March 21, 2007
Single-ended design with Digital Amplifiers
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By
Kevin Belnap, Marketing Manager, Texas Instruments
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One of the biggest advantages of digital amplifiers is flexibility of the digital data path for design reuse. Because the signal is kept in the digital domain until it is practically at the speakers, there is much more flexibility in signal routing. This flexibility also applies to processing both on-the-fly and on the production line with stuffing options and/or firmware changes. One common mode where the digital amp is used is called single-ended operation. This article discusses basics of single-ended design and the engineering tradeoffs involved.
Typically, the digital amplifier has a two-stage architecture " a pulse-width modulation (PWM) processor, followed by the power stage as shown in Figure 1. The logic-level PWM processor accepts the audio data, usually in IIS format. It performs audio processing and converts the pulse-code modulated (PCM) data into PWM data. Typically, the PWM processor is controlled through IIC to change volume, tone controls, or other audio processing functions like equalization. Most PWM processors have another key feature which is the ability to change signal routing, even "on-the-fly." This allows the designer flexibility in PCB routing, or the ability for the user to route content to different speakers. The power stage takes the 3.3V PWM signal, translates it to a higher voltage, and applies it to the speaker through the MOSFET H-bridge and second-order LC filter.
Figure 1: Digital amplifier datapath showing the power stage H-bridge
The power stage contains a MOSFET H-bridge shown in Figure 1. This is where the MOSFETs are used as switches to apply the +V voltage across the speaker, either in a positive or negative direction. Bridge-tied load (BTL) is the normal configuration for most stereo power stages where the speaker is connected between the two MOSFET half-bridges. This is shown in Figure 1. Single-ended (SE) refers to when each speaker is driven by a single MOSFET half-bridge. The channel count is doubled in SE mode when compared to BTL mode, but the power per channel is cut to about 25 percent for a given output load. In single-ended mode, the +V voltage is applied to the speaker in a positive direction when the PWM signal is "high." When the PWM signal is "low," the speaker is connected to ground.
Single-ended operation for a digital amplifier shown in Figure 2 and is not much different than single-ended operation for a linear audio amp. The main difference is that the reconstruction filter (second order LC filter) sorts out the higher frequency components from the PWM signal, leaving the baseband audio signal. Applying the audio signal directly across the speaker results in a large DC voltage across the speaker, which is equal to PVDD/2. Because the speaker impedance has a predominant inductive component, this is equivalent to placing a large DC voltage across an inductor and causes the current to increase linearly to a very large value, possibly damaging the speaker. A large capacitor (DC blocking cap) is placed between the amplifier and the speaker to filter out this DC component. However, this cap also attenuates the lower audio frequencies and has a "3dB point of approximately 1/(2 RspC) where Rsp is the impedance of the speaker. To pass more frequency bandwidth through the speaker, a larger capacitance can be used. However, this comes at a tradeoff of cost and PCB area.
Figure 2: Single-ended digital amplifier with DC blocking cap configuration
In the single-ended configuration just discussed, the audio signal is referenced to ground. In other words, one speaker terminal is connected to ground. Another way to achieve DC blocking is to use a split-cap configuration where the audio signal is now referenced to PVDD/2, shown in Figure 3. From an AC perspective, there is no difference between Figure 2 and Figure 3 " as long as Csm = Cb/2. In addition to the capacitance being cut in, half the current rating of Cs can be half that of Cb, and the ESR can be twice that of Cb, with no change in audio or thermal performance.
Figure 3: Single-ended digital amplifier with split-cap configuration
The biggest advantage of the split-mode configuration over a blocking cap is that the PSRR is increased. The graph shown in Figure 4 is an actual PSRR measurement of the TAS5086/5142 evaluation module (EVM) from Texas Instruments. The TAS5142 power stage is configured as single-ended in this EVM. Some might be surprised that an open-loop, single-ended amplifier can have this much PSRR. This is explained by the fact that voltage changes in PVDD ((ΔPVDD) also will be at the mid-point of the split-cap (ΔPVDD/2). So across the speaker changes in PVDD are cancelled out.
Figure 4: Single-ended PSRR from the TI TAS5086/5142 EVM
The SE split-mode configuration requires that two other design issues be addressed. As previously mentioned, the audio signal after the reconstruction filter has a DC value of PVDD/2. If the capacitors Cs are ideal, both will charge up to PVDD/2 " and there will be no DC component across the speaker. However, since both capacitors are not ideal and will have a tolerance, the DC voltage will not be equal to PVDD/2. Thus, the speaker will have a DC voltage across it when the audio signal is first applied to the speaker resulting in a pop when powered up. Another related issue occurs because the split-caps take a finite time to charge with a time constant of RC. As long as the MOSFETs do not begin switching until the split-caps are charged, this is not a problem. In practice, this is difficult to do, and large pop noises can occur.
The solution to both of these issues can be a power stage with a half-bridge specifically dedicated to quickly charging this voltage to PVDD/2, which is available with the TAS5186A. With a 50 percent duty cycle, the DC voltage output is PVDD/2 and the split-caps are quickly and accurately charged. Another way to charge the split-caps quickly is to use an operational amplifier (op amp). The use of an op amp is a proven method for split-cap charging when a dedicated half-bridge is not available.
In practice, single-ended amplifier audio performance data, including power-on pops, signal-to-noise ratio (SNR), PSRR, and THD+N, are quite good with only a small degradation in audio performance from that of BTL.
About the author
Kevin Belnap is a Marketing Manager for the High Performance Analog Home Audio group at Texas Instruments. His previous experience includes strategic and tactical marketing for digital amplifiers. Kevin holds a Bachelor's Degree in Electrical Engineering and a Master's degree in Business Administration, both from Brigham Young University.
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