DC-DC｜Application

Important Points in the Design of a Power Supply Using a Floating Type Linear RegulatorInput/output capacitor design and ripple prevention for linear regulator ICs

2025.01.28

table of contents

This article describes the basic characteristics and points to note when designing linear regulator ICs. In addition to the relationship between input/output voltage difference, transient response, and ripple rejection ratio, the article details the essentials of output and input capacitor selection and placement. It also addresses the prevention of ripple voltage increase in floating type and operating characteristics of overcurrent protection, and provides practical advice for building stable power supply circuits.

Relationship of the Input/Output Voltage Difference of a Linear Regulator IC to Transient Response and Ripple Rejection Ratio

The specifications and characteristic graphs discussed here pertain to the BA1117, but in essence are common to all linear regulator ICs. A linear regulator IC cannot operate if the stipulated dropout voltage (voltage difference between input and output) is not secured. The minimum value of the input voltage is the voltage obtained by adding to the output voltage the dropout voltage at the load current to be used, which can be read off from the “dropout voltage vs. output current” graph on the data sheet. In the case of the BA1117 which is used as an example, if the output current is set to 0.5 A, we can read from the graph that the dropout voltage is about 1.1 V (typ value, at Ta=25°C). Hence if the output voltage is 5 V, the minimum input voltage is 6.1 V.

Dropout voltage vs. output current

At this minimum input/output voltage difference, the device can operate in DC fashion, but other control functions cannot be fully executed. For example, when there is load fluctuation, a large current cannot be supplied from input to output in a short time under conditions of a small input/output voltage difference. In other words, load response is delayed. The delay in responsiveness also appears as a lowering of the ripple rejection ratio (PSRR) characteristic. This can be seen by examining the input and output voltage values under stipulated conditions of these specifications, and related graphs; one example is shown below. We see that for a 5 V output, the characteristic when the 1.2 V dropout voltage is added to obtain a 6.2 V input, and the characteristic when 2 V or more is added to result in an input voltage of 7 V or higher, are dramatically different.

Effect of input/output voltage difference on PSRR characteristic

When emphasis is placed on efficiency, efforts will be made to minimize the input/output voltage difference, but it is possible that the expected characteristic cannot be obtained. In this example, input voltages for which the necessary performance is obtained with respect to load response and PSRR are studied, in order to find a compromise between efficiency and the different device characteristics.

Points to Note Regarding Linear Regulator Output Capacitors

An output capacitor is necessary to maintain the stability of the regulator. Below are points to note pertaining to output capacitors, using the BA1117 as an example. Nearly all the points do not apply specifically to the BA1117, but are valid for linear regulators in general.

Points to Note Regarding Linear Regulator Output Capacitors

Positioning on a mounting board

In order to stabilize the feedback loop, the output capacitor COUT is connected as close as possible to the IC VOUT pin, and similarly is connected close to ground. This is true of linear regulators other than the BA1117 as well.

Capacitance

The capacitance of an output capacitor for a BA1117 should be at least 22 µF. If the capacitance is too small, there is the possibility of oscillation. A larger capacitance improves the feedback loop stability and the transient response characteristic of the output. The minimum value for the capacitance differs depending on the linear regulator IC; please consult the data sheet for the IC to be used.

There is no maximum value for the capacitance, but a number of issues should be studied. By increasing the capacitance, the charging time of the output capacitor when the power supply is on (the time until the preset value is reached) is longer, and the discharge time when power is off (roughly the time until ground potential is reached) is also longer. It should be confirmed that these longer times do not pose problems for the power supply requirements of the device or circuits being powered.

In addition, it is conceivable that when the input power supply is turned off the output and input voltages may be inverted (so that the output voltage is higher than the input voltage), resulting in damage caused by large current reverse flow within the IC. Such situations can be avoided by adding a reverse current bypass diode (see figure above). The diode used must conduct at lower voltages than the voltage at which the output transistor of the IC conducts due to input/output voltage inversion, and so a diode with an effective forward voltage VF must be selected. Some ICs are provided with such a function; the data sheets for individual devices must be studied.

ESR (Equivalent Series Resistance)

The ESR of an output capacitor for a BA1117 should be 5 Ω or lower. If the ESR is high, the feedback loop becomes unstable and oscillation occurs. The ESR value is based on the IC by itself and the results of evaluations using a resistive load; in actual use, the ESR value will vary depending on the wiring impedance of the board, the input-side power supply impedance, and the load impedance. It is always necessary to mount the circuit in actual equipment and thoroughly confirm that oscillation does not occur.

In general, oscillation may occur not only when the ESR of the output capacitor is too large, but also when it is too small, causing loop instability. The ESR upper and lower limits differ depending on the IC; each device must be checked using the data sheet.

Aluminum Electrolytic Capacitor

An aluminum electrolytic capacitor can be used as an output capacitor. Such capacitors are inexpensive, and can have large capacitance. However, some aluminum electrolytic capacitors undergo sudden decreases in capacitance and increases in ESR at low temperatures, so caution must be exercised.

Ceramic Capacitor

When using a ceramic capacitor as an output capacitor, it is recommended that an X5R or X7R, with satisfactory temperature characteristics, be used. Z5U, Y5V, and F capacitors undergo large changes in capacitance, and so should not be used. The capacitance value declines according to tolerances, temperature characteristics, and DC bias characteristics, and so a device should be selected such that operation values do not fall below the minimum values stated on the data sheet under the conditions of use. Moreover, DC bias characteristics depend on the case size as well, and there is a tendency for capacitance drops to increase as the size is made smaller.

Output Load Transient Response

When there are sudden fluctuations in the output load current, transient fluctuations may occur in the output voltage. When there is a need to reduce such voltage fluctuations, the capacitance of the output capacitor should be increased. However, if the output capacitance is increased, the amount of charge that charges the output capacitor from the input side also increases, so that if the load responsiveness of the input-side power supply is not satisfactory, there may similarly be a transient drop in the input voltage. In order to prevent this, the capacitance of the input capacitor should be made large enough to be comparable to that of the output capacitor.

Points to Note Regarding Linear Regulator Input Capacitors

An input capacitor is required in order to suppress potential fluctuations in the input power supply during circuit operation so as to stabilize the input to the linear regulator IC. Particularly when input wiring (the thin film wiring of the circuit board) is long or when the input power supply impedance is high, an input capacitor is effective for securing the stability of the LDO input power supply.

Below are points to note pertaining to input capacitors, using the BA1117 as an example. Similarly to the output capacitor, nearly all the important points here are not limited to the BA1117, but apply to linear regulators in general.

Points to Note Regarding Linear Regulator Input Capacitors

Positioning on a Mounting Board

The input capacitor CIN is connected very close to the VIN pin, and similarly connected to a nearby ground. It should be positioned within 1 cm of the IC. As explained previously, an objective is to make the input power supply impedance as small as possible, and so the input capacitor is positioned in this way to eliminate the effects of parasitic components of the board insofar as possible.

Capacitance

　
In the BA1117 circuit example here, a 10 µF input capacitor is used, but in cases where the output current changes suddenly, a method is used in which the capacitance of the output capacitor is increased to reduce transient voltage fluctuations. However, if the instantaneous current supply capacity on the input power supply side is insufficient, the input voltage may drop. To prevent this, the value of the input capacitor should be increased to be comparable to the capacitance of the output capacitor. If a bulk (large-capacitance) capacitor is needed, an aluminum electrolytic capacitor or the like should be connected in parallel with a ceramic capacitor.

ESR (Equivalent Series Resistance)

Because a purpose of the input capacitor is to decrease the input power supply impedance, it is preferable that the ESR be small. When there are no sudden changes in the input current to the IC, other types of capacitors may be used.

Preventing Increases in the Output Ripple Voltage of a Floating Type Linear Regulator

This concerns floating type linear regulators having an ADJ pin. As the output voltage rises, input ripple is amplified, and so the IC ripple rejection ratio worsens. As a measure to deal with this, a capacitor CADJ can be added between the ADJ pin and ground to suppress ripple amplification, thus preventing worsening of the ripple rejection ratio.

Preventing Increases in the Output Ripple Voltage of a Floating Type Linear Regulator

As the equation below indicates, in order to prevent ripple rejection worsening, it is necessary that the impedance of CADJ be lower than R1 at all ripple frequencies.

\(\displaystyle \frac{1}{2\pi \times f_{RIPPLE} \times C_{ADJ}} < R_1\)

fRIPPLE ：Ripple frequency［Hz］

The graph below shows the changes in ripple rejection ratio with CADJ for the BA1117; the effect of the CADJ can be seen.

Preventing Increases in the Output Ripple Voltage of a Floating Type Linear Regulator

Points to Note Relating to Loads and Startup of Linear Regulators

The overcurrent protection (OCP) function of the BA1117 has a drooping characteristic, and so startup is possible for almost any load. However, startup may in some cases not be possible when large currents (inrush currents) flow in the circuit at startup. This is because if the load current exceeds the IC output (supply) current, the output voltage cannot rise, so that IC startup becomes impossible. Loads to which attention must be paid include motors, large-value capacitors, constant-current loads, and the like.

The graph below shows the relation between the output current IO and the output voltage VOUT (in this example, set to 1.5 V) for the BA1117. The BA1117 output current specifications stipulate a 1.0 A minimum current and 1.7 A typical current. The graph shows a drooping or droop-type overcurrent protection characteristic in which the output current is held essentially constant as the output voltage falls. In the case of this drooping characteristic, even when the current is limited, the output current continues to flow at essentially the limiting value, so that startup is normally possible even in the cases of the above-mentioned loads.

Points to Note Relating to Loads and Startup of Linear Regulators

There are some linear regulators that adopt overcurrent protection with a foldback type characteristic. A foldback characteristic is one in which both the output voltage and the output current are decreased, so that if an inrush current flows at startup and overcurrent protection is activated, the circuit is fixed in a state in which the output voltage cannot rise. In the case of loads like those discussed previously in particular, it is necessary to check whether this poses a problem for startup.

    DC-DC

    Basic

    Design

    Evaluation

    Application

    Product Information

    FAQ

    THIS WEBSITE USES COOKIES.
    By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.
    Privacy Policy
    OK