A three-terminal regulator is a compact, easy-to-use, fixed-voltage regulator packaged in a single integrated circuit. To use the regulator, it is necessary only to make external connections to the three terminals: Vin, Yo, and ground. These devices are widely used to provide local regulation in electronic systems that may require several different supply voltages. For example, a 5-V regulator could be used to regulate the power supplied to all the chips mounted on one printed-circuit board, and a 12-Y regulator could be used for a similar purpose on a different board.
The regula torn ‘\~:2.ntwell use the same unregulated input voltage, say, 20 V. A popular series of three-terminal regulators is the 780017900 series, available from several manufacturers with a variety of output voltage ratings.National Semiconductor specifications for their 78oo-series regulators, which carry the company’s standard LM prefix and which arc available with regulated outputs of +5 Y, +12 Y, and +15 Y. The last two digits of the 7800 number designate the rated output voltage. For example, the 7805 is a + 5-V regulator and the 7815 is a +15-Y regulator. The 7900-series regulators provide negative output voltages. Notice that the integrated circuitry shown in the schematic diagram is considerably more complex than that of the simple discrete circuits discussed earlier. It can be seen that the circuit incorporates a zener diode as an internal voltage reference. The 780017900 series also has internal current-limiting circuitry. Important points to note in the 7800-scries specifications include the following:t. The output voltage of an arbitrarily chosen device might not exactly equal its nominal value. For example, with a 23-Y input, the 7815 output may be anywhere from 14.4 V’to 15.6 Y. This specification does not mean that the output voltage of a single de, .ce will vary over that range, but that one 7815 chosen at random from a large number will hold its output constant at some voltage within that range.
2. The input voltage cannot exceed 35 V and must not fall below a certain minimum value, depending on type number, if output regulation is to be maintained. The minimum specified inputs are 7.3, 14.6, and 17.7 V for the 7805, 7812, and 7815, respectively.
3. Load regulation is specified as a certain output voltage change (il Vo) as the load current (10) is changed over a certain range. For example, the output of the 7805 will change a maximum of 50 mV as load current changes from 5 mA to 1.5 A, and will change a maximum of 25 mV as load current changes from 250 mA to 750 mA.
Input whose de value may have long-term variations between 19 V and 23 V.
1. What is the maximum peak-to-peak ripple voltage that can be tolerated on the input? ‘
2. What maximum peak-to-peak ripple voltage could appear in the output of the regulator if the input has the ripple found in (1)? (Assume that the de value of the input is 23 V and the load current is 0.5 A.)
Solution
1. The ripple voltage musf not cause the input to fall below the minimum required to maintain regulation, which is 17.7 V for the 7815. Under worst-case conditions, when the de value of the input is 19 V, the decrease in Vin due to ripple cannot, therefore, exceed 19 – 17.7 = 1.3 V. Thus, the maximum tolerable peak-to-peak ripple is 2(1.3 V) = 2.6 V p-p.
2. The 7815 specifications show that ripple rejection under the given operating conditions is a minimum of 54 dB. Thus,
ADJUSTABLE INTEGRATED·CIRCUIT REGULATORS
As the name implies, an adjustable voltage regulator is one that can be set to maintain any output voltage that is within some prescribed range. Unlike threeterminal regulators, adjustable Ie regulators must have external components connected to them to perform voltage regulation.
The 723 integrated-circuit regulator is an example of a popular and very versatile adjustable regulator. It can be connected to produce positive or negative outputs over the range from 2 to 37 V, provide either current limiting or fold back limiting, be used with an external pass transistor to handle load currents up to 10 A, and be used as a switching regulator. Figure 17-43 is a block diagram of the regulator.
Note the terminal labeled VHEF at the output of the voltage reference amplifier. This is an internally generated voltage of approximately 7 V that is available at an external pin. To set a desired regulator output voltage, the user connects this 7-V output, or an externally divided-down portion of it, to one of the inputs of the
error amplifier. The error amplifier is a comparator that compares the externally connected reference to a. v.oltage proportional to Vo’ Depending on whether the reference is connected to the non inverting or the inverting input. the’ regulated output is either positive or negative. For normal positive voltage regulation, the unregulated input is connected to the terminals labeled VI and Vc, and V- is connected to ground,
Note the transistor labeled “current limiter” in Figure 17-43. By making external resistor connections to the CL and CS terminals, either the current-limiting circuit of Figure 17-32 or the fold back circuit of Figure 17-35 can be implemented. The current limiter performs the function of Q2 in each of those figures. Of course, the terminals can be left open if no limiting is desired.
Manufacturer’s specifications for the 723 regulator, along with some typical applications. Figure 17 -45 shows the 723 regulator connected to maintain its output at any voltage between -:-2 V and +7 V. The output is determined from From the specifications in Figure 17-44, we see that VREf· may be between 6.95 V and 7.35 V. Therefore, the actual value produced by a given device should be
measured before selecting va’ues for R, and R2,·if a very accurate output voltage is required. Notice that the full (undivided) output voltage Vo is fed back to the inverting input (INV) through R3• For maximum thermal stability. R, should be set equal to RI II R2
The circuit shown in Figure 17-45 is connected to provide current limiting, where
The 100-pF capacitor shown in the figure is used to ensure circuit stability. When the circuit is connected to provide foldback limiting, a voltage divider (identified as resistors R3 and R4 in Figure 17-35) is connected across Vour in Figure 17-45. The CL terminal on the 723 regulator is then connected to the middle of the divider, instead of to Your
