LP38692MP-ADJ datasheet(11/13 Pages) NSC

11 / 13 page

Application Hints (Continued)

load changes above that frequency. This means the effective

output impedance of the IC at frequencies above 100 kHz is

determined only by the output capacitor(s).

In applications where the load is switching at high speed, the

output of the IC may need RF isolation from the load. It is

recommended that some inductance be placed between the

output capacitor and the load, and good RF bypass capaci-

tors be placed directly across the load.

PCB layout is also critical in high noise environments, since

RFI/EMI is easily radiated directly into PC traces. Noisy

circuitry should be isolated from "clean" circuits where pos-

sible, and grounded through a separate path. At MHz fre-

quencies, ground planes begin to look inductive and RFI/

EMI can cause ground bounce across the ground plane. In

multi-layer PCB applications, care should be taken in layout

so that noisy power and ground planes do not radiate directly

into adjacent layers which carry analog power and ground.

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).

LP38692MP-ADJ Datasheet(PDF) 11 Page - National Semiconductor (TI)