Supply Voltage Restrictions on Output Driver Performance
At 65-nm and 40-nm processes, many integrated audio codecs use 2.5 V I/O transistors for the analog circuitry and actually overdrive the 2.5 V devices to 3.3 V in order to increase the audio performance. However, at 28-nm processes, most SoC designs will migrate to the 1.8 V I/O transistors. Overdriving the 1.8 V transistors to 2.5 V or 3.3 V is not widely supported today. The net result is that the supply voltage and the transistors for the line and headphone drivers are limited to 1.8 V.
By restricting the supply voltage to 1.8 V, there are fundamental limits to the audio output performance. For the line output drivers, the output voltage swing is now limited to 0.54 Vrms compared to 1.0 Vrms available with a 3.3 V supply. The headphone drivers are limited to only 12 mW of output power on a 32 Ohm headphone, where in previous generations at 65- and 40-nm processes, the headphone drivers were able to deliver 40 mW from the higher supply (see "Headphone Power Requirements" sidebox).
Headphone Power Requirements
Audio signals have a wide dynamic range, usually with very pronounced peaks, as seen in the figure below. Therefore, an audio driver must have sufficient output power to support all listening levels without saturating the peaks. For a typical listening level, the peak power must be at least 15 dB above the average power.
For example, a typical 32 Ohm headphone has about 95 dB of sensitivity. That means that it produces a sound level of 95 dB SPL (sound pressure level) with 1 mW input signal. In order to produce 100 dB SPL to support loud music range as seen in the table below, with 15 dB of peak-to-average ratio, the headset output driver must be capable of delivering a peak power of 100 mW. Delivering 100 mW into a headset can cause damage to the ear over the long term. As a result, to deliver a comfortable and full listening experience, the typical adopted peak power for headset drivers is approximately 40 mW.
Typical audio signal (classic music)
Sound level examples
Synopsys tested thirteen recent models of mobile multimedia devices, including smartphones, tablets, MP3 players and notebook PCs to evaluate the actual audio performance of devices that are available in the market today. Of the thirteen, 55% fit well within the performance supported by a 1.8-V supply and three were slightly above in terms of the line output rms voltage swing. As shown in Figure 4, a line driver output of 0.54 Vrms and a 12 mW headphone driver will perform as well or better than many of the devices on the market today.
However, the other samples tested used a dedicated audio integrated circuit (IC) to deliver higher output power in line with the 40 mW required for a better listening experience. Both the smartphones and tablets have publicly available teardowns that identify a separate, dedicated audio codec IC. In these cases, the use of an external audio IC removes the supply voltage limitation of the 28-nm process, but comes at the expense of an additional component that consumes more system power, area, and cost.
Figure 4 - Headphone driver output power of a sample of commercially available mobile multimedia devices, tablets and smartphones - Test performed in the Synopsys Lab
In addition, there are consumer electronics that absolutely require a higher output drive level, either due to system requirements or perceived market differentiation, which makes it necessary to support the classical power requirements of 40 mW. In this case, there are two main options for accessing a 3.3 V supply:
- The first is to tap into the 3.3 V supply used for the USB interface. A vast majority of mobile multimedia SoCs will have at least one USB interface (and often several) and therefore have a 3.3 V supply. Because the supply is used for the high-speed USB interface, there may be limits to the current load that it can support without impacting the USB performance.
- The second option is to generate the 3.3 V supply with a charge pump that takes the existing 1.8 V supply and creates a negative 1.8 V supply (Figure 5). Because the line output and headphone drivers require a relatively low current, the switches required for the charge pump can be small. An additional advantage of the negative supply is that the output drivers will be centered at ground, creating a True-Ground configuration that allows the audio codec output to be directly connected to other devices without the need for large direct current (DC) blocking capacitors.
Figure 5 - True-ground output driver configuration provides an output signal centered at ground and does not require a DC blocking capacitor
In both options, the 1.8 V devices need to be properly cascaded to withstand the 3.3 V supply.