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Aeluma Inc of Goleta, CA, USA (which specializes in high-performance, scalable technologies for AI, mobile and consumer, defense & aerospace, and quantum) has executed an agreement with Sumitomo Chemical Advanced Technologies of Phoenix, AZ, USA (SCAT, a subsidiairy of Japan’s Sumitomo Chemical Co Ltd) to accelerate development and production of photonics wafers for the AI datacom market. The agreement will initially leverage existing metal-organic chemical vapor deposition (MOCVD) capabilities at SCAT’s manufacturing facility, and provides a path for increasing capacity in the future...

At European Microwave Week (EuMW 2026) at EXCEL, London, UK (4–9 October), Altum RF of Eindhoven, The Netherlands (which designs RF, microwave and millimeter-wave semiconductors) is showcasing its featured products and technical expertise in booth #L80 during the exhibition on 6–8 October...

Beyond low-power design and beyond multi-threshold-voltage libraries, clock and power gating, and voltage-frequency scaling lies the strange land of ultra-low-power systems. Here, power comes from tiny, semi-permanent batteries or energy scavenging, and it comes in microwatts or nanowatts, not milliwatts.

For system-on-chip (SoC) designers, this is the alien realm of ultra-low voltage (ULV): supply voltages near or below transistor threshold voltages. In this realm, many things are unfamiliar to conventional SoC designers. Foundational IP must be different. Familiar-looking tool flows may hide important differences in tools and skills. New challenges appear—threats that could sink a project.

ULV design explored

All differences in ULV design begin with the definition: ULV circuits use a VDD near or below the threshold voltage of the process’s MOSFETs. This leads to several important effects.

First, of course, is energy savings, the whole reason for ULV design. ULV works because instantaneous power is quadratic in supply voltage. So, the instantaneous power consumed when a circuit is active, and hence the energy required to complete a task, can be significantly lower at lower VDD.

Usually, designers will talk not about power consumption, but about the energy required to complete an operation, typically in nanoJoules. This is a preferred metric because, in ultra-low-power systems, ULV SoCs are usually quiescent for long periods, wake to perform specific tasks, and then reenter their trance. This activity pattern makes peak and average power figures poor indicators of energy consumption and, therefore, battery life or drain on energy scavengers.

But this energy benefit comes with challenges. Near threshold voltage, a MOSFET gate exerts only weak control over channel current. Leakage can be high, and the difference between ION and IOFF is small.

Maintaining the separation—the values of 1 and 0—throughout each net’s stages and across clock trees becomes a fundamental undertaking for the design team. The separation is already low, and other factors conspire against it. Interconnect parasitics, noise, and process variations can prevent circuits from working reliably. So, ULV design requires special measures.

Foundational IP

One of the first challenges designers will face is that conventional low-power libraries will not work for most ULV designs. Cells in standard libraries may be optimized for performance or packing density, but not for operation at ultra-low voltages. Some cells in the library are likely to fail or become unreliable simply because they cannot maintain the distinction between 1 and 0, even under ideal conditions. Add in parasitics, noise, and variations, and the library cells cannot cope. New, custom cell designs are needed.

In addition, some cells, such as high-fan-in gates, may work correctly but impose so much delay that they become useless. Such cells need to be eliminated from libraries. Other cells—ULV-level shifters, in particular—must be designed for the specific voltage range the design requires.

SRAM is another problem. The standard 6T SRAM cell usually cannot perform reliable reads and writes at ULV. Adding custom assist circuitry can help, as can simply enlarging the bit cell. The dual-rail design will work better (Figure 1). And some designers avoid the issue by making the SRAM a high-voltage island surrounded by level shifters, sacrificing energy efficiency for simplicity.

Figure 1 The dual-rail SRAM design highlights array on HV, periphery on ULV, and level shifter on the WL path. Source: Faraday Technology

Fine-grained characterization

Cells that work at ULV are necessary, but far from sufficient. Traditional delay modeling at a few process corners is hopelessly inadequate to achieve timing closure in a ULV design. In this voltage region, delay can vary exponentially with voltage, often in a non-Gaussian distribution. A traditional approach using a few corners would result in hopelessly large design margins.

We have found that fine-grained characterization of the foundational libraries, using voltage increments no larger than 50 mV and often finer, is a minimum requirement. This data must be captured in an advanced format such as Liberty Variation Format with Moments (LVFM). This allows statistical timing tools—using knowledge of the intended supply voltage range of the finished system—to produce design margins that are not overly pessimistic.

So, use knowledge of the intended supply voltage range of the finished system to produce design margins that are not overly pessimistic (Figure 2).

Figure 2 This data must be captured in an advanced format such as LVFM. This lets statistical timing tools produce design margins that are not overly pessimistic. Source: Faraday Technology

The bottom line for design teams is that ULV SoC design requires custom ULV foundational libraries, characterized over the expected operating range with very fine granularity.

Tool flow

Fortunately, ULV SoC design can use standard modern synthesis and place-and-route tools compatible with the LVFM format (Figure 3). But the standard flow still requires additional ULV expertise. For example, the synthesis tool must support the custom ULV libraries.

Figure 3 Standard Tools must possess critical ULV expertise. Source: Faraday Technology

All tools must recognize that, at ULV, timing will be extremely sensitive to routing paths due to both parasitic impedance and noise. Static timing tools must be variation-aware and able to consume LVFM data to exploit the libraries’ fine-grained characterization and minimize design margins.

Special attention must also be given to noise: crosstalk, supply transients, and even substrate noise. This may come down to the designers’ skill in floorplanning and in guiding placement and routing tools to anticipate and avoid noise sources.

The concluding steps

The ULV SoC design requires additional work to reach signoff. But then, both silicon bring-up and manufacturing test also have special requirements at ULV. Test equipment needs high accuracy at low voltages and, of course, a low-noise floor. So, load boards must be designed with extra care about leakage and parasitics.

During both silicon evaluation and manufacturing test, pay special attention to the power-on reset sequence across the entire operating voltage range, as it’s particularly vulnerable in ULV designs. Also, carefully test those ULV-level shifters.

Finally, the OSAT organization or internal manufacturing test facility must use the advanced binning strategy. Because process variations—even across a wafer—are so significant at ULV, it’s necessary to perform accurate voltage-speed binning (Figure 4). This separates chips that meet the requirements across the intended operating voltage range from those that work only at higher voltages.

Figure 4 Accurate voltage binning separates chips that meet requirements across the intended operating range from those that work only at higher voltages. Source: Faraday Technology

Why ULV expertise matters

ULV design facilitates SoCs that can operate almost indefinitely on tiny batteries or scavenged power. Custom libraries, fine-grained characterization, advanced design tools, and ULV-experienced designers make these projects achievable. Moreover, deep foundry and OSAT relationships make volume production realistic.

Some large design groups have these resources, expertise, and relationships in-house and can confidently undertake ULV SoC designs independently. But in many cases, an organization will want to deploy its assets across the overall low-energy system rather than to the specialized needs of the SoC design. In these cases, a traditional SoC design team can partner with an organization with extensive ULV experience.

The right ULV expert must be ideally positioned to partner with a traditional SoC design team entering ULV land. The land is indeed strange and strewn with risks. But with the right partner, it’s the land through which the path to technical and commercial success lies.

C.H. Chien has dedicated 33 years to IC design; his industry experience includes IP development and IC design flow. He also has 10 years of experience managing overseas R&D teams. Chien worked as a director at MediaTek and GlobalFoundries before joining Faraday Technology Corp.

Related Content

• Low power design is here to stay
• Scaling the Power Wall: Low-Power SOC Design
• Designing with ultra-low voltage MOSFET arrays
• The why, where and what of low-power SoC design
• Low Power Design Techniques, Design Methodology, and Tools

The post A design path to success exists for ultra-low-voltage SoCs appeared first on EDN.

Taking various considerations into account, the theoretical advantage of the spread bit PWM can be challenging to translate into practice.

A recent Design Idea (Reference 1) addressed the implementation of digital-to-analog converters using both common “clustered-bit” and less common “spread-bit” pulse-width modulation (PWM) techniques. Within a repetitive clustered PWM cycle, all of the ones appear in succession, as do all of the zeroes. In the spread PWM, the ones and zeroes are distributed more or less evenly within a cycle.

Wow the engineering world with your unique design: Design Ideas Submission Guide

The clustered PWM tends to concentrate energy at lower frequencies, while the spread PWM moves energy towards the higher frequencies, allowing for faster-settling analog ripple-suppression filters. Given PWMs with individually-customized filters of the same complexities, clocks and cycle periods, the latter has clear advantages. Such is the case for hardware-based implementations such as FPGAs. However, while most microcontrollers can advance a clustered-bit PWM with the speed of their CPU clock, they cannot do so for a spread PWM. Effective clocks for these PWMs are slower because a microcontroller must implement them with multiple CPU instructions.

Clocks are further slowed by any code required to support features other than the PWM. Note that all code blocks must execute in invariant periods of time to prevent jitter from degrading the accuracy of the output. So including interrupts in the code is problematic. Another concern is the error which accrues to unequal rise and fall times, leading to mismatched logic one and zero durations. In a clustered cycle, there is only one rising and one falling transition, leading to an error considerably less than one bit. Unfortunately, the multiple transitions in a spread cycle make this type of PWM more subject to this type of error, which will be worst at a 50% duty cycle.

This all being said, the prior article investigated a claim that a microprocessor-implemented spread-bit PWM with a single resistor-capacitor pair filter outperformed a clustered-bit PWM with a three component-pair filter. This claim was shown to be true only for PWMs of 16 bits or more, and then only for a microcontroller which supported no features other than the PWM. But it’s unfair to tie one hand behind the back of the spread PWM, limiting it to first-order filters. This Design Idea compares spread and clustered alternatives driving individually customized filters of the same third order three-R/C-pair complexities.

The rules of the game

The b-bit PWMs discussed here can be thought of as having repetitive sequences of length N = 2b, where 1/N is the PWM resolution. N is an integer, but b needn’t be one for the clustered type. For the spread PWM, however, b typically is an integer for reasons of coding efficiency. See a discussion of why and of one way to code a spread-bit PWM in Reference 2.

Analog filters are employed to suppress PWM waveform-induced ripple. They exhibit a settling time in response to a duty cycle (DC) change. Some particular DC change will yield the maximum settling time TS to within VST of some fully settled voltage, and some duty cycle DC will produce the absolute maximum steady state error Vrip due to the ripple.

I’ve required that Vrip = ½ · 1/N and that VST = 1/N. Optimized-for-settling-time clustered-bit PWM filters are covered in Reference 3 and are easily designed. For the design of optimized spread-bit 3rd order filters (Figure 1), I’ll briefly describe the math involved at the end of his Design Idea.

Figure 1 These first (left) and third order (right) low-pass analog filter structures are buffered with op amps because their inputs employ resistors of high values. This is done to limit the errors imposed by the unequal resistances (Reference 4) of the logic high and low outputs of ICs such as the 74AC04 which drive the filter inputs.

Comparisons

The graph in Figure 2 is based on the data in Table 1. We see the expected superiority in settling time of the spread approach in comparison to the clustered alternative for PWMs with the same b (and therefore N) values and a clock frequency of 1 MHz. This relationship also holds for different frequencies as long as the clocks are identical.

Figure 2 In this comparison of b-bit PWMs clocked at 1 MHz, spread and clustered PWMs are investigated with individually optimized third order filters compliant with the information in the “Rules of the game” section. Additionally, the spread with a first order ( one R, one C ) filter (see Figure 1) is shown for reference. With third order filters, the improvement in settling time of the spread over that of the clustered PWM is apparent and grows with b and N.

Number of bits	Settling time (mS), spread, first order filter	Settling time (mS), spread, third order filter	Settling time (mS), clustered, third order filter	Spread/clustered improvement, third order filters
2	5.00E-03	3.09E-03	3.32E-03	1.08
3	1.80E-02	7.31E-03	1.01E-02	1.39
4	5.20E-02	1.65E-02	2.93E-02	1.77
5	1.34E-01	3.89E-02	8.32E-02	2.14
6	3.29E-01	9.29E-02	2.34E-01	2.52
7	7.78E-01	2.00E-01	6.53E-01	3.27
8	1.80E+00	4.23E-01	1.81E+00	4.28
9	3.74E+00	8.89E-01	4.98E+00	5.60
10	8.42E+00	1.87E+00	1.36E+01	7.28
11	1.87E+01	3.94E+00	3.71E+01	9.40
12	4.13E+01	8.32E+00	1.00E+02	12.07
13	9.03E+01	1.87E+01	2.71E+02	14.47
14	1.85E+02	4.02E+01	7.28E+02	18.11
15	4.00E+02	8.40E+01	1.95E+03	23.21
16	8.61E+02	1.74E+02	5.21E+03	29.88
17	1.84E+03	3.61E+02	1.39E+04	38.36
18	3.94E+03	7.50E+02	3.68E+04	49.09
19	8.01E+03	1.56E+03	9.75E+04	62.46
20	1.70E+04	3.43E+03	2.58E+05	75.14
21	3.59E+04	7.15E+03	6.80E+05	95.12
22	7.58E+04	1.52E+04	1.79E+06	117.43
23	1.59E+05	3.11E+04	4.71E+06	151.57
24	3.23E+05	6.33E+04	1.24E+07	195.18

Table 1 The data in this table forms the basis of the graphs shown in Figure 2.

A comparison based on identical clock rates would be appropriate if the two PWM alternatives were implemented in hardware, such as with an FPGA. But a microcontroller implementation of the spread, unlike that of the clustered, requires code execution. Comparatively, a microcontroller’s spread clock frequency is lower than that of the clustered (which requires no code to support an initialized, constant duty cycle PWM), especially if the microcontroller is performing tasks in addition to the spread PWM implementation.

Using the data

Defining a PWM whose full-scale output is “1” starts with specifying its 1 / N = 2-b resolution. With a 1 MHz clock, a PWM’s b bits correspond to points on each of the settling time curves in Figure 2. These are the maximum settling times TS, 1MHz to an error of 1/N. For a desired settling time of Tdes ≠ TS, 1MHz, the clock frequency must be changed. Defining a frequency scaling factor FSF equal to TS, 1MHz / Tdes, the PWM clock PWMclk becomes FSF · 1MHz.

If the clustered PWM has been selected, the spreadsheet in Reference 5 can be used to design the filter. Its parameter peak-peak Ripple, Fraction Frac of Full Scale should be set to 1/N, and the parameter PWM frequency, Hz to PWMclk/N. This spreadsheet executes the job with the press of a button, and it also runs an LTspice simulation of the filter’s worst-case transient response and ripple with the press of another button.

But if the spread PWM with a third order filter is selected instead, things are more complex (the first order spread PWM was discussed in Reference 6). Table 2 supplies three pairs of resistor and capacitor values to be used to implement a third order filter. These values will need to be modified to meet certain requirements. Recalling FSF, the actual component values are the table’s capacitor values divided by FSF · ZSF and the table’s resistor values multiplied by ZSF. ZSF is a positive, unit-less impedance scale factor which can be selected to meet requirements’ needs.

Number of bits	N ( 1/resolution)	r1, ohms (c1 = 10nF)	r2, ohms (c1 = 10nF)	r3, ohms (c1 = 1nF)
2	4	163.1000885	71.68292585	427.1087404
3	8	343.433862	271.9382005	828.4900535
4	16	716.8099067	681.854738	1497.328378
5	32	1444.093716	1398.000657	2966.654472
6	64	2816.731051	2186.864174	6876.068023
7	128	5619.092479	4103.426809	14167.24162
8	256	11229.51857	7684.085243	29082.22862
9	512	22464.62272	15027.67815	58621.97919
10	1024	45100.99781	27402.70118	120390.8368
11	2048	90360.13359	53513.94391	242063.9576
12	4096	182059.6186	99434.32312	490140.5768
13	8192	368094.4499	184124.8578	986470.2649
14	16384	736189.1063	368249.819	1972941.083
15	32768	1466466.446	755969.6418	3940496.058
16	65536	3005457.305	1320906.538	7870364.119
17	131072	5976310.761	2716759.439	15771899.07
18	262144	11952685.17	5433547.811	31543966.12
19	524288	24043631.67	10567240.53	62962842.83
20	1048576	47816245.75	21736693.78	126190392.7
21	2097152	95636270.97	43475105.66	252390759.7
22	4194304	191158658.6	86898441.34	504480973.5
23	8388608	384771591.2	169108145.2	1007597919
24	16777216	741230589.8	427594752.1	1990907707

Table 2 This table’s prototypical component values can be as-needed modified to implement a third order, spread-bit PWM filter (see text and Figure 1).

One of the requirements is that r1 should be large enough to swamp out the errors due to the difference rdiff between the logic high and logic low resistances of the digital ICs driving r1. A little math shows that this means that r1 > rdiff · (N -1) / 2 for an error of less than 1/ (2·N) = Vrip.

Don’t drive the circuit directly from a microcontroller, whose outputs generally won’t swing adequately close to the rails because of voltage drops across the IC’s bonding wires that accrue from the device’s supply currents. Consider buffering the output with a 74AC04. Five 74AC04 inverters connected in parallel and powered from 3V or more have a maximum rdiff of 9 ohms and a far lower typical value.

Another requirement is that the sum of r1, r2 and r3 should not be so large as to incur voltage drops in excess of Vrip due to op amp input current. All resistors should be metal film. As for the capacitors, ceramic NPO / C0G and polyester film types are sufficiently stable with temperature and voltage. Capacitor values should be more than 330pF so as to swamp out PCB and op amp input capacitances.

The freedom to choose a ZSF value might not be sufficient to meet the listed requirements. The addition of an op amp buffer stage (see Figure 3) between the 74AC04 and the filter transfers the filter’s r1 > rdiff · (N – 1) / 2 requirement to the buffer stage where it can be more easily met; the filter is now being driven from a low dynamic, constant- impedance op amp output.

Another way to ease requirement satisfaction is to sum the outputs of a “least significant” and a “most significant” PWM, also shown in Figure 3. This relaxes the r1 > rdiff · (N – 1) / 2 requirement to r1 > rdiff · (sqrt(N) – 1) / 2 and speeds settling time by a factor of sqrt(N).

Figure 3 In this circuit, ra, ca and U1 provide a means to eliminate the spread filter’s r1 > rdiff · (N – 1) / 2 requirement. Optionally, the entire circuit allows the addition of contributions from the outputs of separate PWMs weighted by factors of 1/257 and 256/257. It is recommended that the MS PWM be buffered by five 74AC04 inverters in parallel and the LS PWM by a single inverter.

A spread third order filter math overview

This section is supplied for completeness and can be skipped if desired.

The following is the transfer function of a third order low-pass filter, all of whose poles are constrained to have identical real parts so that they contribute more or less equally to the overall settling time.

H(s) = .5 · ω03 / Q / [ ( s + .5·ω0/Q ) ·  ( s2 + s·ω0/Q + ω02) ]

A worst-case settling time occurs when the filter input transitions at t = 0 from DC = 1 to DC = 0. The time domain transient response is calculated using the following equation.

ytr(t) = e-a·t · [ ( cos(a·β·t) – 4·Q2 ) / β2 –  sin(a·β·t) / β ]

where a = .5·ω0/Q  and  β = sqrt(4·Q2 – 1)

The biggest ripple occurs, perhaps surprisingly, with the input of a single one (or zero) in a cycle.

x(t) = 1

for (k · N) · T    ≤   t   ≤    (k · N + 1) · T, k = 0, 1, 2…, and T = 1/PWMcllk

x(t) = 0

otherwise.

This is well approximated by a truncated Fourier series.

x(t) = 1/N + (2/N) · Σ sinc ( π·k/N ) · cos ( 2·π·k·(t – T) / (N·T) )

where k = 0, 1, 2… 15.

The resulting steady state output ySS(t) is obtained by multiplying the amplitude and time-delaying each harmonic in x(t) by amounts determined by H( 2·π·k·j / (N·T) ). The total output y(t) is the sum of ytr(t) and ySS(t).

To design the filter, the maximum absolute values of ySS(t) – 1/N are constrained to be less than .5/N. They are examined for Q values between .5 and 2, and solved in each case for ω0. That Q, ω0 value pair is selected which corresponds to the smallest settling time of y(t) – 1/N to an absolute error less than 1/N.

The numerical values of H(s) now being determined, its analytic form expressed in terms of r1, r2, r3, c1, c2, and c3 is examined and solved through numerical techniques to obtain the resistor values (for the capacitor values shown) that appear in Table 1.

Conclusion

There’s no question that the spread PWM offers a substantially shorter settling time than the clustered alternative when same b-bit PWMs are driven by identical clocks and succeeded by topologically similar but individually optimized filters. The ratio of improvement increases with the number of PWM bits. If a PWM is to be implemented in hardware such as an FPGA, the spread PWM is probably the better choice.

The caveat comes when a microcontroller is doing the implementation. A clustered PWM can run at the CPU clock rate. The spread PWM effective clock is slower, with this type requiring the execution of X instruction cycles, including that needed to implement an infinite loop. So, the spread PWM clock is the CPU clock divided by X, and its filter’s settling time will be increased by that factor X.

Clock period and settling time are further increased if functions other than the PWM are to be implemented. And care must be taken to ensure that PWM code execution occurs at a consistent rate; a jittery clock will degrade accuracy. Accordingly, implementing interrupts is problematic.

Another concern is the error which accrues to unequal rise and fall times, leading to unequal durations of logic ones and zeroes. In a clustered cycle, there is one rising and one falling transition only, leading to an error considerably less than one bit. Unfortunately, the multiple transitions in a spread cycle make this type of PWM more subject to this type of error, which will be worst at a 50% duty cycle.

When considering PWMs with larger numbers of bits (16, for instance), minimized settling times favor the approach of using a pair of resistors to sum the contributions of two independent 8-bit PWMs. This PWM pair’s filter can settle 256 times faster than a single 16-bit PWM’s filter can do. Most microcontrollers support a pair of independent clustered PWMs running off the same counter, an approach which could be duplicated in an FPGA.

But spread PWMs, whether in hardware or on a microcontroller, demand entirely independent means of support. When these considerations are taken into account, it can be seen that the theoretical advantage of the spread bit PWM can be challenging to translate into practice.

References:

1. Implementing a DAC: The battle of the PWMs
2. Ibid
3. Custom design PWM filters easily
4. Ibid, see the SN74AC04-induced errors section.
5. Ibid
6. Implementing a DAC: The Battle of the PWMs

Christopher Paul has worked in various engineering positions in the communications industry for over 40 years.

Related Content

• Implementing a DAC: The battle of the PWMs
• Custom design PWM filters easily
• A nice, simple, and reasonably accurate PWM-driven 16-bit DAC
• Parsing PWM (DAC) performance: Part 1—Mitigating errors, Part 2—Rail-to-rail outputs, Part 3—PWM analog filters, and Part 4 – Groups of inhomogeneous duty cycles

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