Power DesignLine Blog
Don't diss the LDO
Bill Schweber
6/23/2010 10:49 PM EDT
Yet some designers scoff at them as being too inefficient, unable to provide more than about an amp or two, and just not sophisticated enough for today's advanced supply needs. Like the late comic Rodney Dangerfield, too often "they can't get any respect around here." This is despite the fact that the LDO is easy to use, does one thing, does it well if you exercise modest care, and doesn’t try to step beyond its basic function.
Let's take the last presumed weakness first. Although the LDO looks simple from the outside̫after all, it's a three-terminal device, and usually requires just an external capacitor or two in use–internally they are quite complex, with temperature compensation, thermal and load protection, stability compensation, and all sorts of other goodies. (Some of the LDOs released in the past year add additional clever features such as the ability to parallel them for higher output current, without trivial additional circuitry.)
As for efficiency: yes, they are less efficient than a well-designed switching DC/DC supply. But sometimes, that extra efficiency is not worth the cost in components, PCB real estate, and other operating factors.
And yes, if you need tens of amps, an LDO is not the way to go. But many applications need far less current, especially in multirail application where there is a primary supply while a few other special supply-rail voltages are needed.
The LDO's other well-know virtue is its low noise. Again, many application can tolerate tens of millivolts of supply noise, but there are quite a few–especially related to transducer and I/O–-that cannot. An LDO may be the nest choice in terms of total system performance, EMI/RFI, and other factors.
I'm not saying that the LDO is the right choice, period, end of story. But rather than assume it is not, check the specs, look at the real costs of its inefficiency versus noise versus filtering versus footprint versus all the other factors that go into supply-subsystem execution. Maybe the LDO is the better choice within certain limits, maybe not. I do know that vendors sell a lot of them each year, and are announcing new ones all the time, so there must be something virtuous about them, right? ♦
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kendallcp
7/4/2010 8:42 AM EDT
When you pick a regulator vendor, consider the support they provide for simulation. Many people agonize about finding the best simulation models for their op-amps, but few people actually model what's happening on their supply rails, if they are using linear regulators. Not all three-terminal regulators are the same, either in the noise that they spit out or in the way they respond to variations in current demand!
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green_is_now
7/9/2010 6:35 PM EDT
It can be just as efficient as a multistage SMPS if a 1st stage smps is creating a voltage just above the LDO limit. If small currents then 2nd dc2dc switching and conduction losses elimination may cover LDO losses for small loads. If 1st stage is a output needed for another rail even better.
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Duane Benson
7/14/2010 7:35 PM EDT
There are still plenty of applications that are just fine with a three-terminal LDO. Even plenty that do well with a non-LDO 78XX series regulator. Sure there are tight tolerance applications that need something newer and more efficient, but it's hard to find something easier to implement than a little three-terminal linear regulator. It may not have the cachet, but it's quick to design in and, in the right place, will just work solidly.
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