“Low dropout (LDO) linear regulators have been widely used in noise-sensitive applications for decades. However, with the latest precision sensors, high-speed and high-resolution data converters (ADC and DAC), and frequency synthesizers (PLL/VCO) continue to challenge traditional LDO regulators to generate ultra-low output noise and ultra-low output noise. With high power supply ripple rejection (PSRR), noise requirements have become increasingly difficult to meet.

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Low dropout (LDO) linear regulators have been widely used in noise-sensitive applications for decades. However, with the latest precision sensors, high-speed and high-resolution data converters (ADC and DAC), and frequency synthesizers (PLL/VCO) continue to challenge traditional LDO regulators to generate ultra-low output noise and ultra-low output noise. With high power supply ripple rejection (PSRR), noise requirements have become increasingly difficult to meet.

For example, when powering the sensor, power supply noise will directly affect the accuracy of the measurement results. Switching regulators are commonly used in power distribution systems to achieve higher overall system efficiency. In order to construct a low-noise power supply, LDO regulators usually perform post-regulation on the output of relatively high-noise switching converters without the need for huge output filter capacitors. The high-frequency PSRR performance of the LDO regulator becomes critical.

ADI’s LT3042 is the industry’s first linear regulator with only 0.8μV rms output noise and 79dB PSR at 1MHz. Two similar devices LT3045 and LT3045-1 can provide higher ratings and additional features. All these devices are positive LDO regulators. When the system has bipolar devices (such as operational amplifiers or ADCs), a negative LDO regulator must be used in the design of the polar power supply. LT3094 is the first negative LDO regulator with ultra-low output noise and ultra-high PSRR. Table 1 lists the main features of the LT3094 and related devices.

Table 1. Characteristics of LT3094 and low noise LDO

typical application

The LT3094 has a precision current source reference, followed by a high-performance output buffer. The negative output voltage can be set by a −100µA precision current source flowing through a single resistor. This current-reference-based architecture provides a wide output voltage range (0V to −19.5V) and provides almost constant output noise, PSRR, and load regulation, regardless of the set output voltage. Figure 1 shows a typical application, and the demo board is shown in Figure 2. The overall solution size is only about 10mm×10mm.

Figure 1. −3.3 V output low-noise solution.

Figure 2. The demo circuit shows a −3.3 V miniature solution.

The LT3094 has ultra-low output noise, 0.8µVrms from 10Hz to 100kHz, and 74dB ultra-high PSRR at 1MHz. In addition, the LT3094 has programmable current limit, programmable power good threshold, quick start function and programmable input to output voltage control (VIOC). When the LT3094 performs post-regulation on the switching converter, if the output voltage of the LDO regulator is variable, the voltage across the LDO regulator will remain constant through the VIOC function.

LT3094 avoids device damage through internal protection functions, including internal current limit, thermal limit, reverse current and reverse voltage protection with foldback function.

Direct parallel connection for higher current

LT3094 can be easily connected in parallel to increase output current. Figure 3 shows the solution of using two parallel LT3094s to achieve 1A output current. In order to connect the two devices in parallel, connect the SET pin together and place a SET resistor RSET between the SET pin and ground. The current flowing through RSET is 200µA, which is twice the amount of SET current in a single device. In order to obtain good current sharing characteristics, each output of LT3094 uses a small current resistance of 20mΩ.

Figure 3. Schematic diagram of two parallel LT3094s.

Figure 4 shows the thermal performance of the circuit in Figure 3, where the input voltage is −5V, the output voltage is −3.3V, and it runs at a load current of 1A. The temperature of each device rises to approximately 50°C, indicating that the heat is evenly distributed. For higher output current and lower output noise, there is no limit to the number of devices that can be connected in parallel.

Figure 4. Thermal image of two parallel LT3094s.

Positive and negative dual power supply with variable output voltage

The power supply is usually configured with a switching converter that is adjusted by an LDO regulator to achieve low output noise and high system efficiency. In order to maintain an appropriate trade-off between power consumption and PSRR, the optimal voltage difference between the input and output of the LDO regulator is approximately −1V. It is complicated to maintain this voltage difference in a variable output voltage system, but the LT3094 has a tracking function VIOC, which can maintain a constant voltage across the LDO regulator even if the output voltage changes.

Figure 5 is a schematic diagram of dual power supplies using LT8582, LT3045-1 and LT3094. The LT8582 is a dual-channel PWM DC/DC converter with built-in switches that can generate positive and negative outputs from a single input. The first channel of the LT8582 is configured as a SEPIC to generate a positive output, and the second channel is an inverting converter to generate a negative power rail.In the negative power rail, the voltage across the LT3094 is controlled by the VIOC voltage

Figure 5. Adjustable dual output positive/negative power supply with high ripple rejection and low temperature operation performance.

Among them, VFBX2 is 0mV and IFBX is 83.3µA. Set R2 to 14.7kΩ, then the VIOC voltage can be set to 1.23V for a variable output voltage. When the resistance R1 is 133kΩ, the input voltage of the LT3094 is limited to 16.5V, the calculation is as follows:

The thermal image of the circuit running under 12V input is shown in Figure 6. When the output voltage changes from ±3.3V to ±12V, the temperature rise of the LT3094 remains unchanged. Table 2 lists the voltage and current of all three devices. Figure 7 shows the ±5V power supply transient response under 12V input.

Figure 6. Thermal image of dual power supply at 12 V input.

Figure 7. Dual-supply transient response with 12 V input and ±5 V output.

Table 2. Circuit performance of dual output positive/negative power supply with 12 V input and ±500 mA load

In Figure 5, in addition to the output capacitance of the LT8582, no additional capacitance is placed at the input of the LT3094. Usually, the input capacitance will reduce the output ripple, but this is not the case for the LT3094. If the LT3094 has an input capacitance, the switching current of the switching converter will flow through the input capacitance, resulting in electromagnetic coupling between the switching converter and the output of the LT3094. The output noise will increase, thereby reducing PSRR. If the switching regulator is within two inches of the LT3094, in order to obtain the best PSRR performance, we recommend not to place a capacitor on the input of the LT3094.

in conclusion

LT3094 is a negative LDO regulator with ultra-low noise and ultra-high PSRR. It uses a current reference-based architecture to make noise and PSRR performance independent of output voltage. Multiple LT3094s can be easily connected in parallel to increase load current and reduce output noise. When the LT3094 is used for post-regulation of a switching converter, the VIOC function can minimize the power consumption of the LDO regulator, making it an ideal choice for variable output voltage applications.