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A voltage regulator is one of the most widely used electronic circuitry in any device. A common case is with micro controllers, where a smooth regulated input voltage must be supplied for the micro controller to function smoothly. You may also like this article on Regulated Power Supply.
Voltage regulators are of different types. In this article, our interest is only with IC based voltage regulator. An example of IC based voltage regulator available in market is the popular IC which regulates the output voltage at 5 volts. Now lets come to the basic definition of an IC voltage regulator.
It is an integrated circuit whose basic purpose is to regulate the unregulated input voltage definitely over a predefined range and provide with a constant, regulated output voltage. An IC based voltage regulator can be classified in datwsheet ways. A common type of classification is 3 terminal voltage regulator and 5 or multi terminal voltage regulator.
3A, Step-Up/Down/Inverting Switching Regulator
We have given below the block diagram of a 3 terminal IC based voltage regulator. These datasyeet provide a constant output voltage. A popular example is the IC which provides a constant 5 volts output. A fixed voltage regulator can be a positive voltage regulator or a negative voltage regulator.
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A positive voltage regulator provides with constant positive output voltage. The only difference is in the polarity of output voltages. An adjustable voltage regulator is a kind of regulator whose regulated output voltage can be varied over a range. There are two variations of the same; known as positive adjustable voltage regulator and negative adjustable regulator. LM is a classic example of positive adjustable voltage regulator, whose output voltage can be varied over a range of 1.
LM is an example of negative adjustable voltage regulator. There may be certain conditions where a variable voltage may be required.
Right now we shall discuss how an LM adjustable positive voltage regulator IC is connected. The connection diagram is shown below. The resistors R1 and R2 determine the output voltage Vout. The resistor R2 is adjusted to get the output voltage range between 1. The output voltage that is required can be calculated using the equation:. In this circuit, the value of Vref is the reference voltage between the adjustment terminals and the output taken as 1. The value of Iadj will be very small and will also have a constant value.
Thus the above equation can be rewritten as. In the above equation, due to the small value of Iadj, the drop due to R2 is neglected. The load regulation is 0. The ripple rejection is 80 db, equivalent to 10, The LM series of adjustable voltage regulators is a complement to the LM series devices.
The negative adjustable voltage regulators are available in the same voltage and current options as the LM devices. As shown in the block diagram above, the built-in reference voltage. Vref drives the non-inverting input of the operational amplifier.
There are many stages of voltage gain for the op-amp used here. This high gain helps the op. Thus, the inverting input terminal vaue will also be the.
Thus, the current flowing through the potential divider can be written as.
The resistor R2 shown in the figure is not an external component connected to the IC, but an internal resistor that is built inside the IC during manufacture. Due to the conditions above, the same current flows through R1. Thus the output voltage can be written as. This shows that the output of dattasheet regulator can be controlled by putting desired values for R1 and R2. The IC has a series pass transistor that can handle more than 43166t. Thermal shutdown is a feature that will turn off the IC as soon as the internal temperature of the IC rises above its preset value.
This rise in temperature may mostly be datasheft to excessive external voltage, ambient temperature, or even heat sinking. Because of thermal shutdown and current limiting, devices in the LM series are almost indestructible.
The figure above shows the application of LM IC as a voltage regulator. Pins 1, 2, and 3 are the input, output and ground. When there is quite a distance in cms from the IC to the filter capacitor of the unregulated power supply, there may occur unwanted oscillations within dqtasheet IC due to lead inductances within the circuit. In order to remove this unwanted oscillation, the capacitor C1 has to be placed as shown in the circuit.
Capacitor C2 is sometimes used to improve the transient response of dtaasheet circuit. Any device in the LM series needs a minimum input voltage at least 2 to 3 V greater than the regulated output voltage.
Otherwise, it will stop regulating. Furthermore, there is a maximum input voltage because of excessive power dissipation.
IC Voltage Regulators-with Circuit Diagram – Design & Theory
In switching regulators the output voltage is regulated by controlling the switching time of feedback circuitry which includes a reference voltage ; that is by adjusting the duty cycle. The 31466t discussed above are all linear voltage regulators which need a series pass transistor to regulate in the active region.
Though they are chosen for different purposes, they do have a disadvantage of high power dissipation of the series pass transistor. The main reason is because the series pass transistor is used as a Class A power amplifier which is known for its low efficiency and power loss in the form of heat. The series pass resistor will have to carry a heavy load when the load current increases. This causes the series pass transistor to get bulky with a bulkier heat sink.
This in turn increases the overall cost as well. Such linear regulators will also need a step-down transformer which again increases the size of the overall circuit. The large ripples produced by the circuit should be eliminated and this datasjeet large sized filter capacitors. All these problems can be solved by using a switching voltage regulator.
M1496-A Datasheet PDF
The whole operation is completely different when compared to a linear voltage regulator. Here, the series pass transistor is not used as an amplifier, but as a switch. That is, instead of the transistor working in the active region, it is made to switch between the saturation region or cut-off datashedt.
Thus, the power dissipation is lessened and thus can carry heavy load currents at low voltages with less bulky heat sinks. Thus, this regulator finds its extensive use in personal computers. The basic switching regulator is designed to work in three configurations.
Their circuit diagrams and explanation are given below. Step-down Switching Regulator As shown in the figure above, the rectangular pulses are given to the base of the transistor.
During each cycle of the pulse the transistor changes between saturation and cut-off. This creates a rectangular voltage at the input to LC filter. The ac components of the input voltage to the filter is blocked and the dc component is allowed to pass through the filter. As the transistor keeps switching, the average value will always be lesser than the input voltage. Step-up Switching Regulator As shown in the figure above, the rectangular pulses are given to the base of the transistor.
When the transistor is saturated the current flows through the inductor. When the transistor switches to cut-off a large voltage will be induced across the inductor coil because of the sudden collapse of the magnetic field around it. Thus the current keeps flowing in the same direction. Polarity Inverting Switching Regulatory As shown in the figure above, when the transistor is saturated, the current flows through the inductor.
Since the transistor is cut-off, the only path is through the capacitor. If the direction of charging current through the capacitor is checked, output voltage is found to be negative. A simple switching regulator is designed with the mix of circuits that we already know. The working starts with the relaxation oscillator which generates a square wave. The frequency of the square wave is decided by the values of R5 and C3.
The square wave is given as input to the integrator and produces an output triangular wave. This is given as input to the positive terminal of the triangle to pulse converter. The output pulse will then drive the base of the pass transistor. The duty cycle of these pulses will determine the output voltage.
By controlling the duty cycle of the pulse generator, the duty cycle of the input voltage to the LC filter is controlled. The output of the LC filter is a dc voltage with only a small ripple. A simple voltage divider is used to sample the output of the LC filter. This voltage is then fedback to the comparator circuit where a reference voltage Vref is compared with the output voltage and passed on to the negative input of the triangle to pulse converter. When the output voltage increases, the comparator circuit produces a higher output voltage and thus the inverting input of the triangle to pulse converter will have high value.
This will narrow down the pulses at the base input of the pass transistor. Since the duty cycle is lower, the filtered output voltage is less, which tends to cancel almost all the original increase in the output voltage.