# How to Solve 8 of 10 Design Issues

Steve Sandler finds that eight out of 10 design issues are due to a single problem that is easy to solve.

The vast majority of design issues are directly or indirectly attributable to control loop stability. The most common offenders:

- Linear regulators
- Voltage references
- Op-amps

The control loop stability of these devices can propagate through an entire system. Symptoms include:

- EMI
- Increased circuit noise
- Clock jitter

These are all simple circuits, so why is this a common issue?

**We are not clear on the definition of stability**

In the high-reliability world, including satellite systems, where I do most of my troubleshooting, stability means a minimum phase margin of 30 degrees and a gain margin of 6dB at the end of life, including all component variations and environmental factors. There are guidelines that define the stability margin limits, and one of our tasks is to ensure that margin.

This requirement is not well aligned with the information given out by manufacturers. They generally do not provide quantifiable metrics for the stability of their devices. In many cases, the devices will not meet a 30-degree phase margin even typically when coupled with low-ESR capacitors. The definition of stability in the semiconductor world seems to be whether the circuit has a stability margin that is greater than zero.

**Component manufacturers drive us to it through recommendations**

In some cases, manufacturers provide recommendations. One example I often use is the REF02 voltage reference. The manufacturer's datasheet *highly recommends* the addition of a 0.1uF output capacitor on the voltage reference.

The highly recommended output capacitor is shown in two different schematics in the datasheet.

**Inability to measure directly**

The results of following this recommendation can be seen in the ringing produced by a small signal step load.

The results also show up in a VNA measurement of the reference output impedance.

One reason engineers get into so much trouble with the control loop stability of these types of devices is that, in many cases, the access to the control loop is not available, so it isn't possible to measure stability directly using traditional Bode plot techniques.

Even though measuring the phase margin directly is not possible, a non-invasive stability method shows the phase margin of the recommended circuit to be about 12 degrees in the typical case.

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Niso 8/27/2013 2:53:29 PM

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Steve.Picotest 8/27/2013 3:00:29 PM

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bcarso 8/29/2013 3:35:16 PM

Sometimes the bast way to proceed is local fast shunt regulation, although it entails substantially higher quiescent current. But it can render local current fluctuations small enough to allow grounds to be shared, which is helpful at very high frequencies.

Brad

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Steve.Picotest 8/29/2013 3:43:04 PM

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elucches 8/28/2013 2:36:39 PM

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David Ashton 8/29/2013 4:10:20 PM

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Steve.Picotest 8/29/2013 7:01:01 PM

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felixk1 9/2/2013 5:41:30 AM

I was taught (and I have read in many books on control system theory), that the definition of a fully-stable system is one in which one or more bounded inputs to the control system (i.e. The transfer function), results in one or more bounded outputs (for SISO and MIMO systems). Put another way if a finite input results in a finite output; put yet another way (in terms of the impulse response of said system), if the impulse response of the system tends to zero after "some" time.

You can have marginally stable systems where the impulse response tends to some finite non-zero value but never goes to zero. Hence a system is unstable (in terms of the impulse response), if the impulse response reaches infinity after a certain time. [Ref: "Electronic Devices and Amplifier Circuits with MATLAB computing", Second Edition by Steven T. Karris, Orchad Publications 2008 [ISBN-13: 978-1-934404-14-0, ISB-10: 1-934494-14-4].

Sidenote: The impulse response in the digital domain is simply a vector (of necessary length (n)), that contains a 1 followed by n-1 zeros.

With respect to the ringing due to the output capacitor, this is entirely predictable I thought. Without the capacitor (in combination with the output impedance), you have all frequencies passing through (due to the input of Dirac-delta function equivalents at the start and end of the square wave pulse), resulting in an approximation of a theoretical impulse (we are in the analog (or analogue) world now :)). With the capacitor much of the high-frequency components have been filtered (passed-through to ground) but some low-pass signals have passed through. In any case the system (or more commonly subsystem) still appears fully-stable in theory as the output voltage is tending to zero; the problem comes when this is inputted into the next stage. This is all undergraduate stuff so I must be missing something!?!?

With regards to not being able to measure something in one's design; if you can't measure it you can't test it and if you can't test it then you may be in trouble. One would have to go back and think how do I know the existence of something that can't be measured? In general it is because some [acceptable number of] mathematician/physicist says it has to be there in theory. One may have to look at what the unmeasurable object affects, with a view to being able to measure the effects and work backwards with theory (math).

Regards,

William Knox

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Steve.Picotest 9/2/2013 3:05:43 PM

You are mostly, but not entirely correct. The stability definition you presented is reasonable, the question is the metric for quantifying the margin of stability, which in most cases is the closest proximity of the gain vector to the singular unstable point (1,0).

The ringing is not quite preditcable, as it has much to do with the degree and Q of the open loop, which is often unknown. The issue is particularly troublesome in circuits where the loop is not accessible for measurement. This is often the case with class D monolithic audio amps, voltage references and fixed voltage regulators to name a few. So for example if you look at a LDO datasheet and it says stable for capacitors from 1uF to 100uF what exactly does this mean? Will the circuit ring? If so, how much and do we care?

I'm currently writing a new book for McGraw-Hill on high fidelity measurement and it will address some of these issues and how they propagate through systems. The book should be submitted to the publisher in early 2014.

The point of this article is that we should be concerned about even a little bit of ringing (especially in a high performance system) might rwreak havoc on the performance.