How is very low noise phantom power designed?Detailed schematic and three denoising methods
【Introduction】Professional-grade condenser microphones require a 48 V power supply to charge the internal capacitive sensor, as well as power the internal buffer to provide a high-impedance sensor output. The current of this power supply is very low, typically only a few mA, but because the output level of the microphone is very low and the power supply ripple rejection of the buffer itself is not good, the power supply must be very low noise. Additionally, phantom power must not inject EMI into adjacent low-level circuits, which is always a challenge for compact products.
Q: Is it possible to generate compact ultra-low noise phantom power (48 V) from a 5 V, 12 V or 24 V input?
A: Yes, you need to use a simple boost converter, a filter circuit to reduce EMI, and a small size can be achieved with a little trick.
Professional grade condenser microphones require a 48 V supply to charge the internal capacitive sensor, as well as power the internal buffer to provide a high impedance sensor output. The current of this power supply is very low, typically only a few mA, but because the output level of the microphone is very low and the power supply ripple rejection of the buffer itself is not good, the power supply must be very low noise. Additionally, phantom power must not inject EMI into adjacent low-level circuits, which is always a challenge for 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 demo board DC2628A, whose schematic is shown in Figure 1.
Figure 1. Schematic of the DC2628 demonstration circuit used to build phantom power.
The input EMI filter on this demo board can effectively filter high frequency noise with a switched Inductor in series with the input. On the output side, the situation is not so ideal. The output EMI filter can effectively suppress the noise in the MHz region, but has little effect on the noise in the audio region. These noises are mainly caused by the 30× gain in the feedback loop, which amplifies the reference noise of the LT8362.
One of the ways to remove this noise is to add capacitance at the output. Adding enough capacitance works, but for a 48 V output, the minimum operating voltage for a real capacitor is 63 V, which means 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 on the output. This requires the use of high-voltage LDO regulators, which generally cost more than low-voltage regulators. Additionally, while these regulators have low noise at low output voltages, devices using voltage references suffer from the same reference noise multiplication issues as the LT8362.
The third method is that since the sensitivity of the microphone output is not highly dependent on the supply voltage, there is no need to fully adjust the phantom power. This means that we can put some resistors in series with the output capacitor to increase its effectiveness; however, this only reduces the size of the high voltage capacitor to a certain extent.
A better approach is to make the output capacitor appear larger than it is. We can do this using a traditional method called capacitance multiplication. This simple circuit can be seen in the gray shaded portion of Figure 2.
Figure 2. The same circuit as Figure 1, but with a capacitor multiplier (gray) at the output to suppress audible noise from the switching regulator.
Among them, the 100 μF capacitor controls the ripple of the base current, so its effect on the collector current is 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 1 kΩ (50 mA) load.
Figure 3. Before and after filtering. (a) The noise content of the boost regulator output is about 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 about 80 mV pp, which corresponds to a noise content of about 0.2%. For non-critical applications, this noise content may be sufficient, but after filtering, the output noise performance is significantly improved, around 1 mV pp, as shown in Figure 3b. This equates to approximately 0.002% or 20 ppm noise content, sufficient for the most demanding applications. Figure 4 shows the bench setup.
Figure 4. Bench setup for clean phantom power using demo circuit DC2628.
Transistor SBCP56-16T1G is used to achieve high VCBEO (80 V) and high beta at low current. A high beta allows the capacitance multiplier to have a high apparent capacitance and maintain a relatively constant voltage drop over output current. The output voltage is reduced from 47.8 V with a 2 kΩ load to 47.5 V with a 500 Ω load, which is sufficient for microphone applications. Do not replace another transistor without testing the effect of noise and regulation.
Tested with 16 V input, but performance is similar to 12 V to 24VIN. Some applications may require a boost from 5 V, which can be achieved by reducing the switching frequency of the LT8362 from 2 MHz to 1 MHz, resulting in 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.
● Wide input voltage range: 2.8V to 60V
● Ultra-low quiescent current and low ripple Burst Mode® operation: IQ = 9μA
● 2A, 60V power switch
● Positive or negative output voltage programming with a single feedback pin
● Programmable frequency (300kHz to 2MHz)
● Can be synchronized to an external clock
● Spread spectrum frequency modulation for low EMI
● BIAS pin for higher efficiency
● Programmable undervoltage lockout (UVLO)
● Thermally enhanced 10-pin 3mm x 3mm DFN and 16-pin MSOP packages
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