Don’t underestimate the power of an LDO


I admit it’s hard to think of an LDO as anything beyond its primary function: dropping from one voltage to another. You can pretty much find one, if not multiple, in any application that requires certain rails for operation. Very often in power design they come as an afterthought with much of the attention being devoted to DC/DC converters. As long as the LDO meets the requisite input and output voltage and load requirements, then there’s not much to it, right?

Not exactly. There might have been a time when these three requirements were the only concerns, but that time has since passed. Ever-evolving electronic applications are constantly boasting of higher performance, smaller solutions and extended battery life. LDOs are not exempt from these demands. Despite being a small component in the context of often complex systems, LDOs can make a significant impact that could help push a good product to become a great one.

Like many things in life that seem simple enough at first glance, LDOs become a lot more complex and interesting when you start looking at them a little closer. There are many facets of LDOs that I could discuss at length. For this blog, I am limiting the discussion to power consumption.

It seems as though ‘lower power’ is a sacred maxim within electronics. And why shouldn’t it be? It allows for our smartphones and electric vehicles to run longer without a recharge. It’s why our home appliances aren’t running our electric bills through the roof. And it’s also a method for reducing our impact on the planet. All of these things should be considered desirable and, in fact, are. However, the translation of these high level interests into actual electronic solutions isn’t always so easy at first. To achieve lower power consumption, an entire system with all of its components must be optimized.

LDOs are a part of that effort. However, the impact that an LDO could make in terms of overall power consumption may not be apparent. That is until you take into consideration the various states the LDO can be in: active, shutdown or no-load:

  • When active, or providing current to a load, it’s fairly simple to measure the losses from an LDO:

   Being as the LDO is a linear device, there’s no way to improve these losses unless Vin is brought closer to Vout or vice versa. The feasibility of such an action, however, is limited by the dropout of the given LDO.

 

  • When in shutdown, or disabled, there is no current being supplied to the load and there is no voltage at the output. But, although the LDO is disabled, it still draws trace amounts of current (ISHDN or shutdown current.)  The power losses in this state are determined by the shutdown current::

  

  • Lastly, when the LDO is enabled but not providing any current to the load (no-load), it is considered to be idle. It’s still regulating the desired output voltage, but there’s no current being supplied. In order to maintain this regulation, the LDO draws a small amount of current (IQ or quiescent current) to control the internal circuitry. The losses in this state are determined by the quiescent current: 

Why this all matters, of course, depends on the application itself. Most applications are not always considered ‘on’ or ‘active.’ Take your typical tablet or smartphone, for instance. We usually don’t turn these off when we’re not actively using them. Rather, to preserve battery power, they go into a lower power state until we do something to activate them.  It’s in this low power state that we can say that components like LDOs are not active but aren’t disabled either. Rather they’re in an idle state where they’re still outputting the desired voltage but not supplying a load. They are ready for the moment when they’ll need to power their given rails. Just how often devices are in these low power states varies, but duty cycles of 10% or less are not uncommon. That means that for most of the time, we can expect LDOs to be in this no-load state.

It’s in these no-load states where quiescent current plays a big role. Although the LDO is not actively supplying current to the load, it’s still regulating the output voltage. And much like when someone buys you lunch, it can never be considered free. A small amount of current it still required to keep the internal circuitry running for proper regulation. But how much? Here’s a quick example:

A TPS78233 LDO is being used to bring a 5V rail down to 3.3V. It’s also supplying 100mA to the load.

When active, the LDO is dissipating:


However, when in a no-load state this is reduced to: 

You can see that the differences between the two are pretty stark. That 170mW figure might seem large at first but keep in mind that those losses occur only 5% of the time or less with many applications. Conversely, that 2.5uW figure is what we can expect 95% of the time or more. This is low because we chose the right low IQ LDO. However, if we decided to go with another LDO with, say, an IQ of 50uA, we could expect the losses to increase by a factor of 100. Using an LDO with a higher IQ will only burn your power faster. As part of extending battery life as much as possible, it’s imperative to make sure that this quiescent current is as small as possible.

But, you may ask, why don’t I just disable the LDO when I’m not using it? After all, the shutdown current is much smaller than the quiescent current. Can’t I save more power this way? To this, I answer, that it is often not feasible. In any portable application where the LDO is powered by the battery, it’s simply not possible to disable and enable the device at convenience. It can also be unwise to disable the LDO when looking for a quick response. Turn-on times for LDOs are typically a lot slower than transient load responses.

The application doesn’t even need to be battery-powered to benefit from low IQ LDOs though. Many applications like your TV have a standby mode where LDOs go into this idle state. To meet Energy Star compliance, power dissipation during these standby modes should be minimal. Although LDOs are not the only consideration, they can be lossy if not paid adequate attention. This translates into higher electric bills that have never won any customer’s admiration.

Does this mean that we can count on LDOs to make our system an efficient one? Well, not by themselves. The efficiency of the system is still determined by the overall design. But by choosing the right low IQ LDOs, we can help minimize those losses experienced in the idle state. It’s unfortunate that they’re called ‘low IQ’ LDOs since their implementation is, well, a smart one.

Resources:

Watch a video "How to save power with low IQ LDOs"

Choose a low IQ LDO here