Capacitor Guide

Creation of ESR control-type low-ESL ceramic capacitors

Introduction

Amid the demand for ever-more ground-breaking and products, electronic devices and equipment are continuing to evolve. The central pillar of this evolution are the high-performance LSI chips that are incorporated in the great variety of digital devices and equipment, ranging from communication devices Multilayer ceramic capacitors support these high-performance LSI chips, and large numbers of them are positioned as decoupling devices around the LSI chips in order to absorb load fluctuations during operation and suppress noise.

Until recently, the general thinking behind the basics of noise countermeasures could be expressed as “reducing the impedance in the lines involved.” For this reason, multilayer ceramic capacitors with a low-resistance component known as ESR (equivalent series resistance) have been considered to be devices that are ideally suited as countermeasures against noise, and large numbers of them were incorporated. On the other hand, cases where their use was not applicable arose with some kinds of circuits. Since their ESR is too low, impedance peaks form, which are referred to as anti-resonance, and the decoupling performance at the frequencies where the peaks form deteriorates as a result.
In order to solve this problem, Murata took it upon itself to create the LLR series, a lineup of ESR control-type low-ESL ceramic capacitors with increased ESR values. The “LLR series” is introduced in this article.

Features of LLR series

The feature of the “LLR series” is the deliberately high ESR value, and four levels of resistance from 100mΩ to 1000mΩ are supported. (Table 1)

Ideally, only the ESC (equivalent series capacitance) that indicates the capacitance would represent the electrical characteristics of capacitors. However, losses in the dielectric that is the material used and losses in the internal electrodes give rise to coil components known as ESL (equivalent series inductance) and resistance components known as ESR. This is why the impedance characteristics take on a valley shape as shown in Fig. 1. Multilayer ceramic capacitors are characterized by a low ESR (high Q value), but this may give rise to anti-resonance in capacitors with different capacitance levels or with the capacitance components of LSI packages. To safeguard against this, the “LLR series” incorporates the feature of increased ESR values (reduced Q values) for the capacitors and the provision of impedance characteristics in the shape of a bathtub curve. Furthermore, as a way of minimizing the load fluctuations of the LSI chips, it would be ideal to minimize the ESL values of the capacitors and raise the charging/discharging characteristics. However, the problem with ESR control that changes the internal electrode patterns of the multilayer ceramic capacitors that have been generally available in the past was that the ESL values were also raised at the same time. Now, by using a proprietary design, Murata has succeeded in controlling the ESR values while keeping the ESL values at their original level. This achieves a capacitor performance that functions in a wide band.

Table 1 LLR series lineup
Fig. 1 Capacitor impedance curve

How LLR capacitors are used

(1) Minimization of anti-resonance

Anti-resonance may result if the ESR value of a capacitor is reduced excessively. Cases of anti-resonance occurring in LSI packages are described below. If a normal circuit configuration is to be used for this, various converters are placed between the high-performance LSI chips and the DC-DC converters that serve as the power supplies of the chips (see Fig. 2). Among the on-package capacitors indicated as C01 (capacitors mounted on the LSI package board), the large-capacitance ceramic capacitors, which serve as decoupling devices for the high-frequency region, and the aluminum electrolytic capacitor with a capacitance exceeding 100µF indicated as C02 and C03 are mounted for decoupling in the low- and mid-frequency regions. In the case of this kind of circuit configuration, one place where the effects of anti-resonance are liable to be felt is between the LSI package and on-package capacitors (C01). Anti-resonance is caused by the LC resonance produced by the extremely low-capacitance components of the LSI package and ESL component of the capacitors (C01) mounted on the package.
One method to minimize this anti-resonance is to increase the static capacitance of the LSI package, a second method is to reduce the ESL values of the capacitors, and a third method is to increase the ESR values of the capacitors. Since those aspects determined by the components selected at the equipment design stage are uppermost in the case of methods 1 and 2, increasing the ESR value in method 3 turns out to be the simplest way, and this is the method that is adopted by the “LLR series.”
Figure 3 shows the simulation data yielded when all the capacitors (C01) mounted on the LSI package have been replaced by the “LLR series.” This shows that as the resistance value rises, the anti-resonance peak falls. In the simulation carried out here, the peak value has been reduced to 1/50, and noise reduction in the target frequency range can be expected.

Fig. 2 Example of circuit configuration on an LSI chip-mounted board
Fig. 3 Circuit impedance simulation data

(2) Prevention of DC-DC converter oscillation

At the output side of a step-down DC-DC converter, a capacitor is used in order to minimize the fluctuations in the output voltage (ripple voltage fluctuations). In addition, an LC low-pass filter is formed at the output side in order to smooth the voltage that has been output from the DC-DC converter (see Fig. 4). If a low-ESR multilayer ceramic capacitor is used in the place concerned, oscillation arises. One way of preventing this oscillation is to increase the ESR value of the capacitor to provide a phase margin: This is now achieved by using an LLR series capacitor, thus overcoming the oscillation caused by the low ESR that was the weak point of multilayer ceramic capacitors. As a result, it is now believed possible to expand the range of possibilities at the circuit design stage.

Fig. 4 Step-down DC-DC converter

Conclusion

The characteristics previously demanded of multilayer ceramic capacitors were simply a high capacitance and low ESR for keeping the power lines and I/O lines at low impedance levels. However, in response to the increasingly sophisticated functions of devices and equipment, a new concept has emerged: ESR control. By flexibly supporting new needs such as these, Murata believes that the value of multilayer ceramic capacitors can be increased as passive components that are easier than ever to use.
 By aptly ascertaining the performance required from its LLR series, Murata intends to expand its product lineups.


* For details on Murata’s multilayer ceramic capacitors, go to:
▼LLR series top page 
▼Capacitor web site 

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