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LT1239

Backup Battery Management Circuit

Linear Technology 

LT1239

APPLICATIONS INFORMATION

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:

(VBACKUPBATTERY – 4.85V) × IOUT

IOUT 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:

(VSAT × IOUT)

IOUT 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: (VSAT × ILOAD) = (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:

(VBACKUPBATTERY/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 (PTOTAL × 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.

V1

MAIN

BATTERY

+

PACK

IN 1

OUT 1

IMON1

LT1239

ADJ

E/A (IN)

E/A (0UT)

GND

*R4

C1

300pF

V2

R3 +

604k

10µF

3.4V Li-Ion

CELL

R2

69.8k

R1

681k

3.4V Li-Ion

CELL

R4 >

V1 – V2

MAX CHARGE CURRENT

*THIS RESISTOR IS REQUIRED

BY SOME SAFETY AGENCIES.

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

LT1239 • F05

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.

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