LT1239 Просмотр технического описания (PDF) - Linear Technology

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LT1239

Backup Battery Management Circuit

Linear Technology 

LT1239

APPLICATIONS INFORMATION

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 ( IADJ ×R2). This error is small

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

IN 1 OUT 1

REGULATOR 1

ADJ

R2

50nA

5µA

R1 ≈ 750k

) VOUT = 3.75

1+

R2

R1

+ IADJ (R2)

R2 = (VOUT – 3.75V)

(3.75V/R1) + IADJ

CHOOSE: R1 = 750k

IADJ = 50nA

Figure 1. Adjusting Output Voltage

LT1239 • F01

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

lithium-ion system, use R1 = 750k and IADJ = 50nA.

Then:

R2 = 6.8V – 3.75V = 604k

(3.75V/750k) + 50nA

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.

6.8V

IN 1 OUT 1

REGULATOR 1

R2 = 604k

3.75V

ADJ

R3

69.8k

E/A

E/A (IN)

3.4V

R1

681k

+

10µF

E/A (OUT)

LT1239 • F02

Figure 2. Equalizing Lithium-Ion Cells

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 VIN2 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.

8

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