LP2951A
POWER MANAGEMENT
Applications Information (Cont.)
This can be done by bypassing the upper resistor in the
feedback divider with a small capacitor to provide a more
direct path for AC feedback. The size of this capacitor
can be calculated from the formula:
C
BYPASS
=
2
π
R f
1 corner
where R1 is the upper resistor of the feedback divider
and f
corner
is the frequency above which the increased AC
feedback is to become active. Because the gain of the
error amplifier in the LP2951(A) begins to roll off at about
300Hz, this is generally an optimum choice for corner
frequency.
The reduction of the output noise will be proportional to
the ratio of the two resistors in the feedback divider:
R1
R1
+
R 2
1
and will increase at a rate of 20 dB per decade at
frequencies above the corner frequency chosen, up to
the frequency where the error amplifiers gain has rolled
off to 1 (~100kHz). In order to maintain regulator
stability when using a noise-reducing bypass capacitor, it
will also be necessary to increase the size of the output
filter capacitor by the ratio:
R1
R1
+
R 2
ã
2001 Semtech Corp.
6
www.semtech.com
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