[Shishuo Design] How to design ultra-low noise phantom power supply? Detailed schematic diagram and three noise reduction methods are available for taking away.

Professional-grade condenser microphones require a 48 V power supply to charge the internal capacitive sensor and power an internal buffer to provide a high-impedance sensor output. The current of this power supply is very low, generally only a few mA, but because the output level of the microphone is very low and the power supply ripple suppression performance of the buffer itself is poor, the power supply must have extremely low noise. Additionally, phantom power must not inject EMI into adjacent low-level circuitry, which is always a challenge in compact products.

We can build a high-performance power supply using the LT8362 boost converter, which uses a 60 V, 2 A switch and operates up to 2 MHz in a small 3 mm × 3 mm package. The circuit below is based on the standard LT8362 demonstration board DC2628A, the schematic of which is shown in Figure 1.

Figure 1. Used to build phantom power

Schematic of the demonstration circuit DC2628.

The input EMI filter on this demo board effectively filters high-frequency noise using a switched inductor in series with the input. On the output side, the situation is less ideal. The output EMI filter can effectively suppress noise in the MHz region, but has no effect on noise in the audio region. This noise is primarily caused by the 30× gain in the feedback loop, which amplifies the LT8362’s reference noise.

One way to eliminate this noise is to add capacitance to the output. It works as long as you add enough capacitance, but for a 48 V output, the minimum operating voltage of the practical capacitor is 63 V, which means that the required capacitance is large and expensive.

The second method is to increase the LT8362 output by 1 V or 2 V and add an LDO regulator to the output. This requires the use of a high-voltage LDO regulator, which generally costs more than a low-voltage regulator. Additionally, while these regulators have low noise at low output voltages, devices using a voltage reference suffer from the same reference noise multiplication problem as the LT8362.

A third approach is that since the sensitivity of the microphone output is not highly dependent on the supply voltage, there is no need to fully regulate the phantom power. This means that we can put some resistors in series with the output capacitor to increase its effectiveness; however, this can only reduce the size of the high voltage capacitor to a certain extent.

A better approach is to make the output capacitor appear larger than it actually is. We can do this using a traditional method called capacitance multiplication. This simple circuit can be seen in the gray shaded area of ​​Figure 2.

Figure 2. The same circuit as shown in Figure 1, but with a capacitance multiplier (gray) on the output to suppress audio noise generated by the switching regulator.

Among them, the 100 μF capacitor controls the ripple of the base current, so its impact on the collector current will be amplified by the beta value of the NPN transistor. The impact is very significant. Figure 3a shows the output of the LT8362 circuit at C4 (before filtering) with a load of 1 kΩ (50 mA).

Figure 3. Before and after filtering. (a) The noise content of the boost regulator output is approximately 0.2% when measured at C4 (before filtering). (b) After filtering, the noise content of the output is significantly reduced to 0.002%.

The noise is approximately 80 mV pp, which corresponds to approximately 0.2% noise content. For non-critical applications, this noise content may be sufficient, but after filtering, the output noise performance improves significantly to about 1 mV pp, as shown in Figure 3b. This corresponds to approximately 0.002% or 20 ppm noise content, which is sufficient for the most demanding applications. Figure 4 shows the workbench setup.

Figure 4. Using demonstration circuit DC2628

Bench setup for clean phantom power.

_{Transistor SBCP56-16T1G is used to achieve high V CBEO} (80 V) and high beta at low current . High beta allows the capacitor multiplier to have a high apparent capacitance and maintain a relatively constant voltage drop as the output current changes. The output voltage is reduced from 47.8 V into a 2 kΩ load to 47.5 V into a 500 Ω load, which is sufficient for microphone applications. Don't replace another transistor without testing the noise and voltage regulation effects.

Tested with 16 V input, but performance is similar from 12 V to 24V _IN . Some applications may require boosting from 5 V, which can be accomplished by reducing the LT8362's switching frequency from 2 MHz to 1 MHz, thereby achieving a minimum off-time of 75 ns. This also requires increasing L1 to about 10 μH to 15 μH and doubling the bulk output capacitor C4 to maintain equivalent performance.

Original text reproduced from Analog Devices

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