Ken Yang, Maxim Integrated Products Inc, Sunnyvale, CA
Many low-dropout-voltage regulators see service in electronic systems, but relatively few are designed for lownoise operation. (For example, Maxim¡¯s MAX8887 achieves a noise voltage of approximately 42µV rms. However, certain applications, such as ultra-low-noise instrumentation oscillators, demand even lower levels of power-supply noise. To reach this level, the circuit in Figure 1 combines low-noise components and extra filtering to achieve an output noise floor of only 6 nV/¡îHz .
Voltage reference IC1, a Maxim MAX6126, features low output noise. Lowpass filter R1-C1 further reduces this noise by attenuating noise frequencies above IC1¡¯s 0.16Hz cutoff frequency. The filtered reference voltage drives the inverting terminal of error amplifier IC2, a Maxim MAX4475, which regulates the output voltage by means of Q1, a P-channel power FET source follower. Feedback resistors R2 and R3 determine the output voltage as follows: R2=R3[(VOUT/2.048V)-1]. The simplified noise-analysis diagram illustrates the components¡¯ noise contributions (Figure 2). Lowpass filter R1-C1 attenuates high-frequency noise on the voltage reference¡¯s output. The op amp¡¯s noise current, 0.5fA/¡îHz, is negligible with respect to its voltage noise, 4.5nV/¡îHz . The reference-noise source adds to the op-amp voltage noise because they effectively connect in series. The MOSFET¡¯s noise contribution appears at Q1¡¯s input. The noise at IC2¡¯s inverting terminal equals the noise at its noninverting terminal:
where VN_OUT represents the lowdropout circuit¡¯s output noise,VN_REF represents the reference noise, VN_OPAMP represents the op amp¡¯s input-referred noise, and H(f)represents the R1-C1 lowpass filter¡¯s transfer function. If a noise frequency of interest falls well below the filter¡¯s cutoff frequency, the reference noise is negligible, and the low-dropout circuit¡¯s output noise comprises only the op amp¡¯s noise multiplied by the closed-loop gain. The feedback loop suppresses VN_FET, the MOSFET¡¯s noise contribution, which therefore can¡¯t contribute to the output noise. For frequencies within the loop¡¯s bandwidth, the low-dropout circuit also rejects ripple and noise voltages that the power supply introduces.
Figure 3 shows a plot of noise density versus frequency for the circuit of Figure 1, which exhibits a noise floor of about 6nV/¡îHz at 1kHz. For comparison, the plot shows the noise-measurement instrument¡¯s noise f loor and a typical low-dropout circuit¡¯s much higher noise density-for example, 500nV/¡îHz at 1kHz for the MAX8887 low-noise, low-dropout circuit.
