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Get control of buck regulator high step-down ratios
Here's what you need to know about a recent development called Emulated Current Mode Control that can improve the ability to operate with very high step down ratios while maintaining good noise immunity.
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By
Craig Varga, National Semiconductor
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Page 1 of 4
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Power Management DesignLine
(01/30/2007 5:00 AM EST)
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Many older monolithic buck type switching regulators have incorporated on-chip feedback loop compensation. While this makes for a very simple design procedure, it typically does not allow for optimization of loop dynamics. The result is the need to choose power path components to suit the feedback requirements, a generally less than optimal arrangement. For instance, a given regulator may require that the user select inductor and output capacitor values within a given range in order that the LC double pole frequency occurs where the feedback compensator has a double zero located. While you may get a stable loop, you may not have nearly the ideal values for the power path requirements. The solution is to depend on external, user selected loop compensation. That, however, can become a bit of a problem for the novice user. Feedback loops are considered to be hard to compensate! One innovation that was originally introduced in the 80s is current mode control. That control scheme reduces the order of the output filter by one to a first order system, drastically simplifying the loop compensation task. However, current mode control is not quite the panacea originally hoped for. Noise sensitivity is a major problem.
A recently developed version of current mode control known as Emulated Current Mode Control (ECM) drastically improves the ability to operate with very high step down ratios while maintaining good noise immunity. With input voltage ratings up to 75V, it's possible to have significant design margins in many high input voltage applications, while still being able to produce the kinds of output voltages used by today's digital circuitry. Loop compensation becomes a relatively simple exercise and available software can make the design almost trivial from the user's perspective.
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