VSUBCC_SUB__AUDIO_NOISE_REDUCER

Published:2009/7/20 3:12:00 Author:Jessie | From:SeekIC

The circuit reduces noise and ripple voltage by 40 dB over the 100-Hz to 20-kHz audio range. It provides a clean source of 5-V power for driving audio circuits in portable applications such as cellular phones and multimedia notebook computers. Most linear regulators reject noise only up to 1000 Hz or so, and the bulk of a low-frequency passive filter is unwelcome in portable applications. The circuit shown accepts noisy Vcc in the 4.5- to 6-V range and produces quiet Vcc at a dc level 7 percent lower. For example, it produces 4.65 V at 1 A from a nominal 5-V source, with only 200 μA of quiescent current. The largest capacitor is 10 μF and the resistors can be 0.1 W 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 percent below Vin。 R1 and R3 form a voltage divider that provides the 7 percent attenuation, and C4 helps to form a 93-percent filter replica of Vin at the op amp's inverting input. The op amp's small input bias current (25 nA typical) allows large resistor values for R1 and R3, yet limits the maximum dc error to only 20 mV. The result is a low-pass filter with a 2-Hz corner frequency that provides 20 dB of attenuation at 20 Hz.



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