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Power Semiconductors

Switching Power Supplies Versus LDOs: Making the "Right" Choice

13 June 2016

For many engineers, when they need a low-power DC/DC power converter, the first reaction is "I'll just use a switching power supply; it's more efficient." Despite that quick response, literally thousands of low-dropout regulators (LDOs) are available from dozens of vendors, and billions of LDOs are sold and used every year. So the questions are: "what's the difference between them?" and "which one makes sense when?"

The answer to the second is no surprise: "it depends." There is no right answer, as each DC/DC converter topology brings unique attributes that may be essential, a deal-breaker, or somewhere in between. As in most engineering decisions, there are multiple tradeoffs when deciding between a low-power LDO (under 10 W) versus a switching IC of comparable power.

Reviewing the Topologies

At its core, the objective of the converter/regulator is to take an unregulated DC source at a nominal voltage and transform it into a tightly regulated DC output at a specified, fixed voltage. It must do this despite steady-state and transient changes in the source voltage (within limits) and changes in the output-load demands (again, within limits). A reference voltage, usually established by an internal diode, sets the desired output voltage value.

To maintain the output stability, some feedback is required between actual output voltage and the internal circuitry and reference, so the output is automatically adjusteThe LDO architecture is simple in concept: it continuously compares an internal reference to the actual output, which is driven via a pass (power) transistor, and adjusts the pass device's drive to maintain the output voltage. Image source: Texas Instruments The LDO architecture is simple in concept: it continuously compares an internal reference to the actual output, which is driven via a pass (power) transistor, and adjusts the pass device's drive to maintain the output voltage. Image source: Texas Instruments d by the regulator as the output rises or falls due to changes in the input supply or the load.

In order to understand the issues associated with switchers versus LDOs, a brief look at how they transform an unregulated or lightly regulated DC voltage value into a tightly regulated one is useful:

The LDO is an all-analog circuit, usually packaged as an easy-to-use, three-terminal device (input, output, common). The reference is connected to one input of an op amp, while the output is connected to the other input, Figure 1. The amplifier compares the two inputs and adjusts its output—which will become the LDO output—to keep that output at the reference-voltage value, via a pass-element power device within the IC. Some LDOs can be used in parallel without any additional components to increase the output current they deliver.

The switching supply (also known as switch mode power supply, SMPS, or simply "switcher") is an analog circuit but with digital aspects. First, it chops the input DC at a frequency between 30 kHz and several MHz to create an AC-like waveform, Figure 2. This waveform can now be passed through the primary side a transformer coupling path, and in the process can also be stepped up or down by the transformer turns ratio (in some cases, capacitive coupling is used, but that is less common). The secondary-side voltage, which has the same chopped, AC-like appearance, is then filtered to establish a DC output.

A feedback path from the secondary side to the primary side, usually via another transformer winding or an optical coupler, allows the switcher to adjust some switching parameter to regulate the output as compared to the internal reference voltage. In most cases, the switcher uses some variation of pulse-width modulation of the switching duty cycle to adjust and regulate the output versus the reference voltage.

Switchers come in a wide variety of often confusing internal designs, usually with cryptic descriptions such as SEPIC, short for single-ended primary-inductance converter. Each approach has different characteristics in areas such as light-load versus full-load efficiency, noise, cost, external components and

The switching supply converts a DC to AC, passes it through a step-up or step-down transformer, and then filters it to provide the DC output; a feedback loop controls the switching parameters to maintain the regulated output. Image source: Maxim Integrated The switching supply converts a DC to AC, passes it through a step-up or step-down transformer, and then filters it to provide the DC output; a feedback loop controls the switching parameters to maintain the regulated output. Image source: Maxim Integrated

size, among others. In contrast, there are relatively few basic topologies for LDOs, although vendors have developed some clever variations to highlight and optimize some attribute in a given LDO family.

Key Characteristics Define Each Approach

•Efficiency: this is the parameter that most designers talk about when they think LDO versus switcher. In general, the efficiency of an LDO is in the range of 40% to 60%, while a switcher's efficiency is typically 60% to 90% and often higher. At first glance, that should make the decision a "no brainer," as everyone wants higher efficiency and its benefits of less stress on the DC source supply and reduced heat dissipation.

But the choice is not so easy. Depending on the use duty cycle of the DC output, the LDO's overall efficiency in total energy used (the time integral of power) may actually be better than the switcher's or close to it. In those cases, the LDO may be a simpler or better solution. Some LDOs also have a quiescent mode that reduces their dissipation to near zero, which is useful for applications with low duty cycles, such as Internet of Things devices.

•Ease of use: LDOs are "drop in" regulators, small and easy to locate next to their load, which is useful because in many designs, the shorter distance between regulator and load helps ensure DC-rail integrity. Many designers like to use a switching regulator as an intermediate bus converter to pre-regulate a higher DC voltage, such as 12 V down to 5 V, then use a scattering of LDOs placed close to their load ICs to regulate the 5 V sown to the individual rails, such as 0.9 V, 1.2 V and 3.3 V. From a high-level system perspective, this often gives a good balance between performance, efficiency and cost.

•BOM complexity: LDOs usually need just one or two low-value, non-critical capacitors as their only external components, which simplifies design and minimizes total BOM cost and complexity. In contrast, a switcher IC usually needs four to six external capacitors and inductors, and these passives must have some very specific primary and secondary AC and DC characteristics such as ESR (equivalent series resistance) and DCR (DC resistance). For switchers above about 1 A, the switcher's power FETs are usually discrete devices.

Vendors of lower-power switchers have addressed the issue of external passives. They package the core switcher IC and the needed passives (and even the FETs) in a module which looks, to the user, like it needs no external components, and is easy to source and use as an LDO.

•Size and total footprint: an LDO can be quite small, but the heatsinking it may require—whether via an internal thermal pad and PC board's copper, external tab, or discrete heatsink—may give it a footprint comparable to the switcher. Again, a pre-packaged module that provides the switcher IC and passives often narrows the difference.

The noise output of a standard switching IC (LT8610) is far greater than that of a switcher optimized for ultralow noise performance (LT8614) and shows an improvement of up to 20 dB. Image source: Linear Technology Corp. The noise output of a standard switching IC (LT8610) is far greater than that of a switcher optimized for ultralow noise performance (LT8614) and shows an improvement of up to 20 dB. Image source: Linear Technology Corp. •Noise and operating frequency: switchers inherently generate noise due to chopping action, and this noise may be unacceptable, especially in low-level sensor-related applications; in contrast, LDOS only have the internal noise of their analog components, which is quite low. Some switcher IC vendors have developed special low-noise switching configurations that minimize the tradeoff, Figure 3, but these low-noise devices are not available in all voltage and power ratings.

Switching regulator output spectrum (9-kHz resolution bandwidth) using an LTC6908 with and without spread-spectrum frequency modulation enabled shows a dramatic difference in peak noise, although total noise energy under the curve remains unchanged. Image source: Linear Technology Corp. Switching regulator output spectrum (9-kHz resolution bandwidth) using an LTC6908 with and without spread-spectrum frequency modulation enabled shows a dramatic difference in peak noise, although total noise energy under the curve remains unchanged. Image source: Linear Technology Corp. One way to reduce the noise impact is to increase the switching frequency into the high kHz and even MHz range. This moves the noise spectrum to where it is more easily filtered and so less of a problem, but switchers tend to be less efficient in that upper region. Switchers at the higher frequencies are generally smaller (the passives are smaller and so is the footprint), but the technical demands on the passives are quite stringent; sourcing them may be more difficult. Another noise-reducing approach uses spread-spectrum techniques to randomly dither the switching frequency; this spreads the noise energy (it does not reduce it) but does help meet EMI regulatory mandates, Figure 4.

Step-up and step-down operation: Note that the switcher can provide voltage step-up (boost mode) as well as step-down (buck mode) of the output with respect to the input, while the LDO can only provide an output voltage that is less than the input. Some switchers are combined buck/boost units with seamless crossover between modes. This is used, for example, when DC source, which varies widely, is used (such as from a 3.6 V battery which goes down to 1.5 V), while the output voltage must be somewhere in the model of the battery range (such as 2.4 V). The switcher can fully regulate this as the battery output goes from above the desired output to below it, while an LDO cannot offer this very useful characteristic.

Making the Decision

In general, for lower-voltage outputs (under 12 V) and power levels below about 10 W, the choice between switcher and LDO has many legitimate and intriguing tradeoffs. Above those levels, the switcher is almost always the better choice due to size, cost and efficiency/dissipation issues. Some designs use a combination of switchers and LDOs to take advantage of the best features of each matched to the localized application need, thus yielding a power-supply subsystem that comes closer to getting the best of both worlds while minimizing negative consequences.

References

  1. "Understanding Low Drop Out (LDO) Regulators," Texas Instruments.

  2. "Choosing the Right Power-Supply IC for your Application," Maxim Integrated, Tutorial 737.

  3. "Reduce EMI and Improve Efficiency with Silent Switcher Designs," Linear Technology Corp., Application Note 144.

  4. "Spread Spectrum Frequency Modulation Reduces EMI," Linear Technology Corp.


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