All modern electronic equipment is built on elements that are sensitive to the supply electricity. Not only the correct functioning, but also the performance of the circuits as a whole depends on it. Therefore, first of all, electronic devices are equipped with fixed stabilizers with a small voltage drop. They are made in the form of integrated circuits, which are produced by many manufacturers around the world.
What is a low dropout voltage regulator?
Under the voltage stabilizer (SN) understand such a device, the main task of which is to maintain a certain constant level of voltage on the load. Any stabilizer has a certain accuracy of issuing a parameter, which is determined by the type of circuit and the components included in it.
Internally, the MV looks like a closed system, where in automatic mode the output voltage is adjusted in proportion to the reference (reference), which is generated by a special source. This typestabilizers are called compensatory. In this case, the control element (RE) is a transistor - a bipolar or a field worker.
The voltage regulation element can operate in two different modes (determined by the construction scheme):
- active;
- key.
The first mode implies continuous operation of the RE, the second - operation in a pulsed mode.
Where is the fixed stabilizer used?
Radio-electronic equipment of the modern generation is characterized by mobility on a global scale. Device power systems are built on the use of mainly chemical current sources. The task of developers in this case is to obtain stabilizers with small overall parameters and as little as possible electricity losses on them.
Modern CHs are used in the following systems:
- mobile communication facilities;
- portable computers;
- microcontroller batteries;
- offline security cameras;
- autonomous security systems and sensors.
To solve the issues of powering stationary electronics, voltage regulators with a small voltage drop in a housing with three KT-type terminals (KT-26, KT-28-2, etc.) are used. They are used to create simple circuits:
- chargers;
- household electrical power supplies;
- measuring equipment;
- communication systems;
- special equipment.
What are fixed-type SNs?
All integral stabilizers (included inwhich include fixed ones) are divided into two main groups:
- Hybrid Low Drop Voltage Stabilizers (HID).
- Semiconductor microcircuits (ISN).
SN of the first group is performed on integrated circuits and packageless semiconductor elements. All circuit components are placed on a dielectric substrate, where connecting conductors and resistors are added by applying thick or thin films, as well as discrete elements - variable resistances, capacitors, etc.
Structurally, microcircuits are complete devices, the output voltage of which is fixed. These are usually stabilizers with a low voltage drop of 5 volts and up to 15 V. More powerful systems are built on powerful frameless transistors and a control circuit (low power) based on films. The circuit can pass currents up to 5 amps.
ISN microcircuits are performed on a single chip, because they are small in size and weight. Compared to previous microcircuits, they are more reliable and cheaper to manufacture, although they are inferior to GISN in terms of parameters.
Linear SNs with three pins belong to the ISN. If you take the L78 or L79 series (for positive and negative voltages), then they are divided into microcircuits with:
- Low output current of about 0.1 A (L78L).
- Average current, around 0.5A (L78M).
- High current up to 1.5 A (L78).
Low Dropout Linear Regulator Working Principlevoltage
Typical stabilizer structure consists of:
- Voltage reference.
- Converter (amplifier) error signal.
- A signal divider and a regulating element assembled on two resistors.
Since the value of the output voltage directly depends on the resistances R1 and R2, the latter are built into the microcircuit and a CH with a fixed output voltage is obtained.
The operation of a low dropout voltage regulator is based on the process of comparing the reference voltage with that which is output. Depending on the level of discrepancy between these two indicators, the error amplifier acts on the gate of the power transistor at the output, covering or opening its transition. Thus, the actual level of electricity at the output of the stabilizer will differ little from the declared nominal.
Also in the circuit there are sensors for protection against overheating and overload currents. Under the influence of these sensors, the channel of the output transistor is completely blocked, and it ceases to pass current. In shutdown mode, the chip consumes only 50 microamps.
Low Dropout Regulator Circuits
The integrated stabilizer microcircuit is convenient because it has all the necessary elements inside. Installing it on the board requires the inclusion of only filter capacitors. The latter are designed to remove interference coming from the current source and load, as seen in the figure.
Regarding 78xx series CHs and using tantalum or ceramic shunt capacitors for input and output, the capacitance of the latter should be within 2 uF (input) and 1 uF (output) at any allowable voltage and current values. If aluminum capacitors are used, then their value should not be lower than 10 microfarads. Connect the elements as close as possible to the pins of the microcircuit.
In the case when there is no voltage stabilizer with a small voltage drop of the desired rating, you can increase the rating of CH from a smaller one to a larger one. By raising the level of electricity at the common terminal, it is increased by the same amount at the load, as shown in the diagram.
Advantages and disadvantages of linear and switching regulators
Continuous ICs have the following advantages:
- Realized in one small package, which allows them to be efficiently placed on the PCB workspace.
- Do not require the installation of additional regulatory elements.
- Provides good output parameter stabilization.
The disadvantages include low efficiency, not exceeding 60%, associated with a voltage drop across the built-in control element. With a high power of the microcircuit, it is necessary to use a crystal cooling radiator.
Switching voltage regulators with a small drop are considered more productivefield voltage, the efficiency of which is approximately at the level of 85%. This is achieved due to the operating mode of the regulating element, in which the current passes through it in pulses.
The disadvantages of the pulsed CH circuit include:
- Complexity of schematic design.
- Presence of impulse noise.
- Low stability of the output parameter.
Some Linear Voltage Regulator Circuits
In addition to the targeted use of microcircuits as CH, it is possible to expand their scope. Some variants of such circuits based on the integrated circuit L7805.
Turn on stabilizers in parallel mode
To increase the load current, CH is connected in parallel to each other. To ensure the operability of such a circuit, an additional resistor of a small value is installed in it between the load and the output of the stabilizer.
CH based current stabilizer
There are loads that need to be powered by constant (stable) current, for example, an LED chain.
Scheme for controlling the fan speed in the computer
The regulator of this type is designed in such a way that when initially turned on, the cooler receivesall 12 V (for its promotion). Further, at the end of the charge of the capacitor C1 with a variable resistor R2, it will be possible to adjust the voltage value.
Conclusion
When assembling a circuit using a voltage regulator with a small voltage drop with your own hands, it is important to consider that some types of microcircuits (built on field-effect transistors) cannot be soldered with an ordinary soldering iron directly from a 220 V network without grounding the case. Their static electricity can damage the electronic element!
