Electronic – How To Properly (and efficiently) Regulate Battery Inputs

Switch mode power supplies (SMPS) can be difficult to make by hand, without a custom PCB. You have several issues. Even on a 2 layer custom PCB this can be difficult but is many times harder when you are hacking things together on a perf board or worse. The reason for this is that the stability of a SMPS is highly dependent on the inductance and "routing" of the signals. If a signal is not routed nicely then it can pick up too much noise and the SMPS becomes unstable. This alone is a topic worthy of several books.

Things can be much more difficult when using tools like National Semiconductors WEBENCH. While WEBENCH is useful for getting you "approximately in the right neighborhood", it falls far short of getting you a robust and reliable design. WEBENCH, and tools like it, often don't check for some basic things that become very important when designing SMPS. There is the easy stuff, like are you violating the power specs on resistors or voltage specs on diodes? Or the intermediate stuff like are you violating the duty cycle limits for the SMPS chip? Or the hard stuff like, is the tool properly simulating the MOSFET/Diode/Cap you're using or does it allow you to add/change the snubber circuit? I have seen WEBENCH and tools like it fail to properly take these things into account-- resulting in a circuit that just barely works or even fails catastrophically. WEBENCH will give you a place to start, but it is no replacement for reading and understanding the datasheet and doing the proper homework on the design. You can't just take a WEBENCH design and run with it as if it were a fully tested and robust design.

My advice: If you're building your own custom PCB then make your own SMPS. Add into your schedule a good amount of time to tinker and tweak the SMPS circuit. If you are going with a one-off build then use something like the UBEC.

Advice #2: Linear Tech's LTSpice simulator is an order of magnitude more useful than WEBENCH. It is also good for non-SMPS analog circuits. You will still need to do your homework, but LTSpice will get you much farther along. The downside is that Linear Tech's chips are a little more expensive than National Semi's and others. But, in my opinion, unless you're building thousands of these things then the better software tools is worth the extra cost.

Battery powered projects (particularly those with periodic events spaced quite a bit apart) usually benefit from using a linear regulator.

Looking at your requirements (LiPo 4.2V to Vo + dropout voltage) a linear regulator will be (on average 3.7V battery, regulated output 3.0V) 81% efficient which is close to the SMPS solution anyway.

A switch-mode device uses a lot of power (relatively speaking) just to power itself up; it is overall more efficient where a relatively large load current is being used.

What you need to look at carefully is \$I_q\$ for the regulator; this is the amount of current it draws simply to power the internal circuitry.

The regulator with the lowest \$I_q\$ that can handle the input and output voltages and required current is usually the best, but temperature rise is something to be considered - the small devices cannot dissipate much heat (there simply is insufficient physical packaging to permit it).

As most modern microcontrollers and many interfaces will run quite happily at 3.0V or even 2.7V, you should consider using that as the regulated voltage. If you really want Vo = 3.3V for Vin 3.0V <> 4.2V you would need a buck-boost or some such arrangement. I would stick with a simple LDO regulator and drop the regulated voltage.

Note that regulators designed for large load currents (not the usual case for battery operated projects) will typically have a relatively high \$I_q\$

The most astounding part I have ever seen for \$I_q\$ : TPS783xx

Compare that with a buck SMPS that has a relatively low \$I_q\$ (for an SMPS device).

There are other considerations, but the no-load power dissipation of the regulator needs to be kept to a minimum.

Incidentally, a Li+ / LiPo can be safely discharged (in my experience) to about 2.7V