LT1579CS datasheet(10/20 Pages) LINER

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LT1579

PIN FUNCTIONS

nally clamped to 7V and – 0.6V (one VBE). If unused, this

pin can be left open circuit. Device operation is unaffected

if this pin is not connected.

DROPOUT: The dropout flag is an open collector output

which pulls low when both input voltages drop suffi-

ciently for the LT1579 to enter the dropout region. This

signals that the output is beginning to go unregulated.

The DROPOUT output voltage is 1V when sinking 5mA,

dropping to under 200mV at 20

µA (see curve of Logic

Flag Voltage vs Current in the Typical Performance Char-

acteristics). This makes the DROPOUT pin equally useful

in driving both bipolar and CMOS logic inputs with the

addition of an external pull-up resistor. It is also capable

of driving higher current devices, such as LEDs. This pin

is internally clamped to 7V and – 0.6V (one VBE). If

unused, this pin can be left open circuit. Device operation

is unaffected if this pin is not connected.

BIASCOMP: This is a compensation point for the internal

bias circuitry. It must be bypassed with a 0.01

µF capaci-

tor for stability during the switch from VIN1 to VIN2.

LBI1: This is the noninvering input to low-battery com-

parator LB1 which is used to detect a low input/battery

condition. The inverting input is connected to a 1.5V

reference. The low-battery comparator input has 18mV of

hysteresis with more than 20

µA of sink current on the

output (see Applications Information section). This pin is

internally clamped to 7V and – 0.6 (one VBE). If not used,

this pin can be left open circuit, with no effect on normal

circuit operation. If unconnected, the pin will float to 1.5V

and the logic output of LB1 will be high impedance.

LBI2: This is the noninverting input to low-battery com-

parator LB2 which is used to detect a low input/battery

condition. The inverting input is connected to a 1.5V

reference. The low-battery comparator input has 18mV of

hysteresis with more than 20

µA of sink current on the

output (see Applications Information section). This pin is

internally clamped to 7V and – 0.6V (one VBE). If not used,

this pin can be left open circuit, with no effect on normal

circuit operation. If unconnected, the pin will float to 1.5V

and the logic output of LB2 will be high impedance.

out of the pin (see curve of Adjust Pin Bias Current vs

Temperature in the Typical Performance Characteristics).

A DC load of 3

µA is needed on the output of the adjustable

part to maintain regulation. The adjust pin voltage is 1.5V

referenced to ground and the output voltage range is 1.5V

to 20V.

SHDN: The shutdown pin is used to put the LT1579 into

a low power shutdown state. All functions are disabled if

the shutdown pin is pulled low. The output will be off, all

logic outputs will be high impedance and the voltage

comparators will be off when the shutdown pin is pulled

low. The shutdown pin is internally clamped to 7V and

– 0.6V (one VBE), allowing the shutdown pin to be driven

either by 5V logic or open collector logic with a pull-up

resistor. The pull-up resistor is only required to supply

the pull-up current of the open collector gate, normally

several microamperes. If unused, the shutdown pin can

be left open circuit. The device is active if the shutdown

pin is not connected.

SS: The secondary select pin forces the LT1579 to switch

power draw to the secondary input (VIN2). This pin is

active low. The current drawn out of VIN1 is reduced to

µA when this pin is pulled low. The secondary select pin

is internally clamped to 7V and – 0.6V (one VBE), allowing

the pin to be driven directly by either 5V logic or open

collector logic with a pull-up resistor. The pull-up resistor

is required only to supply the leakage current of the open

collector gate, normally several microamperes. If sec-

ondary select is not used, it can be left open circuit. The

LT1579 draws power from the primary first if the second-

ary select pin is not connected.

BACKUP: The backup flag is an open collector output

which pulls low when the LT1579 starts drawing power

from the secondary input (VIN2). The BACKUP output

voltage is 1V when sinking 5mA, dropping to under

200mV at 20

µA (see curve of Logic Flag Voltage vs

Current in the Typical Performance Characteristics). This

makes the BACKUP pin equally useful in driving both

bipolar and CMOS logic inputs with the addition of an

external pull-up resistor. It is also capable of driving

higher current devices, such as LEDs. This pin is inter-

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