2021.09.01

Points of this article

・Step response characteristics can be regarded as comprehensive characteristics due to a number of factors, and so rather than considering the value of just one component, optimization will require study of other components as well.

・The capacitance of the output capacitor C_{O} also affects the step response characteristic.

・As a practical matter, optimization depends on striking a balance between R_{ESR} and C_{O}.

In the previous article, the relationship between step responses and R_{ESR} was described, and as a point to be noted it was indicated that in addition to R_{ESR}, if the value of the output capacitor C_{O} changes, the characteristics will change. In this article, in the interest of a deeper understanding of this point, we present an example of measurement data for step response waveforms with different C_{O} values under the previous measurement conditions.

The step response waveform shown in the previous article was an example of a case in which R_{ESR} in the circuit diagram was changed between 0 Ω and 1 Ω, with the output capacitor C_{O} at 22 μF. Below are shown waveforms when R_{ESR} is changed with C_{O} = 22 μF as in the previous article, and with C_{O} = 44 μF. It should be noted that the time axis is changed from area ③ in each waveform.

Upon comparing the waveform data, we see that, even when R_{ESR} is initially the same, if C_{O} is different, the ringing states are quite different. Moreover, rather similar results are observed for C_{O} = 22 μF, R_{ESR} = 0.1 Ω and for C_{O} = 44 μF, R_{ESR} = 50 mΩ, and the results are also similar for C_{O} = 22 μF, R_{ESR} = 0.2 Ω and for C_{O} = 44 μF, R_{ESR} = 0.1 Ω. It almost seems as though if C_{O} is doubled and R_{ESR} is halved, that is, if the product of C_{O} and R_{ESR} is the same, the step response characteristic is approximately the same; but there is not a great difference among waveforms for which R_{ESR} = 0.2 Ω or greater.

From these waveforms, we can see that, as noted in the preceding article, the step response characteristic changes depending not only on R_{ESR} but also on the value of C_{O}. Moreover, we also see that the change is not linear. Further, as explained in the previous article, there are also other factors that have an effect, such as the load current value, the slew rate of the current step, and the type of linear regulator IC. Hence it is not possible to choose component values that work in all cases, and values that are confirmed to be optimal for the circuit conditions should be employed.

The sizes of MLCCs (multi-layer ceramic capacitors) differ depending on the capacitance, and low-value resistors used as R_{ESR} also differ in size depending on the resistance value; both differ in price accordingly. These matters must also be considered when optimizing the step response with an appropriate balance between C_{O} and R_{ESR}.

This article concludes the series on “Easy Stabilization/Optimization Methods for Linear Regulators”.

This is a hand book for understanding the basics of linear regulators, such as operating principles, classification, characteristics by circuit configuration, advantages and disadvantages. In addition, typical specifications of linear regulators, efficiency and thermal calculations are also explained.

This is a hand book for understanding the basics of linear regulators, such as operating principles, classification, characteristics by circuit configuration, advantages and disadvantages. In addition, typical specifications of linear regulators, efficiency and thermal calculations are also explained.