11

LT1239

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:

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:

monitoring circuitry. Comparator Output Saturation Voltage

vs Output Current can be found in the Typical Performance

Characteristics.

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:

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

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

ADJ

E/A (IN)

E/A (0UT)

R3

604k

LT1239 • F05

+

+

10

µF

MAIN

BATTERY

PACK

R4 >

V1 – V2

MAX CHARGE CURRENT

R2

69.8k

R1

681k

3.4V Li-Ion

CELL

3.4V Li-Ion

CELL

*R4

C1

300pF

*THIS RESISTOR IS REQUIRED

BY SOME SAFETY AGENCIES.