Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

This application note introduces the performance of power supply suppression feedback used in the CS44800/44600 digital class-D PWM amplifiers, and its feedback measurement process, such as power supply suppression performance testing and the relationship between feedback performance and frequency. This document also introduces the power supply to illustrate how the PSR feedback will maintain the amplified audio level even if the power supply voltage drops.

This application note introduces the performance of power supply suppression feedback used in the CS44800/44600 digital class D PWM amplifiers, as well as the feedback measurement process, such as power supply suppression performance testing and the relationship between feedback performance and frequency. This document also introduces the power supply to illustrate how the PSR feedback will maintain the amplified audio level even if the power supply voltage drops.

Since no mechanism has been developed to provide real-time feedback to suppress unwanted tones (ripples) generated by the power supply or noise coupled to the power rail due to noise, most of the currently available digital class D amplifiers are called “open loop” “system. Ripple current flowing through large decoupling capacitors. Since the high-voltage side of the power MOSFET is directly connected to the power rail, all tones and related harmonics generated on the power rail are coupled to the output of the audio amplifier channel.

It is now possible to reduce power supply costs and use other power supply technologies. “Open-loop” digital Class D PWM amplifiers prohibit the use of traditional low-cost, reliable and low EMI radiation noise unregulated linear power supplies. In view of the published application notes of Class D amplifier systems on the market, it is recommended to combine the use of large, expensive, low-ESR electrolytic capacitors with a switch mode power supply, which has very low output voltage ripple and strict regulation for load and line changes ability. The CS44800 / 44600 PSR circuit eliminates these dependencies and allows the use of inexpensive power supply alternatives.

PSR feedback measurement program

Power suppression performance test

The following test platform for the performance measurement graph includes CDB44800 (half bridge channel), Vpower = +40 V, and a resistive load of 6Ω. Set channel A of the input audio stream to channel 3 on the development board, and set channel B to channel 8.

1. Use unregulated power supply (if available). Any type of power supply can be used to view PSR suppression, but the actual suppression due to PSR feedback can be seen more clearly with a lightly regulated power supply than a highly regulated power supply.

2. Set the board under test for 2-channel operation with 6Ω resistance load, and execute the appropriate script file to play the amplified audio from the board and perform PSR calibration.

3. Using a digital audio source (such as Audio Precision), set channel A to -1 C dBFS, a 1 kHz sine wave, and channel B to 0 dB, a 60 Hz sine wave. Channel B is used to simulate the harmonics generated by a cheap, poorly regulated power supply. The 60 Hz switch input of channel B will produce a corresponding ripple voltage on the rail, which will be coupled to the channel under test, in this case channel A.

4. When only the PWM output of channel A is turned on, use an analog analyzer (such as Audio Precision) to perform amplitude and frequency FFT on channel A. The 1 kHz tone should have an amplitude of -1 dBFS. Figure 1 shows the result of enabling only one channel A as the baseline.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

[通道A = 1 kHz,-1 dBFS,通道B =禁用,PSR反馈禁用]

5. Turn on the PWM output of channel B, and perform the FFT of the amplitude and frequency of channel A. The 1 kHz tone should have an amplitude of -1 dBFS (side tone with modulation) and a 60 Hz tone and related harmonics. Figure 2 shows the FFT of Channel A with the PWM output of Channel A and Channel B enabled. The original 1 kHz tone is displayed as -1 dBFS, and the coupled 60 Hz tone from channel B is displayed as -50 dBFS. The full-scale 60 Hz tone played on the channel B MOSFET device will generate a related 60 Hz ripple current on the supply voltage rail. This ripple current and the equivalent series resistance (ESR) of the capacitor will produce discrete tones on the power rail. Due to the nonlinearity of the system, please pay attention to the 2, 3, 4, 5 harmonics at 120 Hz, 180 Hz, 240 Hz, 300 Hz and other frequencies. Since the power MOSFET switches at a frequency of 384 kHz to modulate all these tones to the audio output of channel A, these discrete tones will also be modulated. These tones are displayed as symmetrical and equidistant on each side of the 1 kHz tones. tone. The amplitude and frequency of each modulation tone can be easily calculated using standard FM modulation formulas.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

The relationship between FFT amplitude and frequency

[通道A = 1 kHz,-1 dBFS,通道B = 60 Hz 0 dBFS,禁用PSR反馈]

6. Enable PSR feedback (set CS44800 / 44600 bit 5 in register 34h to 1b). Perform amplitude and frequency FFT on the output of channel A. The amplitude of the 1 kHz tone should be -1 dBFS, but the amplitude of the 60 Hz tone and modulated sidetone will be greatly reduced. Figure 3 shows the result of enabling the channel and PSR feedback at the same time.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

The relationship between FFT amplitude and frequency

[通道A = 1 kHz,-1 dBFS,通道B = 60 Hz,0 dBFS,启用了PSR反馈]

Figure 4 shows the results of Figures 2 and 3 as overlays.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

The relationship between FFT amplitude and frequency

[图2和3叠加图]

7. To show the effect of this noise modulation on low-level audio signals, set the amplitude of the 1 kHz tone on channel A to -60 dBFS. Channel B maintains a 60 Hz, 0 dBfs signal. Figure 5 shows the same test as above. The blue trace is the FFT of channel A output when PSR is turned off. The magenta trace represents the output of channel A with PSR feedback enabled.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

The relationship between FFT amplitude and frequency

[通道A = 1 kHz,-60 dBFS,通道B = 60 Hz,0 dBFS,启用/禁用PSR反馈]

PSR feedback performance and frequency

The bottom graph in Figure 6 shows the relationship between the amount of suppression that PSR feedback will provide and the frequency of power supply noise (usually in the low frequency range). The frequency of channel B varies from 20 Hz to 20 kHz, and this frequency causes a ripple voltage on the measured channel (channel A). This figure shows the relationship between the amplitude of the coupled noise from the power rail to channel A and the frequency of the coupled noise.

Analysis of Power Supply Rejection Feedback Used in CS44800/44600 Digital Class D PWM Amplifiers

Disable PSR and enable PSR

[ChannelA=”On”butmuted(zerodata)sweepthechannelBfrequencyat0dBFS”[通道A=“开”,但已静音(零数据),以0dBFS扫描通道B频率”

Power supply voltage drop test

To demonstrate how PSR feedback compensates for sagging in the power rail, use CDB44800 to do the following:

Using the above settings, calibrate and enable PSR, set Vpower to +40 V, and play a 1 kHz tone with a peak-to-peak amplitude of approximately 10 V (or any amplitude required). Use one channel of the oscilloscope to monitor the sine wave output of channel A. Use another channel of the oscilloscope to monitor the DC voltage on Vpower.

Reduce Vpower to +30V. Even if the Vpower power supply drops from +40 V to +30 V, the peak-to-peak value of the sine wave output from channel A will remain constant.

This test shows how the PSR feedback will maintain the amplified audio level (common when playing low frequency audio) even if the power supply voltage drops. When playing a full-scale signal, the maximum voltage rail droop compensation is limited to 10% of the nominal voltage rail. As the amplitude of the signal being played decreases, more sag in the voltage rail can be compensated.

The Links:   SKM145GAL123D NL128102BC29-01B

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