LT3045EDD

The LT3045EDD is an ultra-low noise, ultra-high PSRR linear regulator from Analog Devices, representing the state-of-the-art in clean power delivery for noise-sensitive analog and RF circuits. This device is part of ADI’s LTC precision power management portfolio, engineered to provide exceptionally quiet supply rails for the most demanding applications.

What makes the LT3045 exceptional is its extraordinary noise and rejection performance. With just 0.8µV RMS output noise integrated from 10Hz to 100kHz and a spectral noise density as low as 0.8nV/√Hz at high frequencies, this regulator produces one of the cleanest output voltages available in a monolithic LDO. Even more impressive is its PSRR (power supply rejection ratio) of 78dB at 1MHz — far exceeding traditional LDOs that typically roll off their rejection above 100kHz. This makes the LT3045 highly effective at cleaning up high-frequency ripple from switching power supplies.

The LT3045 operates from 1.8V to 15V input and provides an adjustable output from 0V to 13.2V using a single SET resistor. Unlike traditional LDOs that use a resistive divider (which adds noise), the LT3045 uses a current-reference architecture where a precise 50µA current flows through an external resistor to set the output voltage. This approach, combined with internal noise filtering, achieves the ultra-low noise performance.

Additional features include programmable current limit, precision enable threshold, output noise reduction via an external bypass capacitor, comprehensive protection features (reverse battery, reverse current, thermal shutdown, short-circuit), and stability with ceramic output capacitors.

The LT3045 employs several innovative techniques to achieve its industry-leading noise and PSRR performance:

1. Current-Reference Architecture: Unlike traditional LDOs that use a voltage reference and resistive divider, the LT3045 uses a precision 50µA current reference. This current flows out of the SET pin through an external resistor R_SET, establishing the output voltage: VOUT = 50µA × R_SET. This architecture eliminates the noise contribution of a resistor divider network.

2. Low-Noise Bandgap Reference: An advanced bandgap reference generates the reference current with extremely low noise. Special circuit techniques, including chopper stabilization and noise filtering, minimize the reference noise to the nanovolt level.

3. High-PSRR Error Amplifier: The error amplifier is designed with multiple gain stages and advanced frequency compensation to maintain high power supply rejection ratio even at high frequencies (up to and beyond 1MHz). This is achieved through careful design of the power supply rejection at each stage.

4. PNP Pass Element: A PNP (or PMOS) pass transistor provides the series regulation. The P-type device allows low dropout voltage and good high-frequency performance. The dropout voltage at 500mA is typically 260mV.

5. Noise Reduction (NR) Pin: An external capacitor connected to the NR pin filters the reference voltage, further reducing output noise. Larger NR capacitors reduce noise down to the 0.8µV RMS level but increase start-up time.

6. Protection Features: The LT3045 includes reverse battery protection (diode in series with input), reverse current protection (prevents current from flowing backward from output to input), programmable current limit (set by a single resistor), and thermal shutdown.

7. Stability: The device is stable with ceramic output capacitors, which are preferred for low ESR and small size. A minimum output capacitance ensures stability across the full operating range.

• RF transmitter/receiver power supplies for low phase noise
• High-speed ADC and DAC reference and analog supply rails
• Low-jitter PLL and VCO power in communications equipment
• Medical imaging devices (MRI, ultrasound) requiring clean power
• High-end audio and professional audio equipment
• Post-regulation for switching supplies feeding sensitive analog circuits