Design your power supply SMALLER

So, let’s say you’re almost done with your latest and greatest application. All the bugs have been rooted out and it’s working like a charm. It’s almost ready to hit the main stage but there’s one last thing: power. After all, we can’t expect everyone to power their application with a lab supply, right?

It’s not unusual that power comes as an afterthought. Systems are rarely, if ever, designed around power management; it’s the other way around. This sequence of events can cause panic, especially when there’s not a whole lot of space left on the PCB. The only recourse we have is to place the smallest power ICs we can find.

LDOs are a popular choice for applications that are stressed for space. This includes portable applications like fitness bands, smart watches, and other wearables. However, even line-powered applications like set-top boxes and routers don’t have infinite amounts of space and need to optimize when possible. Engineers will often opt for LDOs because of the small chip sizes.

But, should that be the only consideration?

It certainly is one of the most important. There are other factors that we need weigh like power consumption, noise characteristics, accuracy, etc., but the chip first needs to fit within the application to make it a plausible solution.

The physical dimensions of the IC begin to tell the story but are by no means comprehensive. As with any other IC, one needs to always consider external components. With an LDO, these include the resistor network, the input capacitor, and the output capacitor.

Figure 1. An example PCB with various passive elements

Let’s first talk about the resistor network. Fixed LDOs offer three main advantages over adjustable LDOs by internalizing the resistor divider:

  1. Space. Fixed LDOs forgo the need to place resistors or route them on your board. This may sound negligible but even 0402 resistors (1.0mm x 0.5mm) can take up precious real estate.
  2. Cost. Although resistors are cheap compared to silicon, they aren’t free. We must also consider placement costs. Reducing component count can save you $$$ especially when you’re looking to produce your application at volume.
  3. Accuracy. As part of the internalization of the resistors, manufacturing ensures that they are trimmed for particular tolerances. As a result, if the datasheet specifies 1% accuracy over temperature, as is the case with the TPS7A3725, that’s the tolerance that can be expected. This is not true for adjustable LDOs. Instead, we must consider both the accuracy of the reference voltage AND the tolerance of the external resistors. TI has a great app note on this very subject which you can view here.

Fixed LDOs are a great option if you’re looking to generate a common rail like 1.8V, 3.3V, 2.5V, 5V, etc. (However, a fixed version may not be available if you need an odd voltage like 3.75V.) When possible, using a fixed option should be a no-brainer. 

Input capacitors are a little bit trickier. Input capacitors help improve line transients, attenuate upstream noise, help stabilize the input rail if there are parasitics, or filter inductance upstream from the LDO. However, these concerns may or may not be applicable in a given application. Identifying the cases where they are not needed can provide an opportunity to forgo an additional passive component.

Portable applications are a good example of devices that can potentially avoid the use of an input capacitor. An example of this would be an LDO with a battery for an input rail, and low inductance between the battery and the LDO.  The battery can provide a very stable input source and minimal input inductance. This means that any load transients subjected to the output of the LDO do not result in large input voltage deviations.  Like the advantages of a fixed LDO, dropping the input capacitor can save on both space and cost. However, due diligence must be paid to ensure that the input capacitor is not necessary for the functionality of the application.

                The last and, perhaps, the most interesting external component to consider is the output capacitor. It’s traditionally been important for several reasons: it helps ripple rejection, tames load transients, and, yes, ensures output voltage stability. Better ripple rejection and improved load transient response are benefits to having an output capacitor.  However, an output capacitor has traditionally been vital to achieve stability. Without one, you can expect to have something closer to an oscillator than an LDO.

Well, that was the case until technology proved otherwise. TI now offers LDOs that are capable of providing a stable output without the need for output capacitors. TLV713 and TLV716 are good examples. They’re fully capable of operating with or without output capacitors. 

The implications drawn from this are equally impressive.

Take the TLV71333P for example: the IC itself is a 1x1mm QFN. It does not require external resistors because it’s a fixed voltage LDO. Check. If it’s used in a portable application where noise or line transients aren’t concerns, we can ditch the input capacitor. Check. It’s an LDO that’s capable of operating without an output capacitor. Of course, our load transient response and PSRR will improve with an output cap but these may not be necessary for our application. Therefore, we can design in the IC without the output cap. Check.

What we have here is a total solution size of 1x1mm.

To visualize the space savings, take a look at Figure 2. Although the additional components may seem trivial in size, land patterns and routing quickly eat up space.

Figure 2: A typical layout for an LDO power supply

This is the bottom line. There may be other LDOs out there that have a smaller chip size than 1x1mm but it’s impossible to find one with a smaller total solution size capable of sourcing 150mA. And, not only is it smaller, but it’s a more cost-effective solution.

Power may be an afterthought but it need not necessarily be a nuisance.  Cap-free LDOs are capable of addressing power needs while consuming the minimum space possible. Of course, research must be done to ensure that a Cap-Free LDO is the right fit for your application. Load transients or excessive ripple may very well dictate that it makes sense to add an output capacitor. Regardless, these LDOs offer another option in your back pocket when trying to optimize your power supply.

***Note: We will be giving 3 people a few of our EVM’s next week on Twitter. Just keep an eye out for images using the hashtag #IspywithTI and retweet for entry. Contest is now closed. Winners announced here.

As always, if you have any thoughts or questions about this topic or just want to say hello, please post a comment in the section below.