LP2951A

© 1998 SEMTECH CORP.

150mA ADJUSTABLE

VOLTAGE REGULATOR

March 17, 1998

652 MITCHELL ROAD NEWBURY PARK CA 91320

4

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

where R1 is the upper resistor of the feedback divider

and f

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,

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

2

R

1

R

1

R

+

2

R

1

R

1

R

+

corner

R

2

1

π