A 700+-mW class D design with direct battery hookup in a 90-nm process

A "battery connect" compatible class D (switching) amplifier which is fully integrated in a 90-nm digital CMOS process is presented. The integration of the amplifier requires no additional masks, processing, or cost. This paper includes a brief description of the circuit techniques that enable direct battery (2.7-5.4 V) connection and allow support >6.7 V_P2P (700 mW into 8 Ω) output swing from a 4.2-V supply using devices that operate solely with low gate voltages. The achieved SNR over an audio (20 Hz to 20 kHz) bandwidth >98.5 dB and the total harmonic distortion (THD) is better than 0.03% at 500 mW. Efficiency is greater than 75% above 375 mW. The power supply rejection ratio, which is a crucial parameter in modules connected directly to the battery, is measured at 70 dB at 217 Hz. The area of the switching amplifier is <0.44 mm², where the power devices occupy approximately 20% of the total.