[Shishuo Design] How to generate low noise voltage? This article tells you~

Latest update time：2021-09-03 00:56

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Linear regulators are particularly well suited for filtering the voltage produced by switching regulators. Switching regulators always produce a certain amount of output voltage ripple. In many applications that process very small signals, this ripple can be disruptive. Passive components are often used to filter the output voltage of a switching regulator, but passive filters such as LC filters (see Figure 1) have some disadvantages.

Depending on the cutoff frequency required for the filter, the space requirements can sometimes be quite large and the inductor cost can be high. However, the biggest disadvantage of passive filters is that the filter adds some losses and an output voltage that varies with the operating current (like V _OUT in Figure 1 ). Therefore, the DC regulation accuracy of the generated voltage is quite low.

[Figure 1. Passive filter at the output of a switching regulator]

Linear regulators are often used instead of passive LC filters to clean up the voltage generated by a switching regulator. They usually have a high power supply rejection ratio (PSRR). This means that most of the input ripple of the linear regulator is blocked, so the output ripple of the linear regulator is very small. In addition, the output voltage of the linear regulator has its own closed loop, so it is better and more accurately regulated.

Another important specification of the linear regulator is different from the passive LC filter, that is, its intrinsic noise generated by its internal reference voltage and internal error amplifier. In applications that require low interference (low noise), the influence of the intrinsic interference generated by the linear regulator cannot be ignored. For this reason, ultra-low noise linear regulators can be used.

The concept of using a linear regulator to filter the voltage generated by the switching regulator is shown in Figure 2. This scheme applies not only to positive supply voltages, but also to negative supply voltages. For example, the following typical application circuit: Bipolar signals between –5 V and +5 V must be accurately measured and digitized using an analog-to-digital converter. The input stage of the signal path requires a low-noise bipolar power supply with positive and negative supply voltages.

For positive supply voltages, there are many linear regulators on the market that offer high PSRR and low interference performance. Especially for very critical applications, ADI offers many solutions. However, for negative supply voltages, only a few linear regulators are suitable for such applications. For example, the new LT3094 can be used to filter negative supply voltages at very high PSRR and with very low noise levels, which is very important for applications that need to respond sharply to power supply interference. Figure 3 shows the PSRR of the LT3094 at different frequencies. For a switching regulator with a switching frequency of 1 MHz, the PSRR of the LT3094 linear regulator is about 75 dB. The noise level of the LT3094 between 10 Hz and 100 kHz is only 0.8 μV rms.

[Figure 2. Linear regulator used as an active filter at the output of a switching regulator]

[Figure 3. Power supply voltage rejection ratio of the LT3094 linear regulator for negative supply voltage]

Linear regulators can filter out supply ripple for both positive and negative supply output voltages. This filtering is also useful for negative supply voltage ranges. Since there are limited low noise types for negative supply voltages, it can be difficult to find suitable parts. With the introduction of ADI’s new Power product, the LT3094, customers now also have a product option that has very low noise and very high PSRR.

LT3094 Product Details

Ultra-low RMS noise: 0.8μVRMS (10Hz to 100kHz)
Ultra-low spot noise: 2.2nV/√Hz at 10kHz
Ultra-high PSRR: 74dB (at 1MHz)
Output current: 500mA
Wide Input Voltage Range: −1.8V to −20V
Single capacitor improves noise and PSRR specifications
100μA SET Pin Current: ±1% Initial Accuracy
Output Voltage Set by Single Resistor
Programmable current limit
Low dropout voltage: 235mV
Output voltage range: 0V to −19.5V
Programmable Power Good and Fast Startup
Bipolar Precision Enable/UVLO Pin
VIOC pin controls upstream regulator to minimize power dissipation and optimize PSRR
Minimum output capacitance: 10μF (ceramic)
12-Lead MSOP and 3mm x 3mm DFN Packages

Original article from Analog Devices

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