Scope of application
- 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.
This circuit stabilizes the current through one or more LEDs, almost independently of the supply voltage. Its main advantage is the very low voltage drop, which can be less than 100 mV. The design may find application in LED strips, where the voltage can vary along the length due to resistive drop, and small changes in voltage lead to significant changes in current and brightness. And also in, where every volt counts.
LED current stabilizer circuit
The voltage drop in the resistor R circuit does not exceed 40 mV. The rest depends on the parameters of Q3.
The nominal LED current here is 7.2 mA at 9 V. Increasing the voltage to 20 V causes a current change of only +15%, due to dynamic resistance.
The value of resistor R1 is selected for a blue/white LED with a voltage drop in the range of 2.9 - 3.4 volts. To maintain the desired level at a different voltage drop, change the value of R1 in proportion to the change in voltage drop.
The current through the LEDs is inversely proportional to the value of R. The current can be roughly changed by using this resistor, and fine-tuned by changing R1.
To obtain good thermal stability, Q1 and Q2 must be in thermal contact. Ideally, they should be on the same chip, but good results are obtained when they are pressed against each other.
The circuit works well not only with one LED. The maximum number of LEDs in a line depends only on the parameters of the circuit components.
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 ones; 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 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.
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 “ordinary” 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.
In case lack of transistor assemblies discrete transistors can be used. In my version, instead of the KR198NT5 assembly, 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.
Based on powerful switching field-effect transistors, linear voltage regulators can be built. A similar device was previously described in. By slightly changing the diagram, as shown in Fig. 1, it is possible to improve the parameters of the described stabilizer by significantly (5...6 times) reducing the voltage drop across the control element, which is the IRL2505L transistor. It has a very low channel resistance in the open state (0.008 Ohm), provides a current of up to 74 A at a housing temperature of 100 ° C, and is characterized by a high slope characteristic (59 A/V). To control it, a small gate voltage (2.5...3 V) is required. The maximum drain-source voltage is 55 V, gate-source voltage is ±16 V, the power dissipated by the transistor can reach 200 W.
Like modern microcircuit stabilizers, the proposed module has three pins: 1 - input, 2 - common, 3 - output. The DA1 microcircuit is used as a control element - a parallel voltage stabilizer KR142EN19 (TL431). Transistor VT1 serves as a matching element, and zener diode VD1 provides a stable voltage for its base circuit. The output voltage value can be calculated using the formula
Uout=2.5(1+R5/R6). 
The output voltage is adjusted by changing the resistance of resistor R6. Capacitors ensure stable operation of the stabilizer. The device works as follows. As the output voltage increases, the voltage at the control input of the DA1 microcircuit increases, as a result of which the current through it increases. The voltage across resistor R2 increases, and the current through transistor VT1 decreases. Accordingly, the gate-source voltage of transistor VT2 decreases, as a result of which the resistance of its channel increases. Therefore, the output voltage decreases, restoring to its previous value. 
The regulating field-effect transistor VT2 is connected to the negative wire, and the control voltage is supplied to it from the positive wire. Thanks to this solution, the stabilizer is capable of providing a load current of 20...30 A, while the input voltage can be only 0.5 V higher than the output voltage. If you plan to use the module with an input voltage of more than 16 V, then transistor VT2 must be protected from breakdown using a low-power zener diode with a stabilization voltage of 10...12 V, the cathode of which is connected to the gate, and the anode to the source.
The device can use any n-channel field-effect transistor (VT2), suitable for current and voltage from the list given in, preferably highlighted in yellow. VT1 - KT502, KT3108, KT361 with any letter indices. The KR142EN19 (DA1) microcircuit can be replaced with TL431. Capacitors - K10-17, resistors - R1-4, MLT, S2-33.
The connection diagram for the stabilizer module is shown in Fig. 2.
With a large load current, transistor VT2 dissipates a lot of power, so an effective heat sink is necessary. Transistors of this series with letter indices L and S are installed on the heat sink using soldering. In the author’s version, a housing from a faulty transistor KT912, KP904 is used as a heat sink and at the same time a supporting structure. This case was disassembled, its upper part was removed so that a gold-plated ceramic washer with a transistor crystal and stand-off leads remained. The crystal is carefully removed, the coating is tinned, after which the VT2 transistor is soldered to it. A printed circuit board made of double-sided foil fiberglass is soldered to the coating of the washer and the terminals of transistor VT2 (Fig. 3). The foil on the back side of the board is entirely preserved and connected to the metallization of the washer (drain of transistor VT2). After setting up and checking the stabilizer module, the board is glued to the case. Pins 1 and 2 are pads on the printed circuit board, and pin 3 (drain of transistor VT2) is a metal pin-stand on a ceramic washer. 
If you use parts for surface mounting: TL431CD microcircuit (Fig. 4), transistor VT1 KT3129A-9, transistor VT2 IRLR2905S, resistors P1-12, then some of them can be placed on a printed circuit board, and the other part can be mounted directly on the ceramic washer of the housing . The appearance of the assembled device is shown in Fig. 5. The voltage regulator module has no galvanic connection with the base (screw) of the case, so it can be directly placed on the heat sink, even if it is connected to the common wire of the powered device. 
It is also permissible to use the housing from faulty transistors of the KT825, KT827 series. In such a package, the transistor crystals are attached not to a ceramic, but to a metal washer. It is to this that the transistor VT2 is soldered, having previously removed the crystal. The remaining parts are installed in the same way. The drain of transistor VT2 in this case is connected to the housing, so the module can be directly installed on a heat sink connected to the negative wire of the load power supply.
Setting up the device comes down to setting the required output voltage with trimming resistor R6 and checking for the absence of self-excitation over the entire output current range. If it occurs, it must be eliminated by increasing the capacitance of the capacitors.
LITERATURE
1. Powerful field-effect switching transistors from International Rectifier. - Radio, 2001, No. 5, p. 45.
2. Necheev I. Voltage stabilizer on a powerful field-effect transistor. - Radio, 2003, No. 8. p. 53, 54.
I. NECHAYEV, Kursk
“Radio” No. 2 2005
