

The power dissipation in R4 during fault conditons can be

significant. it will be equal to:

INL

– V

BATTERY

)

Power resistors with ratings greater than 0.25W or fusable

resistors may be required.

Thermal Considerations

The power dissipation of this device is made up of several

components.They are the power dissipation of each regu-

lator, the comparator and the error amplifier. The largest

component will be due to the power in regulator 1, when

the charge current for the batteries is the highest and the

input voltage to regulator 1 is at the maximum. In most

systems this condition only occurs for a short period after

the backup battery has been completely discharged. Both

regulators have thermal limiting circuitry which limits the

power in the regulator when the junction temperature

reaches about 100°C. The thermal limit temperature is set

low because the device is designed to work with batteries

specified to run at ambient temperatures below 60°C. The

power in regulator 1 can be limited with external resistors

placed in the feedback loop as shown in Figure 5. In

lithium-ion systems these resistors are required for safety

reasons.

The power in regulator 1 will be equal to:

[(V

MAINBATTERY

– V

BACKUPBATTERY

) × I

CHG

] – (I

CHG

× R4)

Note that for circuits with a current limiting resistor (R4)

the worst-case power point occurs when I

CHG

is equal to

the maximum charging current/2.

Example: [(24V – 6.8V) × (71mA/2)] – [(71mA/2) × 240]

= 300mW

This is the only significant component of power dissipation

in the device and this condition will only occur when the

LT1239

APPLICATIONS INFORMATION

WUUU

backup batteries have been completely discharged. Once the

backup batteries are charged the power in regulator 1 drops

significantly. The power in regulator 2 when regulator 2 is

providing power to the memory will be equal to:

BACKUPBATTERY

– 4.85V) × I

OUT

is the current needed to power the memory and power

monitoring circuitry.

Example: (6.8V – 4.85V) × 30mA = 58.5mW

The power in the comparator when the comparator is provid-

ing power to the memory will be equal to:

SAT

× I

OUT

)

OUT

is the current needed to power the memory and power

monitoring circuitry. Comparator Output Saturation Voltage

vs Output Current can be found in the Typical Performance

Characteristics.

Example: (V

SAT

× I

LOAD

) = (0.15V × 30mA) = 4.5mW

Note that power for memory will be supplied by either

regulator 2 or the comparator. The power in the error

amplifier when the cells are unequalized will be equal to:

BACKUPBATTERY

/2) × 3mA

Example: (6.8V/2) × 3mA = 10.2mW

This component goes to zero when the cell voltages are

equalized.

The thermal resistance of the LT1239 is 120°C/W when

the device is mounted to a PC board with at least one

ground or power plane. The junction temperature rise will

be equal to the total power in the device multiplied by

120°C/W or (P

TOTAL

× 120°C/W). For 300mW dissipation

the junction temperature rise will be (300mW × 120°C/W)

= 36°C. Given that the thermal limit temperature is ap-

proximately 100°C, this allows for a maximum ambient

temperature of roughly 60°C before the device thermal

limits. This temperature is near the maximum ambient

allowed for most battery types.

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.

Figure 5. Adding a Protection Resistor for Lithium-Ion Charger

LT1239

IN 1

GND

OUT 1

MON1

ADJ

E/A (IN)

E/A (0UT)

604k

LT1239 • F05

10µF

MAIN

BATTERY

PACK

R4 > V1 – V2

MAX CHARGE CURRENT

69.8k

681k

3.4V Li-Ion

CELL

3.4V Li-Ion

CELL

*R4

300pF

*THIS RESISTOR IS REQUIRED

BY SOME SAFETY AGENCIES.

LT1239

TYPICAL APPLICATIONS

NiCd Backup System with 5mA Trickle Charge

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

(408) 432-1900

●

FAX

: (408) 434-0507

●

TELEX

: 499-3977

LT/GP 0695 10K • PRINTED IN USA

 LINEAR TECHNOLOGY C ORPORATION 1995

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LT1239

IN 1

SHDN1

IN2

SHDN2

OUT 1

ADJ

MON1

E/A (IN)

E/A (0UT)

OUT 2

MON2

LT1239 • TA03

10µF

MAIN

BATTERY

PACK

NiCd

BACKUP

BATTERIES

165k

5µF

10µF

MEMORY POWER

MONITORING

CIRCUITRY

5V POWER

SYSTEM

1µF

GND

LT1239

IN 1

SHDN1

IN2

SHDN2

OUT 1

ADJ

MON1

E/A (IN)

E/A (0UT)

OUT2

MON2

LT1239 • TA02

10µF

MAIN

BATTERY

PACK 6V NiCd

BACKUP

BATTERY

750k

650k

5µF

10µF

MEMORY POWER

MONITORING

CIRCUITRY

5V POWER

SYSTEM

GND

50Ω

NiCd Backup System with 20mA Charge Current

S Package

16-Lead Plastic DIP

Dimension in inches (millimeters) unless otherwise noted.

PACKAGE DESCRIPTION

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

SO16 0893

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

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.

MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).