Application Note 83
March 2000
Performance Verification of Low Noise,
Low Dropout Regulators
Silence of the Amps
Jim Williams and Todd Owen, Linear Technology Corporation
Introduction
In an increasing trend, telecommunications, networking,
audio and instrumentation require low noise power supplies. In particular, there is interest in low noise, low
dropout linear regulators (LDO). These components power
noise-sensitive circuitry, circuitry that contains noisesensitive elements or both. Additionally, to conserve power,
particularly in battery driven apparatus such as cellular
telephones, the regulators must operate with low input-tooutput voltages.1 Devices presently becoming available
meet these requirements (see separate section, “A Family
of 20µVRMS Noise, Low Dropout Regulators”).
Noise and Noise Testing
Establishing and specifying LDO dropout performance is
relatively easy to do. Verifying that a regulator meets
dropout specification is similarly straightforward. Accomplishing the same missions for noise and noise testing is
considerably more involved. The noise bandwidth of interest must be called out, along with operating conditions.
Operating conditions can include regulator input and
output voltage, load, assorted discrete components, etc.
Low noise performance is effected by numerous subtleties; changes in operating conditions can cause unwelcome surprises.2 Because of this, LDO noise must be …