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LT3045

3045fa

For more information www.linear.com/LT3045

applicaTions inForMaTion

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

tection features expected in high performance regulators.

Included are short-circuit protection, safe operating area

protection, reverse battery protection, reverse current

protection, and thermal shutdown with hysteresis.

Output Voltage

TheLT3045incorporatesaprecision100µAcurrentsource

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

amplifier’sinvertinginput.Figure1illustratesthatconnect-

ing a resistor from SET to ground generates a reference

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.

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

ence allows for a wide output voltage range from 0V (us-

ing 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 voltages 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, refer

to the Electrical Characteristics Table for details on offset

voltage, SET pin current, output noise and PSRR variation

with the error amp input pair. Table 1 lists many common

outputvoltagesandtheircorresponding1%RSETresistors.

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 voltage reference as used in conventional regulators is

that the regulator always operates in unity gain configura-

tion, independent of the programmed output voltage. This

allows the LT3045 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.

Minimize board leakage by encircling the SET pin with a

guard ring operated at a potential close to itself — ideally

tied to the OUT pin. Guarding both sides of the circuit

board is recommended. Bulk leakage reduction depends

100µA

EN/UV

PGFB

GND

OUT

LT3045

ILIM

10µF

4.7µF

VIN

5V ±5%

0.47µF

VOUT, 3.3V

IOUT(MAX), 500mA

3045 F01

33.2k

SET

OUTS

Figure 1. Basic Adjustable Regulator

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