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LP38690, LP38692

SNVS322M – DECEMBER 2004 – REVISED DECEMBER 2015

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10.4 RFI/EMI Susceptibility

Radio frequency interference (RFI) and electromagnetic interference (EMI) can degrade any integrated circuit’s

performance because of the small dimensions of the geometries inside the device. In applications where circuit

sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must

be taken to ensure that this does not affect the device regulator.

If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes

from the output of a switching regulator), good ceramic bypass capacitors must be used at the IN pin of the

device.

If a load is connected to the device output which switches at high speed (such as a clock), the high-frequency

current pulses required by the load must be supplied by the capacitors on the device output. Since the bandwidth

of the regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above that

frequency. This means the effective output impedance of the device at frequencies above 100 kHz is determined

only by the output capacitors.

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 should 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 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.

10.5 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 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 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/

√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. 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 decreases the chance of fitting the die into a smaller package. Increasing the current drawn by the

internal reference increases the total supply current (GND pin current).