LP2951 datasheet(19/30 Pages) MICROCHIP

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DS20005736B-page 19

LP2951

4.4

Reducing Output Noise

In reference 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 method is relatively

inefficient, as increasing the capacitor from 1µF to

220 µF only decreases the noise from 430 µV to

160 µVrms for a 100 kHz bandwidth at 5V output.

Noise can be reduced fourfold by a bypass capacitor

across R1, since it reduces the high frequency gain

from 4 to unity. Pick the resulting frequency from

Equation 4-2:

EQUATION 4-2:

or about 0.01 µF. When doing this, the output capacitor

must be increased to 3.3 µF to maintain stability. These

changes reduce the output noise from 430 µV to

100 µVrms for a100 kHz 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 4-1:

ERROR Output Timing.

FIGURE 4-2:

Adjustable Regulator.

CBYPASS

2R1 200Hz



-----------------------------------

OUTPUT

VOLTAGE

INPUT

VOLTAGE

ERROR

NOT

VALID

NOT

VALID

1.3V

4.75V

* SEE APPLICATIONS INFORMATION

*SEE APPLICATIONS

INFORMATION

NOTE: PINS 2 AND 6 ARE LEFT OPEN

LP2951

OUT

GND

ERROR

OUTPUT

SHUTDOWN

INPUT

100kΩ

OUT

1.2

30V

100

3.3μF

1.23V

REF

OUT

REF

= V

x (1 +

)

OFF

LP2951

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