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LT3045-1

30451fa

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

APPLICATIONS INFORMATION

The LT3045-1 is a high performance low dropout linear

regulator featuring LTC’s ultralow noise (2nV/√Hz at

10kHz) and ultrahigh PSRR (76dB at 1MHz) architecture

for powering noise sensitive applications. Designed as a

precision current source followed by a high performance

rail-to-rail voltage buffer, the LT3045-1 can be easily paral-

leled to further reduce noise, increase output current and

spread heat on the PCB. The device additionally features

programmable current limit, fast start-up capability and

programmable power good.

The LT3045-1 is easy to use and incorporates all of the

protection features expected in high performance regula-

tors. Included are short-circuit protection, safe operating

areaprotection,reverse-batteryprotection,reverse-current

protection, and thermal shutdown with hysteresis.

In addition to the LT3045 feature set, the LT3045-1 incor-

porates a VIOC tracking function to control an upstream

switching converter to maintain a constant voltage across

the LT3045-1 and hence minimize power dissipation.

Output Voltage

The LT3045-1 incorporates a precision 100µA current

source flowing out of the SET pin, which also ties to the

error amplifier’s inverting input. Figure 1 illustrates that

connectingaresistorfromSETtogroundgeneratesarefer-

ence voltage for the error amplifier. This reference voltage

is simply the product of the SET pin current and the SET

pin resistor. The error amplifier’s unity-gain configuration

produces a low impedance version of this voltage on its

noninverting input, i.e. the OUTS pin, which is externally

tied to the OUT pin.

Figure 1. Basic Adjustable Regulator

The LT3045-1’s rail-to-rail error amplifier and current

reference allows for a wide output voltage range from 0V

(using a 0Ω resistor) to VIN minus dropout — up to 15V.

A PNP-based input pair is active for 0V to 0.6V output

and an NPN-based input pair is active for output volt-

ages greater than 1.3V, with a smooth transition between

the two input pairs from 0.6V to 1.3V output. While the

NPN-based input pair is designed to offer the best overall

performance,refertotheElectricalCharacteristicsTablefor

details on offset voltage, SET pin current, output noise and

PSRR variation with the error amp input pair. Table 1 lists

many common output voltages and their corresponding

1% RSET resistors.

Table 1. 1% Resistor for Common Output Voltages

VOUT (V)

RSET (kΩ)

2.5

24.9

3.3

33.2

49.9

121

150

The benefit of using a current reference compared with a

voltagereferenceasusedinconventionalregulatorsisthat

the regulator always operates in unity gain configuration,

independent of the programmed output voltage. This al-

lows the LT3045-1 to have loop gain, frequency response

and bandwidth independent of the output voltage. As a

result, noise, PSRR and transient performance do not

change with output voltage. Moreover, since none of the

error amp gain is needed to amplify the SET pin voltage

to a higher output voltage, output load regulation is more

tightly specified in the hundreds of microvolts range and

not as a fixed percentage of the output voltage.

Since the zero TC current source is highly accurate, the

SET pin resistor can become the limiting factor in achiev-

ing high accuracy. Hence, it should be a precision resistor.

Additionally, any leakage paths to or from the SET pin

create errors in the output voltage. If necessary, use high

quality insulation (e.g., Teflon, Kel-F); moreover, clean-

ing of all insulating surfaces to remove fluxes and other

residues may be required. High humidity environments

may require a surface coating at the SET pin to provide

a moisture barrier.

100µA

EN/UV

PGFB

VIOC

GND

OUT

LT3045-1

ILIM

10µF

4.7µF

VIN

5V ±5%

0.47µF

VOUT, 3.3V

IOUT(MAX), 500mA

30451 F01

33.2k

SET

OUTS

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