LT3045 datasheet(21/36 Pages) LINER

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LT3045-1

30451fa

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

APPLICATIONS INFORMATION

LT3045-1incorporatesfaststart-upcircuitrythatincreases

the SET pin current to about 2mA during start-up.

As shown in the Block Diagram, the 2mA current source

remains engaged while PGFB is below 300mV, unless the

regulator is in current limit, dropout, thermal shutdown

or input voltage is below minimum VIN.

If fast start-up capability is not used, tie PGFB to IN or to

OUT for output voltages above 300mV. Note that doing

so also disables power good functionality.

ENABLE/UVLO

The EN/UV pin is used to put the regulator into a micro-

power shutdown state. The LT3045-1 has an accurate

1.24V turn-on threshold on the EN/UV pin with 130mV

of hysteresis. This threshold can be used in conjunction

with a resistor divider from the input supply to define an

accurate undervoltage lockout (UVLO) threshold for the

regulator.TheEN/UVpincurrent(IEN)atthethresholdfrom

the Electrical Characteristics table needs to be considered

when calculating the resistor divider network:

VIN(UVLO) =1.24V • 1+

REN2

REN1

⎛

⎝

⎜

⎞

⎠

⎟+IEN •REN2

The EN/UV pin current (IEN) can be ignored if REN1 is less

than 100k. If unused, tie EN/UV pin to IN.

Programmable Power Good

As illustrated in the Block Diagram, power good thresh-

old is user programmable using the ratio of two external

resistors, RPG2 and RPG1:

VOUT(PG_THRESHOLD) =0.3V • 1+

RPG2

RPG1

⎛

⎝

⎜

⎞

⎠

⎟+IPGFB •RPG2

IfthePGFBpinincreasesabove300mV,theopen-collector

PG pin de-asserts and becomes high impedance. The

power good comparator has 7mV hysteresis and 5µs of

deglitching.ThePGFBpincurrent(IPGFB)fromtheElectrical

Characteristicstablemustbeconsideredwhendetermining

the resistor divider network. The PGFB pin current (IPGFB)

can be ignored if RPG1 is less than 30k. If power good

functionality is not used, float the PG pin. Please note that

programmable power good and fast start-up capabilities

are disabled for output voltages below 300mV.

Externally Programmable Current Limit

TheILIMpin’scurrentlimitthresholdis300mV.Connecting

a resistor from ILIM to GND sets the maximum current

flowing out of the ILIM pin, which in turn programs the

LT3045-1’scurrentlimit.Witha150mA•kΩprogramming

scale factor, the current limit can be calculated as follows:

Current Limit =

150mA •kΩ

RILIM

For example, a 1kΩ resistor programs the current limit

to 150mA and a 2kΩ resistor programs the current limit

to 75mA. For good accuracy, Kelvin connect this resistor

to the LT3045-1’s GND pin.

In cases where IN-to-OUT differential is greater than 12V,

the LT3045-1’s foldback circuitry decreases the internal

current limit. As a result, internal current limit may over-

ride the externally programmed current limit level to keep

the LT3045-1 within its safe-operating-area (SOA). See

the Internal Current Limit vs Input-to-Output Differential

graph in the Typical Performance Characteristics section.

AsshownintheBlockDiagram,theILIMpinsourcescurrent

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

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

range. If external current limit or current monitoring is

not used, tie ILIM to GND.

Output Overshoot Recovery

During a load step from full load to no load (or light

load), the output voltage overshoots before the regulator

responds to turn the power transistor OFF. Given that there

is no load (or very light load) present at the output, it takes

a long time to discharge the output capacitor.

As illustrated in the Block Diagram, the LT3045-1 incor-

porates an overshoot recovery circuitry that turns on a

current sink to discharge the output capacitor in the event

OUTS is higher than SET. This current is typically about

4mA. No load recovery is disabled for input voltages less

than 2.5V or output voltages less than 1.5V.

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