LP2951 datasheet(16/26 Pages) NSC

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Application Hints (Continued)

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 is the only way noise can be reduced on the

3 lead LP2950 but 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

1, since it reduces the high frequency gain from 4 to unity.

Pick

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 100µV rms for a

100kHz 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.

Typical Applications

1A Regulator with 1.2V Dropout

00854622

300mA Regulator with 0.75V Dropout

Wide Input Voltage Range Current Limiter

00854621

00854609

*Minimum input-output voltage ranges from 40mV to 400mV, depending on

load current. Current limit is typically 160mA.

00854607

*See Application Hints

**Drive with TTL-high to shut down. Ground or leave open if shutdown

feature is not to be used.

Note: Pins 2 and 6 are left open.

FIGURE 2. Adjustable Regulator

LP2951 数据表(PDF) 16 Page - National Semiconductor (TI)