Ultra-low output noise and ultra-high PSRR, do you know about this negative LDO regulator?

For example, when powering a sensor, power supply noise can directly affect the accuracy of the measurement results. Switching regulators are often used in power distribution systems to achieve higher overall system efficiency. To build a low-noise power supply, an LDO regulator often post-regulates the output of a relatively noisy switching converter without the need for bulky output filter capacitors. The high-frequency PSRR performance of the LDO regulator becomes critical.

The LT3042 from ADI is the industry's first linear regulator with only 0.8μV rms output noise and 79dB PSR at 1MHz. Two similar devices, the LT3045 and LT3045-1, offer higher ratings and additional features. All of these devices are positive LDO regulators. When a system has bipolar devices, such as op amps or ADCs, a negative LDO regulator must be used in a polarity power supply design. The LT3094 is the first negative LDO regulator with ultra-low output noise and ultra-high PSRR. Table 1 lists the key features of the LT3094 and related devices.

Table 1. Characteristics of the LT3094 and Low Noise LDOs

The LT3094 features 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 nearly constant output noise, PSRR, and load regulation, independent of the programmed output voltage. Figure 1 shows a typical application and the demonstration board is shown in Figure 2. The total solution size is only approximately 10mm × 10mm.

Figure 1. −3.3 V output low noise solution.

Figure 2. Demonstration circuit shows a tiny −3.3 V solution.

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

The LT3094 avoids device damage through internal protection features, including internal current limiting with foldback, thermal limiting, reverse current and reverse voltage protection.

The LT3094 can be easily paralleled to increase output current. Figure 3 shows a solution for achieving 1A output current using two LT3094s in parallel. To parallel the two devices, the SET pins are connected together and a SET resistor, R _{SET ,} is placed between the SET pin and ground . The current flowing through R _SET is 200µA, which is twice the amount of SET current in a single device. For good current sharing, a small 20mΩ ballast resistor is used on each output of the LT3094.

Figure 3. Schematic of two LT3094s in parallel.

Figure 4 shows the thermal performance of the circuit in Figure 3 with an input voltage of −5 V and an output voltage of −3.3 V, operating at a load current of 1 A. The temperature of each device rises to approximately 50°C, indicating that the heat is evenly distributed. There is no limit to the number of devices that can be paralleled for higher output current and lower output noise.

Figure 4. Thermal image of two LT3094s in parallel.

Power supplies are often configured with a switching converter that is post-regulated by an LDO regulator to achieve low output noise and high system efficiency. To maintain the proper trade-off between power dissipation and PSRR, the optimized voltage difference between the input and output of the LDO regulator is approximately −1V. Maintaining this voltage difference in a variable output voltage system is complex, but the LT3094 has a tracking function, VIOC, that keeps the voltage across the LDO regulator constant even as the output voltage varies.

Figure 5 is a dual power supply schematic using the 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 the positive output, and the second channel is an inverting converter to generate the negative rail. In the negative rail, the voltage across the LT3094 is controlled by the VIOC voltage.

Figure 5. Adjustable dual-output positive/negative power supply features high ripple rejection and cool operation .

Where V _FBX2 is 0mV and I _FBX is 83.3µA. Setting R _{2 to 14.7kΩ sets the V IOC} voltage to 1.23V for a variable output voltage . When resistor R ₁ is 133kΩ, limiting the input voltage of the LT3094 to 16.5V, the calculation is as follows:

A thermal image of the circuit running at 12V input is shown in Figure 6. The temperature rise of the LT3094 remains constant as the output voltage changes from ±3.3V to ±12V. Table 2 lists the voltages and currents for all three devices. Figure 7 shows the transient response of the ±5V supply at 12V input.

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

Figure 7. Dual-supply transient response with 12 V input, ±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, no additional capacitors are placed at the input of the LT3094, other than the output capacitors of the LT8582. Typically, input capacitors reduce output ripple, but this is not the case for the LT3094. If the LT3094 had input capacitors, the switching current of the switching converter would flow through the input capacitors, causing electromagnetic coupling of the switching converter to the LT3094 output. The output noise would increase, degrading the PSRR. If the switching regulator is within two inches of the LT3094, we recommend not placing capacitors at the input of the LT3094 for best PSRR performance.

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