Power Management in Noise-Sensitive

Time：2014-03-11　Reading：37

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 P_diss=(V_in – V_out)_*I_load , 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.