Replace Long-in-the-Tooth LDOs with These Neat and Nifty Drop-In, Switching DC/DC Converters!

We've got a few regular features over here at Hackster.io — people who we know are going to deliver something pretty tasty when we see their notifications pop up on Twitter.

One of the more recent hardware hackers to make our radar is Ketan Desai — who is a dab hand when it comes to embedded development, 3D CAD, and more.

His latest project is a take on the quintessential 3-pin LDO — aiming to replace this older, TO-220 transistor-packaged voltage regulator with something a bit more modern day, in hopes of achieving more reasonable losses due to part converter efficiencies, etc.

No no no to LDOs...

So first up, what and why. I guess we'd best take a quick look at the differences between an LDO, and an active, switching DC/DC converter.

An LDO — or low dropout regulator — is a type of linear voltage regulator.

Formed from a pass FET and an operational amplifier, the device forms a a feedback loop that regulates current flow through the FET against the voltage developed across the R1/R2 resistor divider pair on the device output.

Cheap, cheerful, and amazingly simple to drop in and solder down into your circuit, LDOs are plug-and-play for a great many applications.

They are able to down-regulate a higher input voltage, to one of a number of pre-set, fixed output voltages — in the case of fixed output devices, that simply incorporate that output resistor divider into the internal packaging, or in the case of adjustable devices, often able to regulate the output voltage from 0V to somewhere around [Vin - 0.7 V].

Like Warren G and Nate Dogg, these devices know how to regulate.

Linear vs. active

So, we understand the operation of the LDO — in principle. If we consider the implications of this mode of operation, anyone who has taken a course in amplifier demising or even done work with H-bridges, etc. will be familiar with linear and non-linear modes of operation of a FET device.

That is to say, there is a relationship between the voltage applied to the gate of the FET switch, and the amount of current that it conducts. While intrinsic to the operation of the LDO, this design feature means that at large differences of input versus output voltage, there can be significant losses incurred due to heat generated as current passes through the device while the FET is in this non-linear mode of conduction — the device has a non-negligible resistance in this state, and therefore will incur losses due to heating, thanks to the resistance of the non-saturated junction.

In contrast, an active DC/DC converter fully saturates / desaturates the switching junction of the FET many times a second, fully on-and-off, in order to maintain a regulated voltage output.

By varying the duty cycle PWM of the drive to the switching element, different voltages can be obtained on the device output.

With proper filtering, the switching noise can often be snubbed out, such that these devices are even suitable for use in load-heavy applications such as a powering FPGA power domains — usually at some several amperes.

Switchers can even, thanks to some clever application of the principles of electromagnetics, even generate an output voltage that is higher than that of the input; we call this a boost converter, in contrast to downregulation, where this supply topology is called a buck converter — but the various types of active DC/DC are an article in their own right!

The downside with switchers — aside from the aforementioned switching noise that needs to be filtered — is that they require more parts, and some careful layout in order to not compromise the operation of the circuit.

We can see the difference in design above, with the LDO of the Espressif PICO-D4 development board compared against the switcher found on the newer Raspberry Pi Pico. The required inductor and Schottky diode on the coil output are clearly visible bottom and right of the package marked '035' — the controller.

Making old like new.

Desai has decided that these design constraints needn't constrain the design of your circuits, and has opted to incorporate a Recom RPX-1.0 DC/DC switching regulator, along with the required feedback resistors and bulk capacitance into a pin-compatible, size-comparable, drop-in device, designed to replace a 3-pin TO-220 part — and while the BoM might not be breaking any records for number of lines, that doesn't subtract from the simple elegance of this design — these regulators look just beautiful!

The Recom RPX-1.0 Series has some pretty impressive specifications. With an input voltage range of up to 36 VDC, and a current capability of 1A, with output voltages programmable through a resistor divider, they incorporate not only the switching controller, but additionally, a co-packaged, shielded inductor and Schottky diode, further simplifying design integration.

Making it about as simple as possible to lay out a switching DC/DC regulator, the only downside to such an integrated package is the increasing trend of using the most appropriate, arbitrary pad shapes and geometry for the footprint of these parts!

Hell, this one gets a shout out to @CursedFootprint — deservedly so!

Regulators, mount up.

With two nearly identical designs available in the projects GitHub, Desai has provided us with drop-in replacements for both LM1117 and 78xx-based LDO designs. For those wondering what the difference might be, there's a clue in the side-by-side comparison of the device pinouts, shown below.

Not exactly one you'd want to risk getting wrong in a design, and something to be aware of in a world of parts that seem oh-too-similar!

While Desai is also known for dropping photo-realistic renders, the macro shots of his recent boards are practically indeterminable from the renders, or should that be, the renders are indeterminable from the photos... Either way, just look at these beautiful boards!

With some experimentation into encapsulation, Desai turns to a pretty reasonable starting point — a liberal application of hot-melt glue isn't too far removed from some industry standards of encapsulation, perhaps for a slightly different glue composition.

It's a perfectly acceptable encapsulation method, up until it is possibly made loose by repeated thermal expansion / contraction — during device operation. Hey, we said switchers can be more efficient, but there's no such thing as a free lunch in this game — even the best designs will still generate an amount of heat that needs managing.

Hm. Maybe some through-hole vias around the board edge could provide extra points for the hot glue to more securely encapsulate the regulators — we shall see in due time, no doubt!

Get the designs!

If these regulators have you all fired up, go get your fill of the project sources over on Desai's GitHub here.

With OSH Park more than capable to turn out these boards in either Perfect Purple, or After Dark, both shown here in this article, you can add a touch of elegance while retro-fitting these in some spots.

Upgrade your design elegance and efficiency in one fell swoop.

If you aren't already following along with Desai on Twitter, we'd suggest you do just that — his feed is a heady mixture of photo-realistic PCB renders, and polished designs that are almost pixel perfect in comparison, once assembled and photographed.

Greg Davill, watch out! Desai is coming for your titles!