Stabilizer with low voltage drop. Adjustable series regulator with low voltage dropout input

Application area

Powering the circuits from a battery
Cell Phones
Laptops and PDAs
Barcode scanners
Automotive electronics
DC-DC modules
Device reference voltage
Linear low voltage power supplies

Second version of the scheme

This circuit is a low drop regulated power supply with a very low voltage drop across it. Of course, there are many other designs for regulated power supplies, but the MIC2941 chip has a number of advantages.

Depending on the operating mode, the drop is only 40 - 400 mV (compare with 1.25 - 2 V on LM317). This means you can use a wider range of output voltages (including shaping some digital circuits' standard 3.3V from an equally low 3.7V voltage (such as a 3 AA or lithium-ion battery). Note that ICs The MIC2940 series operate with a fixed output voltage, while the MIC2941 can be continuously adjusted.

MIC294x voltage table

Circuit capabilities on MIC2941

Short circuit and overheat protection.
Input diode to protect the circuit from negative voltage or AC current.
Two indicator LEDs for high and low voltage.
Output switch to select 3.3V or 5V.
There is a potentiometer on the board to adjust the voltage from 1.25 V to the maximum input voltage (20V max).
High accuracy of maintaining output voltage
Guaranteed output current 1.25 A.
Very low temperature coefficient
The input of the microcircuit can withstand from -20 to +60 V.
Logically controlled electronic switch.
And, of course, a low voltage drop - from 40 mV.

One of the important parameters of series voltage stabilizers (including microcircuit ones) is the minimum permissible voltage between the input and output of the stabilizer (ΔUmin) at maximum load current. It shows at what minimum difference between the input (Uin) and output (Uout) voltages all parameters of the stabilizer are within normal limits. Unfortunately, not all radio amateurs pay attention to it; usually they are only interested in the output voltage and maximum output current. Meanwhile, this parameter has a significant impact on both the quality of the output voltage and the efficiency of the stabilizer.
For example, for widespread microcircuit stabilizers of the 1_M78xx series (xx is a number equal to the stabilization voltage in volts), the minimum permissible voltage dUmin = 2 V at a current of 1 A. In practice, this means that for a stabilizer on the LM7805 chip (Uout = 5 V) the voltage Uinmin must be at least 7 V. If the ripple amplitude at the rectifier output reaches 1 V, then the value of Uinmin increases to 8 V, and taking into account the instability of the mains voltage within ±10%, it increases to 8.8 V. As a result, the efficiency of the stabilizer will not exceed 57%, and with a high output current the microcircuit will become very hot.
A possible way out of the situation is the use of so-called Low Dropout (low voltage drop) microcircuit stabilizers, for example, the KR1158ENxx series (ΔUmin = 0.6 V at a current of 0.5 A) or LM1084 (Umin = 1.3 V at a current of 5 A ). But even lower values of Umin can be achieved if a powerful field-effect transistor is used as a regulating element. It is this device that will be discussed further.

The diagram of the proposed stabilizer is shown in Fig. 1. Field-effect transistor VT1 is connected to the positive power line. The use of a device with a p-channel is due to the results of tests carried out by the author: it turned out that such transistors are less prone to self-excitation and, moreover, as a rule, their open channel resistance is less than that of p-channel ones. Transistor VT1 is controlled by parallel voltage regulator DA1. In order for a field-effect transistor to open, the voltage at its gate must be at least 2.5 V greater than at the source. Therefore, an additional source is needed with an output voltage that exceeds the voltage at the drain of the field-effect transistor by exactly this amount.
Such a source - a step-up voltage converter - is assembled on the DD1 chip. Logic elements DD1.1, DD1.2 are used in a pulse generator with a repetition rate of about 30 kHz, DD1.3, DD1.4 are buffer elements; diodes VD1, VD2 and capacitors SZ, C4 form a rectifier with doubling the voltage, resistor R2 and capacitor C5 form a smoothing filter.

Capacitors C6, C7 ensure stable operation of the device. The output voltage (its minimum value is 2.5 V) is set with trimming resistor R4.
Laboratory tests of the device prototype showed that with a load current of 3 A and a decrease in the input voltage from 7 to 5.05 V, the output decreases from 5 to 4.95 V. In other words, at the specified current, the minimum voltage drop ΔUmin does not exceed 0.1 V. This allows you to more fully use the capabilities of the primary power source (rectifier) and increase the efficiency of the voltage stabilizer.

The device parts are mounted on a printed circuit board (Fig. 2) made of one-sided foil-coated fiberglass laminate with a thickness of 1.5...2 mm. Fixed resistors - R1-4, MLT, trimmer - SPZ-19a, capacitors C2, C6, C7 - ceramic K10-17, the rest are imported oxide, for example, TK series from Jamicon. In a stabilizer with an output voltage of 3...6 V, a field-effect transistor with an opening voltage of no more than 2.5 V should be used. Such transistors from International Rectifier are usually marked with the letter L (see the fact sheet "Power field-effect switching transistors company International Rectifier" in "Radio", 2001, No. 5, p. 45). When the load current is more than 1.5...2 A, it is necessary to use a transistor with an open channel resistance of no more than 0.02...0.03 Ohm.
To avoid overheating, the field-effect transistor is fixed to a heat sink, and a board can be glued to it through an insulating gasket. The appearance of the mounted board is shown in Fig. 3.

The output voltage of the stabilizer can be increased, but we should not forget that the maximum supply voltage of the K561LA7 microcircuit is 15 V, and the limit value of the gate-source voltage of the field-effect transistor in most cases does not exceed 20 V.

Therefore, in such a case, you should use a boost converter assembled according to a different circuit (on an element base that allows a higher supply voltage), and limit the voltage at the gate of the field-effect transistor by connecting a zener diode with the corresponding stabilization voltage in parallel with capacitor C5. If the stabilizer is supposed to be built into a power source with a step-down transformer, then the voltage converter (microcircuit DD1, diodes VD1, VD2, resistor R1 and capacitors C2, SZ) can be excluded, and the “main” rectifier on the diode bridge VD5 (Fig. 4) can be supplemented with a doubler voltage on diodes VD3, VD4 and capacitor C9 (the numbering of elements continues what was started in Fig. 1).

Publication date: 29.09.2009

Readers' opinions

Seregy / 10/06/2011 - 08:34
What values need to be changed so that Uout becomes 9V?
Nikolay / 07/30/2011 - 22:30
Good scheme, thanks. I used it to stabilize voltage at currents up to 0.5A from a source with a strong voltage drop when the load current increases. The question arose about the own consumption of the control part - it eats a lot :), from 18.6 mA (U input max) to 8.7 mA. I set R3 = 8.2 kOhm (TL431 in nominal mode, I > 1 mA, although the typical minimum current is 450 μA) and the regulating R4 = 50 kOhm. current consumption decreased to 2.3 mA - 1.1 mA. With this modification, you can use capacitors C3-C5 of smaller capacity, I used 10 µF.

Continuous Series Voltage Regulator - Adjustable, Low Dropout

Adjustable series regulator

To adjust the output voltage in the previous circuit, an integral element with an adjustable stabilization voltage (controlled zener diode) can be used as a zener diode. There is another option.

Here is a selection of materials:

Low Dropout Voltage Stabilizer

Both previous circuits work well if the difference between the input and output voltage allows the desired bias to be generated at the base of transistor VT1. This requires at least a few volts. Sometimes it is not practical to maintain such a voltage, for example, because the losses and heating of the power transistor are proportional to this voltage. Then the following scheme applies.

It can work even if the difference between the input and output voltages is only a few tenths of a volt, since this voltage does not participate in the formation of the bias. The bias is supplied through transistor VT2 from the common wire. If the voltage on the trimmer resistor motor is less than the stabilization voltage of the zener diode plus the saturation voltage of the base-emitter junction VT3, then transistor VT3 is closed, transistor VT2 is open, transistor VT1 is open. When the voltage on the resistor motor exceeds the sum of the stabilization voltage of the zener diode and the saturation of the base-emitter junction VT3, transistor VT3 opens and removes current from the base of VT2. VT2 and VT3 are closed.

[Zener diode stabilization voltage, V] = - [Base-emitter saturation voltage VT3, V]

= ([Minimum possible input voltage, V] - [Base-emitter saturation voltage VT2, V]) * * [Minimum possible current transfer coefficient of transistor VT2] /

[Resistor R2 resistance, Ohm] = [Minimum output voltage, V] * [Resistor R1 resistance, Ohm] * [Minimum possible current transfer coefficient of transistor VT3] / / 3

[Transistor power VT1, W] = ([Maximum possible input voltage, V] - [Minimum output voltage, V]) * [Maximum possible output current, A]

[Transistor power VT2, W] = [Maximum possible input voltage, V] * [Maximum possible output current, A] / [Minimum possible current transfer coefficient of transistor VT1]

There is practically no power dissipation on the VT3 transistor and the zener diode.

There is a great need for 5-volt stabilizers with output currents of several amperes and with as little voltage drop as possible. Voltage drop is simply the difference between the DC input voltage and the output voltage, provided that regulation is maintained. The need for stabilizers with such parameters can be seen in a practical example, in which the voltage of a nickel-cadmium battery, equal to approximately 8.2 V, is stabilized at 5 V. If the voltage drop is the usual 2 or 3 V, then it is clear that using such a battery for a long time battery is not possible. Increasing the battery voltage is not the best solution, since in this case there will be pointless power dissipation in the pass transistor. If it were possible to maintain stabilization at a voltage drop of, say, half that, the overall situation would be much better.

It is known that it is not easy to make a pass transistor with a low saturation voltage in integrated circuits of stabilizers. Although it is desirable to control the pass transistor using an IC, the transistor itself must be a separate device. This naturally implies the use of hybrid devices rather than fully integrated circuits. In fact, this is a blessing in disguise because it makes it easy to optimize the saturation and beta voltages of the transistor to achieve the intended goal. In addition, you can even experiment with germanium transistors, which by nature have low saturation voltages. Another factor to consider is that /7l/7 transistors have lower saturation voltages than their prp counterparts.

Using these facts naturally leads to the low-dropout regulator circuit shown in Fig. 20.2. The voltage drop across this regulator is 50 mV at 1 A load current and only 450 mV at 5 A. The need to create a pass transistor was essentially stimulated by the release of the 71123 linear integrated regulator. The silicon /?l/7-transistor MJE1123 was specifically designed for this circuit, but there are several similar transistors available. Low saturation voltage is an important parameter in transistor selection, but high DC gain (beta) is also important for reliable short-circuit current limitation. It turned out that the 2iV4276 germanium transistor allows even lower voltage drops, but probably at the expense of deteriorating short-circuit current limiting characteristics. The resistance of the resistor in the base circuit of the pass transistor (20 Ohms in the diagram) is selected experimentally. The idea is to make it as high as possible with an acceptable voltage drop. Its value will depend on the expected maximum input voltage. Another feature

This stabilizer has a low idle current, about 600 μA, which contributes to a long battery life.

Rice. 20.2. An example of a linear regulator having a low voltage drop. A hybrid circuit is used here because it is difficult to achieve low voltage drop using only ICs. Linear Technology Sofoga!1op.

A similar low-dropout linear regulator from another semiconductor company is shown in Fig. 20.3. The basic characteristics remain the same - 350 mV voltage drop at 3 A load current. Once again, the use of a hybrid circuit provides additional design flexibility. The main difference between different ICs for controlling such stabilizers is the presence of auxiliary functions. The need for them can be assessed in advance in relation to a specific application and an appropriate choice can be made. Most of these ASICs have at least short circuit and overheat protection. Since the pass-through rpr-trshshstor is external to the IC, good heat dissipation is important. Often, to provide additional stabilization, a low-dropout linear regulator is added to an already built SMPS. Moreover, efficiency the system as a whole will remain virtually unchanged. This cannot be said when a conventional intephal voltage stabilizer with 3 terminals is used for additional stabilization.

Your first inclination might be to replicate the two low-dropout circuits just described, using a conventional 3-pin integrated voltage regulator and a pass transistor. However, the quiescent current (the current consumed by the internal circuit of the stabilizer, and which does not flow through the load) will be much higher than when using special circuits. This ruins the very idea of not introducing additional power dissipation in the system.

Rice. 20.3. Another low-dropout linear regulator circuit. The same configuration is used with an external PPR transistor. The selected control IC is the best in terms of required support functions. Cherry Semiconductor Soph.

Sometimes in amateur radio practice there is a need to stabilizer with low voltage drop on the regulating element (1.5-2V). This may be caused by insufficient voltage on the secondary winding of the transformer, dimensional restrictions when the case does not accommodate a radiator of the required size, considerations of device efficiency, etc.

And if the choice of microcircuits for building “conventional” stabilizers is wide enough (such as LM317, 78XX etc.), then microcircuits for building Low-Drop stabilizers are usually not available to everyone. Therefore, a simple scheme on available components may be very relevant.

I present a scheme that I myself have used for many years. During this time, the circuit showed reliable, stable operation. Available components and ease of setup will allow even novice radio amateurs to repeat the design without difficulty.

click to zoom

The circuit resembles a fairly standard one parametric stabilizer, which is supplemented with a GST (stable current generator) to control the base current of the regulating transistor, due to which it was possible to obtain low voltage drop.

The circuit is designed for an output voltage of 5V (set by resistor R4) and a load current of 200mA. If you need to get more current, then instead of T3 you should use composite transistor.

If you need to get a higher output voltage, you will have to recalculate the resistor values.

When lack of transistor assemblies discrete transistors can be used. In my version, instead of assembling KR198NT5, two selected KT361 transistors were used. The KR159NT1 assembly can be replaced with two KT315 transistors, the selection of which is not required.

Since there is practically no information on the Internet on domestic components, I provide the pinout of transistor assemblies for reference.