OUTPUT NOISE

Noise is specified in two ways: Spot Noise or Output Noise

Density is the RMS sum of all noise sources, measured at

the regulator output, at a specific frequency (measured with

a 1Hz bandwidth). This type of noise is usually plotted on a

curve as a function of frequency. Total Output Noise or

Broad-Band Noise is the RMS sum of spot noise over a

specified bandwidth, usually several decades of frequencies.

Attention should be paid to the units of measurement. Spot

noise is measured in units µV/root-Hz or nV/root-Hz and total

output noise is measured in µV(rms)

The primary source of noise in low-dropout regulators is the

internal reference. Noise can be reduced in two ways: by

increasing the transistor area or by increasing the current

drawn by the internal reference. Increasing the area will

decrease the chance of fitting the die into a smaller package.

Increasing the current drawn by the internal reference in-

creases the total supply current (ground pin current).

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