Myths surrounding PSRR specifications

The quest for lower noise feed-through on low drop out linear regulators (LDOs) has driven cell phones, tablets, and portable instrument system designers to seek ever higher power supply ripple rejection ratio (PSRR) specifications.  In recent years many vendors began advertising LDO spins depicting PSRR at higher frequencies in response to customers’ enthusiasm for PSRR beyond 1MHz.  Some introduced “wide bandwidth PSRR LDOs” while others offered ultra-low noise LDOs, acknowledging this wave of demand.  Thus, the domino effect ushered in by the system design community favoring higher and broader LDO PSRR specifications out to 10 MHz and beyond.  In my opinion, it is like the blind leading the blind, with few really knowing the truth and many falling into a pit of specification absurdity.  

Parasitic elements, of internal and external components, significantly affect LDO PSRR

 Parasitic elements, of internal and external components, significantly affect LDO PSRR

Although it is generally true that LDO PSRR figures at higher frequencies relates to improved system performance due to the increase in noise rejection bandwidth, its’ burgeoning popularity has falsely sparked our perspective of improved device performance.  The PSRR specification in the neighborhood of 10 MHz with respect to mobile-power LDO products is often confusing, distorted and unqualified.  Further, the specified parameter’s sensitive validation practice and methodology are also non-standardized.

The confusion around PSRR and noise specifications, as well as product selection, motivated my writing an article on the subject “Demystifying PSRR Specifications for LDOs” in the October 2013 issue of How2Power Today. In it, I try to clarify the LDO PSRR specification across the application’s frequency spectrum of interest. More particularly, I challenged the common myths in hopes that designers will make intelligent LDO selections. Proven solutions in the form of circuit recommendations and LDO device options are provided to help you achieve your high-performance system design goals.

"New voltage regulators are regularly introduced to the market, giving designers more and more choices. While many parts may offer real improvements in performance, the datasheets often lack critical information, and in some cases may contain misleading information. These problems are compounded by confusion on the customer's part about what certain device specifications really mean. All of these factors decrease the likelihood that an engineer will select the optimum component for their application, " says David Morrison, Editor of the How2Power Today newsletter. "The publication of Kern Wong's article will help designers to cut through the hype and confusion surrounding PSRR specifications, particularly as they apply to newer LDO regulators. Kern's article should provide designers with a better understanding of the factors that will determine regulator PSRR in their applications, how PSRR is measured, and the PSRR-related design tradeoffs that must be considered when selecting LDOs."

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