Adjustable output ICs are fairly new developments and have many types. This section introduces several types of commonly used output-capable integrated circuits. Application examples and some characteristic values will be given with the positive voltage regulator μA78MG and the negative voltage regulator μA79MG, which are widely used and very simple to use .
With these ICs, regulated voltages up to 0.5A output current, from + 5V to + 30V and -2.2V to -30V can be produced. These are integrated circuits; internal current limitation, loss of power limitation and protection against over-temperature. The stability of output voltages is better than 1% in all cases against changes in load and input voltages. Figure-1.17 shows a regulator example with an adjustable output.
In this circuit, output voltage V0; is calculated as follows.
In this form, VR is the reference voltage. This value is 5V for μA78MG and -2.2V for μA79MG. The control K input on duty enables the output voltage to be adjusted. The value of the current flowing to the K terminal is only 1μA. If the average current of the voltage divider is set at 1mA, for R2 at 78MG,
For the 79MG greenhouse, for R2;
values. This is because the control input K is the reference voltage in both circuits. In the circuit in Figure-1.17, the functions of the capacitors C1 and C2 are the same as those described in the previous sections. In other words, these capacitors are used without improving the ripple factor of the output voltage and fluctuating against the changes in the input.
The entire soldering cycle time should not exceed 10 seconds. In addition, the input voltage to be applied to the integrated circuit must be at least 2V higher than the maximum output voltage. In the circuit, the same current flows practically from the resistors R1 and R2. (The current flowing through terminal K can be neglected since it is very small.) Thus, the output voltage for 1mA IQ current;
R1 and R2 are values K? . Since the resistor R1 is selected in the example circuit, the value of the output voltage can be set with this resistor.
The output currents of such integrated circuits can be increased by using power transistors. For this, the principles used in constant voltage sources are used. Figure 1.18 shows a circuit that can deliver high output currents.
In this circuit, some of the output current is taken over the T1 transistor so that damage to the inverter is avoided and the output current capacity is increased. Over-current protection has also been done on the same circuit with the help of T2 transistor and RSC resistors. The value of the RSC resistor is selected according to the specified maximum output current value. The R1 resistor in the circuit is used to adjust the value of the output voltage. Negative adjustable voltage sources are also used depending on the same bases.
A symmetrical dc voltage source example is given in figure-1.19. This type of symmetrical voltage sources is called “Dual-Tracking” voltage regulators. In this circuit, the positive and negative voltage regulators are connected in such a way that the output voltages always show the same (absolute) absolute values relative to the chassis.
For example; if the positive output voltage is reduced by 10mV during load current changes, then 10mV is automatically reduced at negative voltage at the same time. Thus, the output voltages are always symmetrical with respect to the earth (chassis).
Manufacturers have produced and are continuing to produce more adjustable voltage regulators for different types of power and types. Detailed information and examples of these ICs can be obtained from the catalogs.