Page 204 - Troubleshooting Analog Circuits

Appendix C

Understanding and Reducing Noise
Voltage on 3-Terminal Voltage

Regulators

Errol1 H. Dietz, Senior Technician,
National Semiconductor Corporation

The usual approach to reducing noise on 3-terminal voltage regulators has been to
simply place capacitors on the output and on the adjust pin for adjustable regulators.
As it turns out, the addition of output capacitance on most voltage regulators may
reduce the noise over a broad frequency range but may increase noise within a
narrow frequency range. Since the output impedance of most 3-terminal regulators is
inductively reactive over a certain frequency range, one can surmise that adding
output capacitance to improve noise performance and transient response can also
have other effects. The examples given in this appendix will use the LM3 17
adjustable voltage regulator, but this information can be scaled and then applied to all
other kinds of 3-terminal voltage regulators.
As shown in Figure C. 1, the output impedance of the LM3 17 over the 1 kHz to
1 MHz frequency range is inductive. This has nothing to do with long wires, but is
simply another way of looking at the fact that the gain of any operational amplifier or

Output Impedance vs.
Frequency

100 1K 1 OK 1 OOK 1M
Frequency (Hz)

Figure C. I. LM3 17’s output impedance vs. frequency at I, = 500 mA.
Reprinted with permission of Electronic Design.
191