10

LT1239

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 6

µA 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 > 57

Ω.

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:

2

R4

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:

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

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