LT3045 datasheet(13/36 Pages) LINER

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LT3045-1

30451fa

For more information www.linear.com/LT3045-1

PIN FUNCTIONS

IN (Pins 1, 2): Input. These pins supply power to the

regulator. The LT3045-1 requires a bypass capacitor at the

IN pin. In general, a battery’s output impedance rises with

frequency,soincludeabypasscapacitorinbattery-powered

applications. While a 4.7µF input bypass capacitor gener-

ally suffices, applications with large load transients may

require higher input capacitance to prevent input supply

droop.ConsulttheApplicationsInformationsectiononthe

proper use of an input capacitor and its effect on circuit

performance,inparticularPSRR.TheLT3045-1withstands

reversevoltagesonINwithrespecttoGND,OUTSandOUT.

In the case of a reversed input, which occurs if a battery

is plugged-in backwards, the LT3045-1 acts as if a diode

is in series with its input. Hence, no reverse-current flows

into the LT3045-1 and no negative voltage appears at the

load. The device protects itself and the load.

VIOC (Pin 3): Voltage for Input-to-Output Control. The

LT3045-1 incorporates a tracking function to control the

switching pre-regulator powering the LT3045-1. The VIOC

pinistheoutputofthistrackingfunctionthatdrivesthepre-

regulator’s feedback (FB) pin to maintain the LT3045-1’s

input voltage at VOUT + VVIOC. This function minimizes

power dissipation while maintaining PSRR performance.

See Applications Information section for details.

EN/UV (Pin 4): Enable/UVLO. Pulling the LT3045-1’s EN/

UV pin low places the part in shutdown. Quiescent current

in shutdown drops to less than 1µA and the output volt-

age turns off. Alternatively, the EN/UV pin can set an input

supply undervoltage lockout (UVLO) threshold using a

resistordividerbetweenIN,EN/UVandGND.TheLT3045-1

typically turns on when the EN/UV voltage exceeds 1.24V

on its rising edge, with a 130mV hysteresis on its falling

edge. The EN/UV pin can be driven above the input voltage

and maintain proper functionality. If unused, tie EN/UV to

IN. Do not float the EN/UV pin.

PG (Pin 5): Power Good. PG is an open-collector flag that

indicates output voltage regulation. PG pulls low if PGFB

is below 300mV. If the power good functionality is not

needed, float the PG pin. A parasitic substrate diode exists

between PG and GND pins of the LT3045-1; do not drive

PG more than 0.3V below GND during normal operation

or during a fault condition.

ILIM(Pin6):CurrentLimitProgrammingPin.Connectinga

resistorbetweenILIMandGNDprogramsthecurrentlimit.

For best accuracy, Kelvin connect this resistor directly to

the LT3045-1’s GND pin. The programming scale factor

is nominally 150mA•kΩ. The ILIM pin sources current

proportional (1:500) to output current; therefore, it also

serves as a current monitoring pin with a 0V to 300mV

range. If the programmable current limit functionality is

not needed, tie ILIM to GND. A parasitic substrate diode

exists between ILIM and GND pins of the LT3045-1; do

not drive ILIM more than 0.3V below GND during normal

operation or during a fault condition.

PGFB (Pin 7): Power Good Feedback. The PG pin pulls

high if PGFB increases beyond 300mV on its rising edge,

with 7mV hysteresis on its falling edge. Connecting an

external resistor divider between OUT, PGFB and GND

sets the programmable power good threshold with the

following transfer function: 0.3V • (1 + RPG2/RPG1). As

discussed in the Applications Information section, PGFB

also activates the fast start-up circuitry. Tie PGFB to IN

if power good and fast start-up functionalities are not

needed, and if reverse input protection is additionally

required, tie the anode of a 1N4148 diode to IN and its

cathode to PGFB. See the Typical Applications section for

details. A parasitic substrate diode exists between PGFB

and GND pins of the LT3045-1; do not drive PGFB more

than 0.3V below GND during normal operation or during

a fault condition.

SET (Pin 8): SET. This pin is the inverting input of the er-

ror amplifier and the regulation set-point for the LT3045-

1. SET sources a precision 100µA current that flows

through an external resistor connected between SET and

GND. The LT3045-1’s output voltage is determined by

VSET = ISET • RSET. Output voltage range is from zero to

15V. Adding a capacitor from SET to GND improves noise,

PSRR and transient response at the expense of increased

start-up time. For optimum load regulation, Kelvin con-

nect the ground side of the SET pin resistor directly to

the load. A parasitic substrate diode exists between SET

and GND pins of the LT3045-1; do not drive SET more

than 0.3V below GND during normal operation or during

a fault condition.

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