Micrel, Inc.
Reducing Output Noise
In some applications it may be advantageous to reduce
the AC noise present at the output. One method is to
reduce the regulator bandwidth by increasing the size of
the output capacitor. This is the only method by which
noise can be reduced on the 3 lead MIC2950 and is
relatively inefficient, as increasing the capacitor from 1µF
to 220µF only decreases the noise from 430µV to 160µV
rms for a 100kHz bandwidth at 5V output.
Noise can be reduced fourfold by a bypass capacitor
across R
1
, since it reduces the high frequency gain from 4
to unity. Pick:
C
BYPASS
≅
1
2
π
R
1
•
200Hz
MIC2950/2951
or about 0.01µF. When doing this, the output capacitor
must be increased to 3.3µF to maintain stability. These
changes reduce the output noise from 430µV to
100µVrms for a100kHz bandwidth at 5V output. With the
bypass capacitor added, noise no longer scales with
output voltage so that improvements are more dramatic at
higher output voltages.
Figure 1. ERROR Output Timing
Figure 2. Adjustable Regulator
February 2010
12
M9999-021610
MICREL [ MICREL SEMICONDUCTOR ]
