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LT1579

APPLICATIONS INFORMATION

Example: The low-battery detector must be tripped at a

terminal voltage of 5.5V. There is a 100k pull-up resistor

to 5V on the output of the comparator and 200mV of

hysteresis is needed to prevent chatter. With a 1M resistor

for R1, what other resistor values are needed?

Using the formulas in Figure 3,

267

= ()

Ω

()

=Ω

.– .

Use a standard value of 2.7M

Ω. With the 100k pull-up

resistor, this gives a sink current and logic flag voltage of

approximately 45

µA at 0.4V. The hysteresis in this case

will be:

Hysteresis

Ω









An additional 133mV of hysteresis is needed, so a resistor

and diode must be added. The value of R3 will be:

VmV

133

10 5

()

Ω

()

=Ω

.– .

– .

A standard value of 10M

Ω can be used. The additional

current flowing through R3 into the comparator output is

negligible and can usually be ignored

Logic Flags

The low-battery comparator outputs and the status flags

of the LT1579 are open collector outputs capable of

sinking up to 5mA. See the curve of Logic Flag Output

Voltage vs. Current in the Typical Performance Character-

istics.

There are two status flags on the LT1579. The BACKUP

flag and the DROPOUT flag provide information on which

input is supplying power to the load and give early warning

of loss of output regulation. The BACKUP flag goes low

when the secondary input begins supplying power to the

load. The DROPOUT flag signals the dropout condition on

both inputs, warning of an impending drop in output

voltage. The conditions that set either status flag are

determined by input to output voltage differentials and

current supplied to the load from each input. Normal

output deviation during transient load conditions (with

sufficient input voltages) will not set the status flags.

Timing Diagram

The timing diagram for the 5V dual battery supply is shown

in Figure 4. The schematic is the same as the 5V Dual

Battery Supply on the front of the data sheet. All logic flag

outputs have 100k pull-up resistors added. Note that there

is no time scale for the timing diagram. The timing diagram

is meant as a tool to help in understanding basic operation

of the LT1579. Actual discharge rates will be a function of

the load current and the type of batteries used. The load

current used in the example was 100mA DC.

Figure 4. Basic Dual Battery Timing Diagram

VIN1

100mA

4.8V

100mA

VIN2

IIN1

IIN2

VOUT

BACKUP

DROPOUT

LB02

LB01

LTC1579 • F03

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