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Important Points in the Design of a Power Supply Using a Floating Type Linear RegulatorExample of Power Supply Circuit Based on a Floating Type Linear Regulator IC

2025.01.28

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This article explains the basics and practical considerations for power circuit design, using a Floating Type Linear Regulator IC as the subject.

About the Linear Regulator IC Used as an Example

For purposes of explanation, the popular “BA1117” linear regulator IC is used as an example. The BA1117 is an adjustable-output low-saturation (low dropout) regulator IC with floating operation, of the type called an LDO. The main specifications and block diagram are shown below. Floating type linear regulators are configured without a ground (GND) terminal.

  • ・Input voltage rating: 15 V
  • ・Input voltage range: VOUT +1.4 V to 10 V
  • ・Reference voltage: 1.25 V
  • ・Output voltage setting range: 1.25 V to 8.6 V
  • ・Output current: 1 A
  • ・Dropout voltage: 1.2 V（at 1 A）
  • ・Output voltage precision: ±1%（not including the precision of external resistors）
  • ・Output current: 5 mA *1 to 1 A
  • ・Operating junction temperature range: -20℃ to +105℃

*1： Includes the current in the voltage setting resistor.
※For specification values, please refer to the most recent data sheet.

About the Linear Regulator IC Used as an Example

The BA1117 is an LDO that uses an NPN transistor in the output stage; the dropout voltage (input/output voltage difference at which operation is possible; the details will be described later) is about 1.2 V. The dropout voltage of an LDO that uses a PNP transistor or MOSFET in the output stage is about 0.5 V, and so some may ask whether 1.2 V means it really is an LDO. But the dropout voltage of standard linear regulators (such as 78xx) is 2 V, and so the BA1117 is classified as an LDO. For details, please refer to “Linear Regulator Basics” on TechWeb.

There are a number of points to note relating to this section. In the explanations, spec values such as “dropout voltage 1.2 V” appear, but these may not necessarily be guaranteed values (maximum or minimum values). Hence in actual design processes, the latest data sheet should always be consulted. Also, in some cases a part of a circuit example may be omitted; these are provided only for reference.

Example of Power Supply Circuit Based on a Floating Type Linear Regulator IC

Below are shown a representative power supply circuit configured using the BA1117, and the package of the BA1117. The formal part number of this IC is “BA1117FP”.

Example of Power Supply Circuit Based on a Floating Type Linear Regulator IC

The circuit is quite simple. There are a total of four necessary external components: an input capacitor CIN, an output capacitor COUT, and resistors R1 and R2 for setting the output voltage. Small chip-type components can be used for all of them.

The BA1117 package is a TO252-3. It is a surface-mount package having a fin (also called a tab) to improve heat dissipation performance, and is a representative package for 1 A output class devices. The functions of the different pins are indicated below.

Pin Name Function
ADJ Output voltage setting pin
A reference voltage of 1.25 V relative to the VOUT pin is generated at the ADJ pin, and the output voltage can be programmed from 1.25 V to 8.6 V by connecting the ADJ pin to a resistive divider between the VOUT pin and ground.
VIN Input pin
Power is supplied to the IC via the input pin. In order to stabilize the input to the IC, connect a capacitor between VIN and ground.
Place the capacitor close to the pin.
FIN＝VOUT Output pin, heat dissipation fin
Supplies power to the load. In order to prevent oscillation, connect a capacitor between VOUT and ground. FIN is connected via the lead frame to the die (IC chip); in order to improve heat dissipation efficiency, soldering to a VOUT plane with a large copper foil area is recommended.

Method for Setting the Output Voltage of a Floating Type Linear Regulator IC

The BA1117, which is used here as an example of a linear regulator IC, is an adjustable-output type device. In the case of an adjustable-output type device, a desired voltage (normally, between the internal reference voltage and a maximum output voltage specified for the IC) is set using an external resistive voltage divider. It should be noted that in addition to adjustable-output type devices, there are also fixed-output type devices. The latter integrate dividing resistors in the IC according to general standard voltages such as 5 V and 3.3 V; ordinarily, the only external components are the input and output capacitors. Adjustable-output type devices and fixed-output type devices are used selectively according to the application.

The output voltage of the BA1117 can be set in the range 1.25 V to 8.6 V. The output voltage can be found using the following equation.

\(V_{OUT} = V_{REF} \times \left(1 + \frac{R_2}{R_1}\right) + I_{ADJ} \times R_2 \, [V]\)

VREF *1： Reference voltage (V)＝1.25V typ
VADJ：ADJ pin current (A)＝60µA typ

Method for Setting the Output Voltage of a Linear Regulator IC

In the BA1117, a reference voltage of 1.25 V is output across the VOUT pin and the ADJ pin. The current I1 in R1 can be calculated from 1.25 V/ R1, and the current in R2 is equal to the current in R1 plus the bias current VADJ at the ADJ pin. The ADJ pin bias current is 60 µA typ (120 µA max), and flows to ground via R2. In order to reduce the output voltage error that arises due to the ADJ pin bias current, it is recommended that the value of R1 be set to 120 Ω. By setting the value of R1 to a low value and increasing the value of I1, the value of VADJ can be ignored.

Moreover, in a PCB layout, the upper side of the output voltage setting resistor should be connected directly to VOUT (FIN) in order to obtain the optimum load regulation performance.

The table below shows resistor value settings for representative output voltages. In these examples, the E24 series is used for nominal resistance values. The same resistor types are used for the resistors R1 and R2. If the types are different, the different tolerances and temperature characteristics will result in changes in the ratio of R1 to R2, increasing the possibility that the output voltage precision may worsen. When using chip resistors of size 0402 mm (01005 inch) or smaller, devices should be selected with attention paid to the resistor rated power and maximum voltage.

Case when setting values using the fewest components

Target value
VO（V）		 R1（Ω） R2（Ω） Calculated value
VO‘（V）		 Error
（%）
1.25 120 0 1.250 0
1.5 120 24 1.501 + 0.10
1.8 82 36 1.801 + 0.05
1.9 120 62 1.900 – 0.02
2 200 120 2.007 + 0.36
2.5 120 120 2.507 + 0.29
3 130 180 2.992 – 0.28
3.3 110 180 3.306 + 0.19
5 120 360 5.022 + 0.43
6 180 680 6.013 + 0.22
7 180 820 6.994 – 0.09
8 150 820 8.133 + 1.66

Case when setting values for high precision

Target value
VO（V）		 R1（Ω） R2（Ω） Calculated value
VO‘（V）		 Error
（%）
1.25 120 0 1.250 0
1.5 120 24 1.501 + 0.10
1.8 82 36 1.801 + 0.05
1.9 120 62 1.900 – 0.02
2 120 68+3.6 2.000 + 0.01
2.5 150 110+39 2.501 + 0.02
3 120 120+47 3.000 – 0.01
3.3 130 130+82 3.301 + 0.04
5 160 430+47 5.005 + 0.10
6 150 510+56 6.001 + 0.01
7 120 510+39 7.002 + 0.02
8 130 680+18 8.003 + 0.04

There is one matter requiring attention. Ordinarily a current of about 10 mA is constantly flowing from VOUT pin to ground via R1 and R2. However, if for example the output voltage is set to 1.25 V, when R1 is left open this current goes to zero. If the load current of the BA1117 goes to zero, negative feedback no longer functions, so that the output voltage rises and operation is not normal. In relation to this, the data sheet indicates the maximum value of the minimum load current (IO(min)) and the minimum value of the output current (IO) among the specifications. In order to prevent such behavior, an R1 of 120 Ω should be installed so that a load current of about 10 mA is always flowing.

Load Regulation and Kelvin Connections in the Floating Type Linear Regulator

Ordinarily, if an output voltage setting resistor is connected on the line from the VOUT pin, optimal regulation is obtained. However, if the load current is large, the wire width is narrow, or there is considerable distance to the load, then the resistance of the PCB copper foil wiring may cause a drop in voltage, and as a result the voltage at the load point may decline. In such a case, load regulation due to these factors is added to the load regulation of the linear regulator IC itself. This approach to load regulation is not limited to floating type linear regulators, but applies to linear regulators in general.

This influence can be alleviated by bringing the lower side of the resistive voltage divider that sets the output voltage as close as possible to the load for connection. This method is well known as Kelvin connections; which eliminates the influence of the voltage drop caused by the large output current ILARGE and the wiring resistance RPARASTIC between the VOUT pin and the load terminals. And by positioning a high-impedance resistive voltage divider close to the IC and drawing out the wiring on the lower-side resistor having low impedance, noise tolerance is obtained. Below is shown an example of Kelvin connections for the BA1117.

Kelvin connection for the BA1117

The IC output capacitor COUT is used for oscillation prevention and so is positioned as close as possible to the IC. In order to deal with sharp load response, a large-value capacitor CBULK should be positioned near the load.

There is a matter demanding attention. Because many linear regulator ICs have a ground pin (the floating type BA1117 does not) and have a reference voltage between the ADJ pin and ground, make kelvin connections that bring the upper side of the resistor divider closer to the load. However, the BA1117 has a reference voltage between the ADJ pin and the VOUT pin, and so the connections are the opposite of the normal connections. The diagram below shows erroneous Kelvin connections for the BA1117. The output voltage in this case is represented by the equation that follows. Compared with the equation for setting the output voltage presented in the previous chapter, for the erroneous connections, a voltage drop due to the large output current ILARGE and the wiring resistance RPARASTIC between the VOUT pin and the load terminals is added to the VREF term. That is, this shows that load regulation is worsened due to the load current (ILARGE), meaning that the effect of the Kelvin connections cannot be obtained..

Erroneous connections in the case of the BA1117

＜Output voltage calculation equation for the above diagram＞
\(V_{OUT} = (V_{REF} + I_{LARGE} \times R_{PARASITIC}) \times \left(1 + \frac{R_2}{R_1}\right) + I_{ADJ} \times R_2 \, [V]\)

Output Voltage Errors of Floating Type Linear Regulator ICs

Output voltage errors are equal to the sum*2 of the tolerance of the BA1117 reference voltage*1 multiplied by the tolerance of the external resistor that sets the output voltage, the tolerance of the ADJ pin current VADJ, the line regulation tolerance, and the load regulation tolerance. When calculating using the maximum values of each of the tolerances, the result is the maximum tolerance; when calculating using minimum values, the minimum tolerance is obtained.

*1：On the data sheet, VO is the symbol for the reference voltage, and so here VO is used.

*2：Not including the load regulation arising from the PCB wiring resistance explained in the previous article.

The maximum and minimum values of the output voltage can be represented using the equations below. Adding the line regulation and load regulation tolerances to these gives the final output voltage error.

＜Minimum value＞
\(V_{OUT({min})} = V_{O({min})} \times \left(1 + \frac{R_{2(min)}}{R_{1(min)}}\right) + I_{ADJ({min})} \times R_{2(min)} \, [V]\)
＜Maximum value＞
\(V_{OUT({max})} = V_{O({max})} \times \left(1 + \frac{R_{2(max)}}{R_{1(max)}}\right) + I_{ADJ({max})} \times R_{2(max)} \, [V]\)

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