By Landa Culbertson, Mouser Electronics
It's best to take a careful, systematic approach when designing the power distribution system in noise-sensitive applications, like broadcasting, to ensure ideal performance. Not only does one need to consider all the usual parameters in a power management design, like input voltage, output voltage, and current, but also how to handle power supply noise to eliminate effects on the system's video and audio signal integrity.
Choose Linear Regulators with Low-Noise and High PSRR Characteristics
Whenever possible, choose linear regulators to directly power the signal conditioning and signal processing components. All voltage regulators introduce noise into the system, but the linear regulator inherently produces less noise compared to its DC/DC conversion cousin, the switching regulator. Linear regulators can also offer good power supply ripple rejection (PSRR), a measure also known as audio-susceptibility. A high PSRR specification at the switching frequency of any switch mode power supply fed into the linear regulator input will help attenuate that switching noise so that it does not get introduced to the signal chain and cause interference issues. This technique is called post-regulation.
Keep in mind that PSRR falls off eventually to 0-dB at higher frequencies. Most switch mode power supplies operate in the 100kHz to 2MHz range, so this is not an issue as it relates to the linear regulator's ability to attenuate the fundamental switching frequency. However, additional filtering may be required to dampen noise at high frequencies.
The task of choosing a linear regulator can be overwhelming considering the plethora of devices in the market, amid a myriad of manufacturers. Texas Instruments alone, after their merger last year with National Semiconductor, now offers over 1300 linear regulators. Mercifully, some companies provide online parametric search tools that allow one to narrow selections based on output noise as well as PSRR. Another way to find a good low noise linear regulator is through an online search using keywords “ultra-low noise” or “high PSRR”, plus “LDO” or “linear regulator”.
Table 1 highlights some linear regulators with exceptional noise specifications.
Unfortunately, it is not practical to use only linear regulators in every situation, as they are not very efficient at power conversion, and thus can get hot. Calculate the power dissipation in the application using Pdiss =(Vin – Vout )* Iload , and compare the wattage against the thermal rating of the package. If it looks like there are going to be heat issues, opt for more thermally enhanced packages like QFN, or it may be time to consider a switching converter instead. Generally speaking, this is the case if the load circuit continuously draws much above 1A. However, there are 1.5-A, 2-A, and 3-A rated linear regulators commonly available.
Switching DC/DC Converters Offer High Efficiency
Selecting a switching DC/DC converter is even more daunting a task than selecting a linear regulator. There are multiple topologies and trade-offs to consider, not to mention the confusion of devices available on the market from a multitude of suppliers. In any case, when faced with a noise sensitive application, implement a switching DC/DC converter with low output voltage ripple, say, less than 30 mVpp. Follow up the switcher with a linear regulator for post-regulation if a more quiet power supply is desired.
One feature to look for in a switching regulator intended for a broadcasting application is high switching frequency. Besides the benefits of smaller package, and better transient response, high switching frequency avoids the frequency bands where noise can be disruptive, including the AM broadcast band.
Another desirable feature is switching frequency synchronization. In a system with multiple converters, similar, but not matching switching frequencies can produce a disturbance called beat frequency phenomenon. The ability to synchronize the switching frequencies of the regulators prevents the formation of beat frequencies. In addition, it helps keep the EMI generated within the system to a predictable set of frequencies.
Also consider that today's designs are implementing FPGAs and DSPs that are pushing the limits of technology, with some unintended consequences. A given device may need power sequencing, soft-start, or power-good indication to operate properly. Modern power management solutions have these capabilities integrated.
Table 2 lists switching dc/dc step-down converters with desirable features for broadcasting applications.
Integration
If board space is at a premium, consider the modern power management solutions that integrate the switching DC/DC converter and post-regulation linear regulator into a single, small package. One such example is the TPS54120 from TI. Introduced only a little over a year ago in 2012, this device combines a high-efficiency switching DC/DC converter with a low-noise, high PSRR low-dropout linear regulator to support 1-A, noise-sensitive applications. The TPS54120 also includes switching frequency synchronization, soft-start, and power-good indication in a space-saving 3.5-mm x 5.5-mm thermally-enhanced QFN package.
Conclusion
In summary, power supply noise reduction is a complex subject. Part of a good power distribution system design is in choosing the right voltage regulators, the considerations of which have been mentioned. However, keep in mind that there are other important factors that are beyond the scope of this article, including proper bypassing, decoupling, damping, PCB layout, and much more. Thankfully, the internet is a great resource for additional information on these topics.
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