LT3094EDD

The LT3094EDD is an ultra-low noise, ultra-high PSRR negative linear regulator from Analog Devices, designed to provide clean negative supply rails for the most demanding precision analog and RF applications. As the negative counterpart to the award-winning LT3045 positive LDO, the LT3094 delivers the same exceptional performance but for negative voltage supplies, enabling complete low-noise dual-supply solutions.

The LT3094 achieves remarkable noise performance of just 0.8µV RMS (10Hz to 100kHz) and PSRR of 74dB at 1MHz — nearly unheard of for negative regulators. This level of performance is critical for high-performance analog systems that require clean negative supplies, such as high-speed data converters, precision instrumentation amplifiers, and low-noise RF circuits.

Using the same current-reference architecture as the LT3045, the LT3094 sets its output voltage with a single resistor connected from the SET pin to ground. A precision 50µA current source flows through this resistor, establishing VOUT = -50µA × R_SET. This approach eliminates the noise contribution of a traditional resistive feedback divider.

The device operates from -1.8V to -45V input and can provide outputs from 0V down to -42V (limited by the input-output differential). It is stable with ceramic output capacitors and includes comprehensive protection features including reverse battery protection, reverse current protection, programmable current limit, thermal shutdown, and short-circuit protection.

The LT3094 is a negative LDO that mirrors the architecture of the positive LT3045 but with P-type and N-type devices swapped:

1. Negative Current-Reference Architecture: The LT3094 uses a precision -50µA current source that sinks current from the SET pin. When a resistor R_SET is connected from SET to ground (the more positive rail), the output voltage is VOUT = -50µA × R_SET. This architecture eliminates the noise that would be introduced by a traditional resistive divider.

2. Low-Noise Bandgap Reference: An advanced bandgap reference operates from the negative supply and generates the reference current with extremely low noise. Chopper stabilization and internal noise filtering achieve nanovolt-level noise spectral density.

3. High-PSRR Error Amplifier: The error amplifier is specially designed to maintain high power supply rejection at high frequencies. Multiple gain stages with optimized power supply rejection at each stage contribute to the 74dB PSRR at 1MHz, far exceeding traditional negative regulators.

4. NPN Pass Element: An NPN (or NMOS) pass transistor serves as the series regulation element between the negative input and negative output. The N-type device allows low dropout voltage (260mV typical at 500mA) and good high-frequency response.

5. Noise Reduction (NR) Pin: An external capacitor connected to the NR pin further filters the reference, reducing output noise to ultra-low levels. The NR capacitor value determines the noise floor and also affects start-up time.

6. Protection Features: The LT3094 includes reverse battery protection (prevents damage if the input supply is reversed), reverse current protection (prevents current from flowing from ground to output when the input is shorted), programmable current limit via a resistor, and thermal shutdown.

7. Stability: The device is stable with low-ESR ceramic output capacitors. The internal compensation is designed to work with a range of output capacitor values, ensuring robust stability across operating conditions.

• Negative supply rails for high-speed ADCs and DACs
• Precision op-amp negative supply for instrumentation amplifiers
• RF power amplifier biasing and negative supply regulation
• Medical imaging equipment requiring clean dual supplies
• Audio equipment with negative rail requirements
• Post-regulation of negative buck-boost or inverting converters