Linear regulators provide a simple, non-switching DC/DC converter solution with low component count, small solution footprint and low output voltage noise. These attributes make them well-suited for applications that have power supply noise constraints, have limited board space and are averse to magnetics. Applications are numerous and include high and low voltage systems, loads that require milliamperes and those that require amperes, and everything in between.
Customers have specific frequency bands of interest when selecting components, depending on the application or system. The low frequency 0.1Hz to 10Hz band typically is of little interest to those using linear regulators. Instead, this frequency band is of interest to users of voltage references in instrumentation products. The next frequency band is from 10Hz to 10kHz. Customers interested in this band often have needs associated with audio equipment and audio frequencies. The next band is the 10Hz to 100kHz range. The 100kHz upper limit is somewhat arbitrary and is based on historical switching regulator frequencies. In addition, the system bandwidth for many applications is in this range. Finally, there are customers who are interested in noise up to 1MHz and beyond; this however is a small subset of the customers. At these frequencies, the noise measurement will be just slightly higher than the 100kHz number due to the fact that at these higher frequencies, the LDO operates beyond its unity-gain bandwidth point and so rather than gaining up the noise, the LDO attenuates the high frequency noise.
To better align with the customer's need to design for and understand noise performance over the entire frequency band of interest, Linear Technology specifies linear regulator output voltage noise spectral density over the 10Hz to 100kHz frequency range. Since noise is integrated over this band, the value gives an accurate representation of the noise performance an engineer can expect from the device. Some competitors however specify the output voltage noise density over a reduced frequency range to make their voltage noise spectral density appear better than it actually is.
Even when testing over a wider frequency range, Linear Technology provides noise values that often are lower than or on par with our competition. Our lower limit frequency limit is 10Hz; some competitors specify a lower limit of 100Hz up to 500Hz. One reason for this is many competitor LDOs are built on fine-line geometry CMOS or BiCMOS processes where the 1/f corner of the process is on the high side; to specify their noise from 10Hz to 100kHz would require them to give a higher number. Many Linear Technology linear regulators use bipolar processes which generally have lower 1/f noise performance.
Linear Regulator Output Voltage Noise Improvements
Unlike a switcher, a linear regulator does not have switching noise. Other than process, linear regulator output voltage noise comes from two primary sources: the reference voltage and the error amplifier.
The plot below shows the mid-frequency (10Hz to 100kHz) noise for the LT1963A 1.5A LDO linear regulator. Note that the peak-to-peak noise is approximately 2.6 divisions or 260µV. Assuming Gaussian white noise, the RMS noise is approximately 1/6 of the peak-to-peak noise, which in this case is 43µV, and this value is very close to the 40µV RMS noise data sheet value.
One way to reduce output voltage noise is to use a linear regulator that allows the internal voltage reference to be bypassed externally. Linear Technology has several linear regulators that have this feature. The following plot shows how increasing the size of the reference bypass capacitor effectively reduces output voltage noise for the LT3062.
To reduce noise contributed by the error amplifier, one can reduce the gain of the error amplifier control loop. This is accomplished by changing the feedback resistors so the ratio of R2:R1 is reduced. An example of this can be seen on the following LT3062 data sheet figure. Note that the output noise decreases as VOUT decreases.
Unfortunately, a designer cannot always change the output voltage value to reduce noise. Another noise reducing technique involves adding a feed-forward capacitor across the upper feedback resistor. This reduces the AC gain of the loop, thereby attenuating high frequency noise. For frequencies above 1MHz, the control loop gain is below one, so the noise will be attenuated rather than gained up.
One final low noise linear regulator family to consider is a current source reference linear regulator like the 1.2V to 36V input, 1.5A output LT3081. This family uses a current source and an external resistor to generate the reference voltage instead of a voltage reference resulting in a constant output voltage noise spectral density over frequency.
Whether you choose voltage reference LDOs or current source LDOs, Linear technology has a family of low output voltage noise linear regulators for your low noise applications. These devices are specified over the 10Hz to 100kHz frequency range and provide output voltage noise spectral density values as low as 20µV/√Hz.