LP38693 Datasheet, PDF (13/15 Pages) National Semiconductor (TI) – 500mA Low Dropout CMOS Linear Regulators Stable with Ceramic Output Capacitors

Application Hints (Continued)
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).
13
www.national.com