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Important Points in the Design of a Power Supply Using a Floating Type Linear RegulatorStartup characteristics for linear regulator ICs

2025.01.28

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This section describes the startup characteristics of a linear regulator when the power supply ( VIN ) is turned on and the characteristics when it is turned off. The on and off behavior of a linear regulator depends on the transition conditions of VIN and the capacitance of the output capacitor. In addition, since it often affects the load device, it is an essential item to check in the evaluation of operation.

Startup characteristics for linear regulator ICs

The figure below schematically shows the startup waveform at the time of a steep VIN turn-on. From the point at which VIN has risen, the linear regulator circuit operation begins. When the output capacitor capacitance is small (up to several dozen µF or so), the inrush current on startup often does not reach the value at which the output overcurrent protection circuit is activated, and so the overcurrent protection function does not act to limit the current. In this case, the output voltage increases with the rise time of the IC internal reference voltage, independently of the value of the output capacitor.

When VIN turn-on is steep and output capacitor value is small

Next is an example of a startup waveform when the VIN turn-on is similarly steep, but here the value of the output capacitor is large (roughly hundreds of µF or more). Because of the large inrush current for charging of the output capacitor at startup, overcurrent protection is activated, and the current is limited. Consequently, the charging current to the output capacitor is limited, and the larger the value of the output capacitor, the longer the time required for startup (the time until the preset value is reached).

Also shown is a table of the relations between the output capacitor value and the output voltage startup time. This is one example in the BA1117 series; results differ according to the linear regulator type and the circuit conditions. The main point here is that larger the output capacitor value, longer the startup time will be, and higher the output voltage setting, the more time is needed for startup.

When VIN turn-on is steep and output capacitor value is large

The final example is of a startup waveform in a case where VIN turn-on is gradual and the output capacitor value is small. Using a BA1117, circuit operation begins when VIN exceeds about 1 V, and the output voltage begins to rise. When the output capacitor value is large as well, the VIN voltage at which circuit operation starts is the same, and the rising output voltage waveform is similar to the above-described case “when VIN turn-on is steep and the output capacitor value is large”. Conditions under which VIN rises gradually are conditions in which observation is easy during the time until VIN reaches the startup initiation voltage. When VIN turn-on is steep, the voltage is passed instantaneously, and so it appears as if startup initiation occurs immediately.

When VIN turns on gradually and output capacitor value is small

Characteristics of Linear Regulators at Power-Off

The figure below is a schematic diagram of the output voltage waveform when VIN is turned off suddenly. When the VIN turn-off is steep, because some charge remains on the output capacitor, the input and output voltages are reversed (the output voltage becomes higher than the input voltage), so that charge on the output capacitor is discharged to the input side via parasitic elements within the IC. Hence the output voltage sharply drops following the input voltage, and when VIN reaches 0 V, the output voltage starts falling gradually, with the on-voltage of the parasitic elements (about 0.7 V) remaining. Thereafter the voltage continues to drop according to the time constant determined by the load resistor. If the load is a simple resistor, the output voltage falling time can be determined using the following equation.

\(T_{OFF} = -C_{O} \times R_{L} \times \ln\left(\displaystyle \frac{V_{C}}{V_{O}}\right) \, [s]\)
CO：Output capacitor　[F]
RL：Load resistance　[Ω]
VO：Output voltage　[V]
VC：Final dropped voltage　[V]

When VIN turns-off is steep

The next figure is an example of the waveform when the VIN turn-off is gradual. The VIN voltage falls, and when the point is reached at which the input and output voltages are reversed, charge on the output capacitor is discharged to the input side via the parasitic elements within the IC. Hence the output voltage falls in a manner that tracks the input voltage, and when VIN reaches 0 V, the output voltage starts falling further gradually with the remaining on-voltage of the parasitic elements (about 0.7 V). Thereafter the output voltage falls in accordance with the time constant determined by the load resistor. The behavior is essentially the same when VIN is turned off suddenly, but to the extent that the turn-off is gradual, the output voltage drop is also gradual.

When VIN turns off gradually

When there is a need for the output voltage to rapidly fall to 0 V after VIN is turned off, a circuit must be provided that forcibly discharges the charge on the output capacitor.

Inrush Current of Linear Regulators

It is the case, not only for the BA1117 series but for devices in general, that when input power is turned on and the output begins to rise (startup), an inrush current flows in order to charge the output capacitor. At this time, even if the output current value exceeds the maximum value of the recommended operating range, current is limited by the internal overcurrent protection (OCP) circuit, so that no problems arise (overcurrent protection is explained separately).

However, it must be confirmed that this current does not cause the junction temperature TJ to rise above 150°C. The junction temperature TJ resulting from an overcurrent over a short time can be estimated using the transient thermal resistance ZTH.

The transient thermal resistance is a thermal resistance that has time as a parameter. Strictly speaking, TJ begins rising (due to heat generation) from the time power is input, and stabilizes after a certain time has passed. The normal thermal resistance θJA is obtained by dividing the steady-state heat generation by the power consumption. On the other hand, the transient thermal resistance is the heat generation after a certain time has elapsed from power input, divided by the power at that time. The formula for calculating TJ using the transient thermal resistance is shown below. This equation has the form of the equation using θJA, but with ZTH substituted for θJA.

\(T_{J} = T_{A} + Z_{TH} \times P \, [\text{℃}]\)

TA：Ambient temperature [℃]
ZTH：Transient thermal resistance (°C/W) between junction and ambient environment [℃/W]
P：IC power consumption [W]

Here P, the IC power consumption, can be calculated using the equation below. This equation is for floating type linear regulator not having a ground pin, such as the BA1117; for regulators having a ground pin, the IADJ term becomes IIN－IOUT. Regulator types having a ground pin will be explained in a separate article.

\(P = (V_{IN} – V_{OUT}) \times I_{OUT} + (V_{IN} \times I_{ADJ}) \, [W]\)

VIN：Input voltage [V]
VOUT：Output voltage [V]
IOUT：Output current [A]
IADJ：ADJ pin current [A]

In cases where IADJ ≪ IOUT, the following equation can be used.

\(P = (V_{IN} – V_{OUT}) \times I_{OUT} \, [W]\)

In order to present a calculation example, conditions are set. In a TO252-3 package, with an ambient temperature of TA =60°C, suppose that an inrush current of 1.5 A flows for 1 ms. The transient thermal resistance during the 1 ms is, from the graph on the right, 2.7°C/W. Suppose that the input voltage VIN is 5 V and the output voltage VOUT is 3.3 V. These values are substituted into the equation initially presented for the junction temperature TJ to calculate the latter.

\(T_J = 60\,℃ + 2.7 \times (5\,V – 3.3\,V) \times 1.5\,A = 66.9\)

TJ is under 150°C, and so we see that there are no problems under these conditions.

Thus in cases where the inrush current flows over a short length of time of about 1 ms, the rise in the chip temperature (TJ) is trivial, and so no significant problems are posed by temperature increases due to the inrush current.

Transient thermal resistance of the TO252-3 package

Overcurrent Protection（OCP）of Linear Regulators

The overcurrent protection function mentioned in the previous capture on ” Inrush Current of Linear Regulators” is explained.

An overcurrent protection circuit functions to prevent IC destruction due to overcurrent when the IC output is shorted to ground. This circuit is also called by the acronym OCP, for Over Current Protection. An important point about this protection function is that it is intended to prevent the IC destruction, and is not intended to protect a load (a device being fed power) or other circuits or equipment. When there is a need to protect other circuits or equipment, it is assumed that fuses or other current-limiting devices will be used.

The overcurrent protection function of the BA1117 series has a drooping characteristic like that shown below. At point A, the overcurrent protection is activated; the output current at this time is approx. 1.7 A (typical value). There is some variation in the lower limit of the output current at which the overcurrent protection is activated, but this lower limit will never be below 1 A, which is the guaranteed value for the output current. Upon detecting an overcurrent, the current limiting circuit is activated, the output current becomes essentially constant, and the voltage falls nearly vertically (point B). Thereafter, so long as the overcurrent continues to flow, this state is maintained. That is, when the load is the cause of the output short-circuit, the load is not protected, as explained above. When the overcurrent state ends, the output voltage is automatically restored.

During the time after the output current exceeds the 1 A guaranteed value until the overcurrent protection is activated (typical value 1.7 A), the IC operates as a linear regulator, but the electrical characteristics are not within the guaranteed range (see the electrical characteristics in the data sheet). Moreover, when operation continues even while exceeding the allowable power dissipation (at absolute maximum rating 1.2 W, TO252-3 package) regardless of the output current value, the thermal shutdown circuit is activated and turns off the output.

Characteristics of overcurrent protection

Thermal Shutdown （TSD）of Linear Regulators

Similarly to the overcurrent protection function, here thermal shutdown, which is a representative protective function for linear regulators, is explained.
Thermal Shutdown (TSD) is a function for protecting an IC from damage due to overheating when the temperature of the IC chip exceeds the absolute maximum rating due to an output short-circuit or an increase in power losses. It should be understood that this protection function is not intended to substitute for protection of a load or other equipment from overheating.

In the BA1117 series, when the chip temperature (junction temperature) exceeds about 155°C (reference value), the linear regulator output is turned off and the output current is shut off to lower the chip temperature. When the chip temperature falls to about 150°C (reference value), the output is again turned on to start supply of the output current. This output on/off operation is repeated until the cause of the excessive increase in the chip temperature is removed. The IC will not be destroyed immediately even if this state continues, but such repeated operation over a long period of time will culminate in IC degradation and destruction, and so should be avoided.

Thermal shutdown characteristics

Equivalent Circuits of Linear Regulators

This capture explains the final topic, equivalent circuits of linear regulators.
Below is the equivalent circuit for the BA1117 series. The equivalent circuit roughly shows internal connections between each of the terminals, and the basic configuration of an output-stage circuit and an input-stage circuit. Actual circuits are more complex than this, and parasitic elements and other entities are present due to the construction of the IC. However, equivalent circuits can be useful for understanding device characteristics and behavior.
BA1117 series devices are LDOs that have a dropout voltage of 1 to 1.2 V (typical values); the reason for this is the Darlington connection of an NPN transistor and a PNP transistor in the output stage. We can also understand in broad terms the operation of the overcurrent protection circuit and the thermal shutdown circuit. While study of these matters is not absolutely essential, they can be used to deepen one’s understanding.

Equivalent circuit

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