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

2025.01.28

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

The minimum value of the input voltage at which a linear regulator IC can operate is the voltage value obtained by reading the input/output voltage difference at the load current to be used from the “input/output voltage difference vs. output current” graph of the data sheet, and adding it to the output voltage. The figure below is excerpted as an example from the data sheet for the BDxxIC0W series of ICs that are being used in explanations. When for example the load current (output current) IO is 0.6 A, the input/output voltage difference Vdrop is about 0.25 V, and the minimum value of the input voltage for a desired output voltage of 3.3 V will be 3.3 V + 0.25 V = 3.55 V. The characteristic values in the graph are essentially typical values.

Example of dropout voltage vs. output current (load current)
Example of dropout voltage vs. output current (load current)

Under the condition of the minimum input/output voltage difference as in the previous example, there are a number of points to be considered. In this case, DC operation is available and the device can output the specified voltage, but controllability will be diminished.

If the input/output voltage difference is small, sufficient current cannot be fed from input to output in a short time, and the load response characteristic declines. If the response characteristic is slow, the PSRR (ripple rejection ratio) characteristic also declines.

If, emphasizing efficiency, the input/output voltage difference is set to the minimum value, depending on the conditions of use, the linear regulator may not be able to exhibit adequate characteristics. It should be confirmed that an input voltage results in the required load response and PSRR, and a compromise between efficiency and various device characteristics should be sought.

Output Control (EN) Pin of Linear Regulator ICs

The EN pin can be used to switch the output on and off. In the BDxxIC0 series ICs, when the EN pin is at low level, VO is turned off and the entire IC is also turned off, so the current consumption goes to zero. When EN is at high level, the IC turns on, and VO turns on. The following is a circuit example.

The following is a circuit example.

To ensure that the IC turns on/off, the voltage specified in the electrical characteristics of the data sheet should be applied to the EN pin (the table below is excerpted from the data sheet).

  Symbol Min. Typ. Max. Unit
EN Low Voltage VEN(Low) 0 – 0.8 Ｖ
EN High Voltage VEN(High) 2.4 – 5.5 Ｖ

※Ta＝25℃、VCC＝3.3V、R1＝16kΩ、R2＝7.5kΩ

As reference values for design, the threshold median value is about 1.7 V, tolerance is approximately ±0.2 V, the temperature characteristic is from 1.85 V to 1.5 V or so (-40°C to +105°C), for an overall value of about 1.3 V to 2.05 V.

The EN pin is an output voltage on/off control pin, and operates as a switch, however, ICs are designed assuming that EN input is switched between high and low in a short time. The EN pin should not be fixed at the midpoint potential of high/low switching (near the 1.3 V to 2.05 V mentioned above). At the midpoint potential, the output voltage could become unstable.

There are no restrictions placed on the order of the rise of VCC and EN. The graphs shown below are examples of output voltage (VO) startup (turn-on) waveforms when VCC is applied with the EN pin voltage (VEN) high (on the left) and when VEN is driven high after VCC is applied (on the right). The two graphs show that, regardless of the rising order, the output is started up as the result of what might be called a logical AND operation. Similarly with turn-off, the order is irrelevant; the output is turned off either when VEN goes low, or when VCC is shut off. In the cases of some other series and manufacturers, the order is specified, and so the data sheet for a given IC must be referenced to check the device operation and conditions.

Output Control (EN) Pin of Linear Regulator ICs

If the output control function will not be used, the EN pin should be connected to VCC. In such a case, a series resistor is unnecessary. A circuit example is shown below.

If the output control function will not be used, the EN pin should be connected to VCC.

The delay time from the instant when the EN pin goes high until startup of the output voltage begins is approximately 70 µs (reference value for design; see the graph below). Here “beginning of startup” means the point at which the output VOUT has risen to 10% of its final value.

The delay time from the instant when the EN pin goes high until startup of the output voltage begins is approximately 70 µs.

If the EN pin is controlled using a mechanical switch, chattering of the switch may result in chattering in the output voltage. In such cases, an RC filter should be inserted ahead of the EN pin so that the chattering waveform does not reach the EN pin (see the figure below).

In such cases, an RC filter should be inserted ahead of the EN pin so that the chattering waveform does not reach the EN pin.

If the wiring between the EN pin and the switch is long, the inductance of the wiring may result in the occurrence of large pulse waveforms, and if the voltage of these pulses exceeds the withstanding voltage of the EN pin, IC destruction can result. Once again, this can be dealt with by inserting an RC filter before the EN pin to lower the peak value of pulse waveforms (see the figure below).

If the wiring between the EN pin and the switch is long, the inductance of the wiring may result in the occurrence of large pulse waveforms, and if the voltage of these pulses exceeds the withstanding voltage of the EN pin, IC destruction can result.

In any case, with the IC mounted on the board, it is necessary to check the effect of an RC filter using an oscilloscope or others, and adjust it with the capacitor value C.

Points to Note Regarding Linear Regulator Output Capacitors

Placement on a Mounting Board

In order to stabilize the loop control of a linear regulator, the output capacitor should be connected as close as possible to the VO pin of the IC, and similarly connected to a nearby ground. It is preferable that the capacitor be as close to the VO pin of the IC as possible, but as a rule of thumb, it should be placed within 3 cm from the IC.

Placement on a Mounting Board

Capacitance

For the BDxxIC0W series, select an output capacitor with an actual capacitance of 1 µF or greater, taking into consideration tolerances and temperature characteristics. If the capacitance is too small, oscillation may occur.

There is no upper limit imposed on the output capacitor value, but a number of issues must be studied. The greater the value of the output capacitor, the longer charging time at power supply turn-on and discharging time at turn-off. Moreover, when the power supply is turned off, the output and input voltages may be inverted, causing a large reverse current to flow back into the IC and damaging it. In such cases, a reverse current bypass diode or reverse current prevention diode must be added to the circuit. Details are given in an upcoming article on “Terminal Protection”.

ESR (Equivalent Series Resistance)

The ESR of the output capacitor should be set to a value within the stable operating range indicated in the graph below. This graph is based on the evaluation circuit shown on the right side of the graph; there is not complete equivalence with the capacitor actually used. The values are based on an individual IC and a resistive load and actually vary with the board wiring impedance, input power supply impedance, and the load impedance. It is always necessary to thoroughly check for oscillation under the conditions of the final product.

ESR (Equivalent Series Resistance)

Ceramic Capacitor

When using ceramic capacitors as output capacitors, X5R and X7R are recommended due to their satisfactory temperature characteristics. Z5U, Y5V and F should not be used due to their large capacitance change (see the figures below).

Ceramic Capacitor

Although the capacitance may be reduced from the nominal value depending on the tolerance, temperature characteristic, and DC bias characteristic, select a capacitor value so that actual capacitance will not be lower than the minimum value (1 µF). DC bias characteristics are such that the decrease in capacitance tends to be larger for smaller capacitor sizes (see the figure below).

DC bias characteristics are such that the decrease in capacitance tends to be larger for smaller capacitor sizes.

Aluminum Electrolytic Capacitor

Electrolytic capacitors are inexpensive and have large capacitance; but because the electrolyte may harden at lower temperatures, their use may result in sudden drops in capacitance and rises in ESR, so caution is necessary. And, if heat generated by the linear regulator IC reaches the electrolytic capacitor, the electrolyte will be heated, adversely affecting the capacitor lifetime. In order to prevent this, either an electrolytic capacitor should be placed in an area where heat has little effect, or the width of the copper foil wiring on the board should be reduced to the lower limit allowed by the current capacity tolerance to suppress (inhibit) heat conduction from the linear regulator IC.

Output Load Transient Response

When there are sharp fluctuations in the load current, transient voltage fluctuations may occur in the output voltage. In order to reduce these voltage fluctuations, an output capacitor with a large value should be used. Because large-value ceramic capacitors are expensive, a small-value ceramic capacitor can be used in parallel with an aluminum electrolytic capacitor, added as a bulk capacitor, to hold down costs. However, if the output capacitance is increased, the amount of charge that charges the output capacitor from the input side increases, so that if the load response of the input-side power supply is poor, there could similarly be transient drops in the input voltage. In order to prevent this, the value of the input capacitor should be increased to be comparable to that of the output capacitor.

Points to Note Regarding Linear Regulator Input Capacitors

An input capacitor is required to stabilize the input to the linear regulator IC by suppressing potential fluctuations at the power supply line during circuit operation. In particular, when the input wiring is long, or when the impedance of the input power supply is high, an input capacitor is effective to ensure the stability of the IC input power supply.

Points to Note Regarding Linear Regulator Input Capacitors

Placement on a Mounting Board

The input capacitor is connected very close to the VCC pin, and similarly connected to a nearby ground. It is desirable that the capacitor be as close as possible to the IC input pin; in any case it should be within 1 cm from the IC. This is in order to eliminate the effect of parasitic components in the board insofar as possible.

Capacitance

For the BDxxIC0W series, select an input capacitor with an actual capacitance of 1 µF or greater. Depending on tolerances, temperature characteristics, and DC bias characteristics, the actual capacitance may be lower than the nominal value, and so the capacitor value should be set such that the actual capacitance never falls below the minimum value (1 µF).

If there are sudden changes in the output current, a method is used in which the value of the output capacitor is increased to reduce transient voltage fluctuations. However, the larger output capacitor value may lead to momentary insufficiency of the current supply capability on the input power supply side, in which case there will be similar fluctuations in the input voltage. In order to prevent this, the value of the input capacitor must also be increased to a value comparable to that of the output capacitor. As a bulk capacitor, an aluminum electrolytic capacitor or similar capacitor may be connected in parallel with a ceramic capacitor.

ESR (Equivalent Series Resistance)

For the purpose of reducing the power supply impedance, a ceramic capacitor with a small ESR is recommended as the input capacitor. As described in the previous section, a large-capacitance electrolytic capacitor with a large ESR may be used as a bulk capacitor. But in such a case also, by connecting a ceramic capacitor with a small ESR in parallel, the combined impedance can be reduced.

Points to Note Relating to Loads and Startup of Linear Regulator ICs

The BDxxIC0 series devices used as examples in these articles have overcurrent protection (OCP) with a foldback characteristic (see the graph below). The foldback characteristic means that both the output voltage and the output current are caused to drop, so that if the load current exceeds the IC output (supply) current at startup, the output voltage cannot rise, and the IC cannot start up. This phenomenon can occur when the load is a constant-current source, or when the output is at a negative potential at startup.

The BDxxIC0 series devices used as examples in these articles have overcurrent protection (OCP) with a foldback characteristic.

When a constant-current load is turned on after the IC output voltage has risen to the specified value, the IC operates without problems; but thereafter, if the thermal shutdown circuit is activated and the output is turned off, the IC cannot restart. Further, if the IC cannot start up, the current of the constant-current load flows through an electrostatic breakdown protection diode between VO and GND within the IC, so that depending on the current value the chip temperature may rise, possibly resulting in IC destruction and solder melting. Hence use with a constant-current load is not recommended.

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