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LP2951 数据表(PDF) 19 Page - Microchip Technology |
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LP2951 数据表(HTML) 19 Page - Microchip Technology |
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19 / 30 page ![]() 2017 - 2022 Microchip Technology Inc. and its subsidiaries 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 1 2R1 200Hz ----------------------------------- = OUTPUT VOLTAGE INPUT VOLTAGE ERROR NOT VALID NOT VALID 5V 1.3V 4.75V * * * SEE APPLICATIONS INFORMATION *SEE APPLICATIONS INFORMATION NOTE: PINS 2 AND 6 ARE LEFT OPEN LP2951 IN OUT GND FB ERROR 5 3 4 1 8 SD V V ERROR OUTPUT SHUTDOWN INPUT IN +V 100kΩ OUT V 1.2 30V 1 R 100 pF 3.3μF 2 R 1.23V REF V 7 OUT V REF = V x (1 + ) 1 R 2 R OFF ON LP2951 |
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