Time to Finally Give Up That LDO for a Tiny Switching Regulator?
I have to ‘fess up: despite all the attention given to switching regulators, I still have a “soft spot” in my heart (and maybe also in my head) for low dropout regulators (LDOs). Among my reasons for this are that LDOs do one thing, they do it well, they do it easily, and they don’t give headaches or surprises. Plus, the first power regulation devices I used were LDOs, and they were good to me as I tried to build real circuits. Even today when I need a rail at a few volts and around an amp of current, my first thoughts are of the basic, three-terminal LDO rather than a switching regulator (aka: a switcher).
It’s not news that LDOs generally have lower efficiency than switchers, but at lower current levels – about an amp – the difference is small and may not be critical in the specific design. Above several amps, that difference is usually large enough that the LDO is not a good choice unless you need a super-low-noise rail, but even there some switchers are actually quite good and have very low noise. Further, while it is possible to parallel some types of LDO with low ohm “bleeder” resistors between them to equalize current draw, this can get complicated in terms of layout, footprint, BOM, and quantifying performance.
Still, LDOs are vital components in the engineer’s BOM bag. They are so easy to use that many designers “sprinkle” them like popcorn around their printed circuit board wherever they need good regulation close to the IC or subcircuit. Doing so minimizes power rail IR drop and noise pickup, along with other ills that appear when a power source is located further from its load.
But the times they are a-changin’
Nonetheless, I am starting to wonder if even those basic design-in opportunities for LDOs may be coming to an end. Encapsulated lower current switching regulators now have attributes that are comparable to those of LDOs with respect to simplicity, size, external passives needed (or not), and overall ease of design-in. If you take a “black box” view of these encapsulated switchers versus LDOs in a circuit, it would not be easy to tell the difference between a 1 amp LDO and a multiamp encapsulated switcher.
Also, while an LDO can only regulate down (buck), there are buck, boost, and even buck/boost switchers with seamless transition between the two modes. That’s an important capability, as many circuits operate from a single Li-ion cell and need buck/boost mode to regulate the battery from fully charged down to somewhat discharged.
Show me the (
Consider the LTM8074 (LTM8074EY#PBF), a silent switcher µModule regulator from Analog Devices (Figure 1). It has a wide input voltage range (3.2 to 40 volts); wide output voltage range (0.8 to 12 volts); delivers 1.2 amps continuous (at 24 volts input and 5 volts output); and reaches 1.75 amps peak output current at 3.3 volts output. Yet what really caught my attention is its small size. As a BGA device it measures just 4 millimeters (mm) × 4 mm × 1.82 mm high.
Figure 1: The LTM8074 is an easy-to-use switching regulator with a tiny footprint for both the module and its overall circuit. (Image source: Analog Devices)
All it needs to function are two capacitors (1 microfarad (µF) and 10 µF) and two resistors, which makes it as easy to drop in and use as a comparable LDO, but with more power in a smaller footprint. The inductor, which we normally associate with a switcher, is embedded within the package, so from the designer’s perspective, it’s not part of the BOM and not taking up additional space. Plus, its EMI emissions are quite low, rivaling those of the LDO (Figure 2).
Figure 2: CISPR22 Class B emissions for the LTM8074 on its DC2753A demo board, with VOUT = 3.3 volts, 1.2 amp load, and no EMI filter. (Image source: Analog Devices)
Another source of these tiny LDO-like switching modules is Texas Instruments with its LMZ10501 (LMZ10501SILR), a 1 amp NanoModule with an input voltage range of 2.7 to 5.5 volts, and output voltage range of 0.6 to 3.6 volts (Figure 3).
Figure 3: The LMZ10501 from Texas Instruments is housed in a 3.00 mm × 2.60 mm package, including its inductor. Externally, it needs just three ceramic capacitors and two resistors to function. (Image source: Texas Instruments)
The LMZ20501 needs just three ceramic capacitors and two resistors, and also integrates the inductor in its 8-pin microSiP package, which is just 3.00 mm × 2.60 mm (Figure 4). One thing is sure: it doesn’t look like a standard IC.
Figure 4: The inductor of the Texas Instruments LMZ10501 is an integral part of the NanoModule physical design, saving on space and BOM. (Image source: Texas Instruments)
These tiny switchers are even very competitive with LDOs at the lower current ranges where the latter has long had the size and ease-of-use advantage. For example, the Maxim Integrated MAXM15462 is a high efficiency, synchronous step-down DC-DC module with integrated controller, MOSFETs, compensation components, and inductor that operates over a wide input voltage range (Figure 5). It accepts an input of 4.5 to 42 volts and delivers up to 300 milliamperes (mA) output current over a programmable output voltage from 0.9 to 5 volts. It needs three capacitors (two at 1 µF and one at 10 µF) and two resistors alongside its diminutive 2.6 mm × 3 mm × 1.5 mm uSLIC™ package.
Figure 5: The MAXM15462 300 mA output switching regulator from Maxim Integrated is competitive in size with even low current LDOs. (Image source: Maxim Integrated)
The advantages of these tiny switchers go beyond being smaller and more efficient than somewhat comparable LDOs. Depending on the device chosen, they include nice (and sometimes necessary) features such as soft start to reduce input inrush current, a “power good” output pin, and programmable undervoltage lockout (UVLO) threshold.
Finally moving on, maybe?
Sometimes you have to let go of your past preferences to benefit from technical advances. I have seen engineers specify twenty-year-old op amps in new product designs primarily because they are familiar and comfortable with their idiosyncrasies. While that is a sensible tactic in some ways, it may deprive the final design of the ability to do more, do it cheaper, or do it smaller.
For power regulators such as LDOs, they may still be used by the millions and billions annually for new design-ins. However, lower power switching regulators offer so much more in terms of enhanced performance, good behavior, ease of use, and efficiency, and do so in a small package. It would almost be professional negligence to not at least consider them despite a first impulse to put that familiar LDO on the BOM.
So, while you’re doing that, I’ll go check on the carburetor in my car; I think the needle valve needs cleaning and the float is misadjusted.