Fixed Voltage Regulators

Positive voltage regulator series with seven voltage options:

The 7800 series consists of three-terminal positive voltage regulators with seven voltage options. [see Figure 1 (a).]

These ICs are designed as fixed voltage regulators and with adequate heat sinking can deliver output currents in excess of 1 A. Although these devices do not require external components, such components can be used to obtain adjustable voltages and currents. These ICs also have internal thermal overload protection and internal short-circuit current limiting.

As shown in Figure 1 (c), proper operation requires a common ground between input and output voltages. In addition, the difference between input and output voltages (V_in - V_out), called dropout voltage, must be typically 2.0 V even during the low point on the input ripple voltage. Furthermore, the capacitor C_i is required if the regulator is located an appreciable distance from a power supply filter. Even though C_o is not needed, it may be used to improve the transient response of the regulator.

Typical performance parameters for voltage regulators are line regulation, load regulation, temperature stability, and ripple rejection. Line or input regulation is defined as the change in output voltage for a change in the input voltage and is usually expressed in millivolts or as a percentage of output voltage for a change in load current and is also expressed in millivolts or as a percentage of output voltage V_o. Temperature stability or average temperature coefficient of output voltage (TCV_o) is the change in output voltage per unit change in temperature and is expressed in either millivolts/^oC or parts per million (ppm)/^oC. Ripple rejection is the measure of a regulator's ability to reject ripple voltages. It is usually expressed in decibels. The smaller the values of line regulation, load regulation, and temperature stability, the better the regulator.

The 7800 regulators can also be used as current sources. Figure 1 (d) shows a typical connection diagram of the 7805C as a 0.5 A current source. The current supplied to the load is given by the equation

I_L = V_R / R + I_Q

where I_Q = quiscent current (amperes)
               = 4.3 mA typically for the 7805C

Referring to Figure 1 (d), V_R = V₂₃ = 5 V and R = 10 ohm; therefore,

I_L = 0.5 A

The output voltage V_o with respect to ground is

V_o = V_R + V_L

where V_L = I_LR_L. The load resistance R_L = 10 ohm; hence V_L = 5 V. Therefore, V_o = 10 V. Since the dropout voltage for the 7805C is 2 V, the minimum input voltage required is given by equation

V_in = V_o + (dropout voltage)
       = 12 V

In short, a current source circuit using a voltage regulator can be designed for a desired value of load current (I_L) simply by selecting an appropriate value for R. Note, however, that V_in depends on size of R_L and also the dropout voltage of the regulator.

Negative voltage regulator series with nine voltage options:

The 7900 series of fixed output negative voltage regulators are complements to the 7800 series devices. These negative regulators are available in the same seven- voltage options as the 7800 devices. In addition, two extra voltage options, -2 V and - 5.2 V, are also available in the negative 7900 series, as shown in Figure 2 (a).

Figure 2 (b) shows the package types in which the 7900 series voltage regulators are available.