AN899| Application Note
Maxim/Dallas > App Notes > AMPLIFIER AND COMPARATOR CIRCUITS FILTER CIRCUITS (ANALOG) POWER-SUPPLY CIRCUITS
AUDIO CIRCUITS
Keywords: low noise, audio power, power supply, audio filter, noise rejection
Jul 01, 2001
APPLICATION NOTE 899
Low Noise Audio Power Supply Circuit
Audio application note describes a simple, compact scheme that provides a low noise, low ripple power source for use in audio amplifiers, microphones, headphone amplifiers, CODECs and other audio circuits. The circuit shown in the figure below reduces noise and ripple voltage by 40dB over the 100Hz to 20kHz audio range. It provides a clean source of 5V power for driving audio circuits in portable applications such as cellular phones and multimedia notebook computers. Most linear regulators reject noise only up to 1000Hz or so, and the bulk of a low-frequency passive filter is unwelcome in portable applications.
Figure 1. This compact circuit actively compensates for power-supply ripple and noise, providing 40dB of attenuation in the 100Hz to 20kHz audio band. The circuit shown accepts noisy VCC in the range 4.5V to 6V and produces quiet VCC at a DC level 7% lower. For example, it produces 4.65V at 1A from a nominal 5V source, with only 200uA of quiescent current. The physical layout is very small: one SOT23 transistor, one uMAX (shrink SO-8) op amp, and a few passive components. The largest capacitor is 10uF, and the resistors can be 0.1W or surface-mount 0805 size. When operating, the circuit acts as a wide-bandwidth buffered voltage follower (not a regulator) whose DC output level is 7% below that of VIN. R1 and R3 form a voltage divider that provides the 7% attenuation, and C4
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