(In this three part series, we look at low noise portable power supplies using different topologies. Using a low noise linear regulator is a no-brainer for many of these applications, but in applications where a linear regulator may not be desired (due to efficiency, power dissipation or other constraints), low noise solutions can also include charge-pump based and inductor-based DC/DC converters. Advances in circuit design combined with innovative design techniques allows solutions once considered too noisy to be used as low noise alternatives.&
Low Noise Linear Regulators
For portable systems that require low output voltage noise, a linear regulator is often the "go to" choice for system power. Ideally, the linear regulator is powered directly from the battery source rather than an intermediate DC/DC converter for lowest noise performance. Depending on the linear regulator, output voltage noise as low as 0.8µVRMS is possible. For these applications, Linear Technology offers extremely low output noise devices such as the LT3042 and the LT3045, 20V input, 0.8µVRMS linear regulators with 200mA and 500mA capability, respectively.
Because of their unique current reference architecture, the LT3042 and LT3045 can easily be paralleled for even higher output current and lower noise. Below is a low noise supply using four LT3045's in parallel to obtain 3.3V with 2A of output current. Every doubling of the number of devices employed reduces the output noise by a factor of √2.
Though powering a device from a battery is ideal from the standpoint of noise, it usually is not ideal when efficiency and system run-time are factored in. More often, the system uses DC/DC conversion for more efficient power conversion. In these cases, the lowest noise solution can be implemented with a switching regulator followed by a post-regulating linear regulator that features both low output voltage noise and high power supply ripple rejection (PSRR); the high PSRR specification is needed because the linear regulator must be able to adequately reject the DC/DC converter's switching noise; and with switching regulator frequencies now ranging up to 4MHz, this is not a trivial endeavor. These high frequencies exceed the linear regulator's control loop bandwidth, and so the PSRR is not determined by the IC, but rather by the parasitics of the device. Not by coincidence the LT3042 and LT3045 also have excellent PSRR, with >75dB at frequencies beyond 2MHz and >50dB out to 10MHz. This allows them to substantially reduce switching frequency noise over a wide frequency range, while simultaneously contributing minimal additive output noise to the system.
If a customer is looking for the best noise performance, the choice is easy. But often the term "low noise" is more qualitative; the power supply noise needs to be low enough to not adversely affect the system's measurement performance and accuracy. For these applications, low noise may be defined by RMS values ranging from microvolts to millivolts.
In Part 2 of the series, we look at low noise charge pumps solutions. Charge pumps can provide cost-effective power solutions but often have noise levels in the hundred millivolt or tens or millivolts range. Innovative engineering from Linear Technology has led to charge pump solutions with output RMS noise lower than 5mV.