Designing Ultra-Low-Noise Positive and Negative Power Supplies for Precision Analog Circuits

Some of today's high-precision analog systems require low-noise positive and negative voltage rails to power precision analog circuits, including analog-to-digital converters (ADCs), digital-to-analog converters (DACs), bipolar amplifiers, etc. How to generate clean, stable positive and negative voltage rails to power noise-sensitive analog components is a design challenge in front of us.

The usual solution is to use a positive buck or boost switching power supply to generate the positive rail, and then use a linear regulator for post-regulation to reduce the voltage ripple formed by the switching power supply. Use an inverting switching power supply to generate the negative rail. Due to the small product range of high-voltage negative low-dropout regulators (LDOs), we generally use a discrete LC filter to attenuate the switching noise. Although this method is effective, it requires the designer to spend time calculating the accuracy and long-term stability of the LC filter.

For example, the reference design shown in Figure 1 uses the TPS54x60 and shows a simpler way to generate clean voltage rails. With this circuit, a switching converter is used to build the positive and negative voltage rails. Two high power supply rejection ratio (PSRR)/low noise LDOs are used for post regulation to eliminate switching noise. The noise performance of the LDOs eliminates the need for an LC output filter.

To create this reference design, a +60V switching converter in a buck-boost configuration is used to generate a balanced +/- output voltage. The positive and negative voltage outputs of the switch are post-regulated using low-noise, high-PSRR LDOs such as the TPS7A30 and TPS7A49. In Figure 2, the switching regulator voltage ripple is about 40mV for the –18V rail and 20mV for the +18V rail. By post-regulating the output of the 300 kHz switching regulator with an LDO, the voltage ripple is greatly reduced. Here, a 60V switching converter is used because the ground pin is referenced to the –18V rail and the maximum VIN is 30V. In this configuration, the maximum voltage that the switching converter must withstand is 48V. Select the LDO for its wide input voltage, low output noise, and high PSRR.

Figure 1 Reference Schematic Diagram

Figure 2 Oscilloscope screenshot showing LDO PSRR performance

In today's medical, test and measurement, and industrial control markets, the need for higher noise performance becomes increasingly important as data converter resolution increases, or as the full-scale range of the signal decreases. Table 1 shows that as the data converter resolution increases, the 1LSB voltage step decreases. Alternatively, as the full-scale voltage swing magnitude decreases, the 1 LSB voltage step decreases. As the 1 LSB voltage value decreases, the impact of power supply noise increases. Power supply noise increases the signal-to-noise ratio (SNR), thereby reducing the effective resolution of the data converter. For example, a 16-bit data converter will behave like a 14-bit data converter due to excessive power supply noise. It is up to the design engineer to decide how to trade off noise and accuracy.

Data Converter Accuracy
Table 1, LSB Compromise Considerations

An easy way to recreate this design is to use the same evaluation boards we used: the TPS54060EVM-590 for the switching circuit; and the TPS7A30-49REVM-567 for the LDO.

Simply connect the switching regulator evaluation board (EVM) to the input of the LDO EVM to begin evaluation. If this solution does not meet the system voltage requirements, download the point-of-load (POL) DC/DC converter tool and modify the switching circuit to meet your specifications. If the LDO board requires a different output voltage, modify the feedback resistor using the LDO data sheet to calculate the value required for the ideal output voltage. Using this method, positive and negative voltage rails can be generated from an input voltage of 12V to 30V and an output voltage range of +/−2.5V to +/−15V.

in conclusion

If you need a simpler solution to generate very low noise stable positive and negative voltage rails to power ADCs, DACs, bipolar amplifiers, etc., consider using the reference design shown in Figure 1. Please evaluate the available tools and application support that make the creation, evaluation, and modification of positive and negative voltage supplies easier. Now you can quickly create the power supply section and spend more valuable design time on the data conversion circuitry that is critical to differentiating your application from the competition.

References
TPS7A3001 and TPS7A4901 low-dropout linear regulator evaluation modules.
TPS54060 split-rail power supply evaluation module.
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About the Author

Pat Hunter is a product marketing engineer at TI, responsible for developing marketing strategies for point-of-load power solutions. Pat graduated from the University of North Carolina, Charlotte, North Carolina with a Bachelor of Science in Electrical Engineering and holds one patent.