LP3871, LP3874

SNVS225H – FEBRUARY 2003 – REVISED JUNE 2015

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In applications where the load is switching at high speed, the output of the device 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 capacitors 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 must be isolated from "clean" circuits where possible, and grounded through a separate path. At

MHz frequencies, ground planes begin to look inductive and RFI/EMI can cause ground bounce across the

ground plane.

In multi-layer PCB applications, care must be taken in layout so that noisy power and ground planes do not

radiate directly into adjacent layers which carry analog power and ground.

8.2.2.5 Output Noise

Noise is specified in two ways:

•

Spot Noise (or Output Noise Density): the RMS sum of all noise sources, measured at the regulator output, at

a specific frequency (measured with a 1-Hz bandwidth). This type of noise is usually plotted on a curve as a

function of frequency.

•

Total Output Noise (or Broad-Band Noise): the RMS sum of spot noise over a specified bandwidth, usually

several decades of frequencies.

Attention must be paid to the units of measurement. Spot noise is measured in units µV/

√Hz or nV/√Hz and total

output noise is measured in µVRMS.

The primary source of noise in low-dropout regulators is the internal reference. In CMOS regulators, noise has a

low frequency component and a high frequency component, which depend strongly on the silicon area and

quiescent current. 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 increases the total supply current

(ground pin current). Using an optimized trade-off of ground pin current and die size, the LP3871 and LP3874

achieve low noise performance and low quiescent-current operation.

The total output noise specification for LP3871 and LP3874 devices is presented in Electrical Characteristics.

The output noise density at different frequencies is represented by a curve under Typical Characteristics.

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