LT1579CS datasheet(12/20 Pages) LINER

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LT1579

APPLICATIONS INFORMATION

Device Overview

The LT1579 is a dual input, single output, low dropout

linear regulator. The device is designed to provide an

uninterruptible output voltage from two independent input

voltage sources on a priority basis. All of the circuitry

needed to switch smoothly and automatically between

inputs is incorporated in the device. All power supplied to

the load is drawn from the primary input (VIN1) until the

device senses that the primary input is failing. At this point

the LT1579 smoothly switches from the primary input to

the secondary input (VIN2) to maintain output regulation.

The device is capable of providing 300mA from either

input at a dropout voltage of 0.4V. Total quiescent current

when operating from the primary input is 50

µA, which is

µA from the primary input, 2µA from the secondary and

a minimum input current of 3

µA which will be drawn from

the higher of the two input voltages.

A single error amplifier controls both output stages so

regulation remains tight regardless of which input is

providing power. Threshold levels for the error amplifier

and low-battery detectors are set by the internal 1.5V

reference. Output voltage is set by an internal resistor

divider for fixed voltage parts and an external divider for

adjustable parts. Internal bias circuitry powers the refer-

ence, error amplifier, output driver controls, logic flags

and low-battery comparators.

The LT1579 aids power management with the use of two

independent low-battery comparators and two status flags.

The low-battery comparators can be used to monitor the

input voltage levels. The BACKUP flag signals when any

power is being drawn from the secondary input and the

DROPOUT flag provides indication that both input volt-

ages are critically low and the output is unregulated.

Additionally, the switch to the secondary input from the

primary can be forced externally through the use of the

secondary select pin (SS). This active low logic pin, when

pulled below the threshold, will cause power draw to

switch from the primary input to the secondary input.

Current flowing in the primary input is reduced to only a

few microamperes, while all power draw (load current and

bias currents) switches to the secondary. The LT1579 has

a low power shutdown state which shuts off all bias

currents and logic functions. In shutdown, quiescent

currents are 2

µA from the primary input, 2µA from the

secondary input and an additional 3

µA which is drawn

from the higher of the two input voltages.

Adjustable Operation

The adjustable version of the LT1579 has an output

voltage range of 1.5V to 20V. The output voltage is set by

the ratio of two external resistors as shown in Figure 1. The

device servos the output to maintain the voltage at the

adjust pin at 1.5V. The current in R1 is then equal to 1.5V/

R1 and the current in R2 is the current in R1 minus the

adjust pin bias current. The adjust pin bias current, 6nA at

°C,flowsoutoftheadjustpinthroughR1toground.The

output voltage can now be calculated using the formula:

OUT

ADJ







 ()( )

.–

The value of R1 should be less than 500k to minimize the

error in the output voltage caused by adjust pin bias

current. With 500k resistors for both R1 and R2, the error

induced by adjust pin bias current at 25

°C is 3mV or 0.1%

of the total output voltage. With appropriate value and

tolerance resistors, the error due to adjust pin bias current

may often be ignored. Note that in shutdown, the output is

turned off and the divider current is zero. The parallel

combination of R1 and R2 should be greater than 20k to

allow the error amplifier to start. In applications where the

minimum parallel resistance requirement cannot be met,

a 20k resistor may be placed in series with the adjust pin.

This introduces an error in the reference point for the

resistor divider equal to (IADJ)(20k).

Figure 1. Adjustable Operation

OUT

ADJ

GND

CFB

COUT

VOUT

1579 • F01

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