LP2951A Datasheet, PDF (4/5 Pages) Semtech Corporation – 150mA ADJUSTABLE VOLTAGE REGULATOR

150mA ADJUSTABLE
VOLTAGE REGULATOR
March 17, 1998
PRINCIPLES OF OPERATION (LP2951 and LP2951A) (cont.)
Reducing Output Noise
In ultra-quiet systems, or when the LP2951(A) is being
used as a reference, it may be desirable to perform
additional output filtering to reduce noise. While this
can be done simply using larger capacitors on the out-
put, that solution tends to be bulky and expensive, and
eventually, with huge capacitors (>1,000µF) may cause
instability in the regulator. Generally, it is more cost-
effective to let the regulator regulate output noise
away. This can be done by bypassing the upper resis-
tor 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 =
1
2 πR ₁f_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 300 Hz, 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 + R2
and will increase 20 dB per decade at frequencies
above the corner frequency chosen, up to the fre-
quency where the error amplifier’s 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+ R2
LP2951A
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