Adjustable Voltage Regulators

Adjustable positive voltage regulators

The LM317 series adjustable three-terminal positive voltage regulators. The different grades of regulators in the series are available with output voltage of 1.2 to 57 V and output current from 0.10 to 1.5 A as shown in Figure 1.

The LM317 series regulators are available in standard transistor packages that are easily mounted and handled [see Figure 2].

The three terminals are V_in, V_out and ADJUSTMENT (ADJ). Figure 3 shows a typical connection diagram for the LM317 regulator. From this diagram it is obvious that the LM317 requires only two external resistors to set the output voltage. When configured as shown in Figure 3, the LM317 develops a nominal 1.25 V, referred to as the reference voltage V_REF, between the output and adjustable terminal. This reference voltage is impressed across resistor R1 and, since the voltage is constant, the current I_ADJ is also constant for a given value of R1. Because resistor R1 sets current I₁, the current I_ADJ from the adjustment terminal also flows through the output set resistor R2.

The LM317 is designed such that I_ADJ is very small and constant with line and load changes. The maximum value of adjustment pin current I_ADJ is 100 uA. Thus, referring to Figure 3, the output voltage V_o is

V_o = R₁I₁ + R₂(I₁ + I_ADJ)

where I₁ = V_REF / R₁

         R₁ = current (I₁) set resistor

         R₂ = current (V_o) set resistor

         I_ADJ = adjustment pin current

Substituting the value of I₁ in above equation and rearranging, we get

V_o = V_REF (1 + R₂ / R₁) + I_ADJ R₂

where V_REF = 1.25 V = reference voltage between the output and adjustment terminals.

However, the current I_ADJ is very small (100 uA) and constant. Therefore, the voltage drop across R₂ due to I_ADJ is very small and can be neglected. In short,

V_o = 1.25 (1 + R₂ / R₁)

Equation indicates that the output voltage V_o is a function of R₂ for a given value of R₁ and can be varied by adjusting the value of R₂. The current set resistor R₁ is usually 240 ohm, and to achieve good load regulation it should be tied directly to the output of the regulator rather than near the load.

Normally, no capacitors are needed unless the LM317 is situated far from the power supply filter capacitors, in which case an input bypass capacitor C₁ is needed [see Figure 4]. A 0.1 uF disc or 1 uF tantalum capacitor C₂ can be added to improve transient response. Output capacitors in the range of 1 to 1000 uF of aluminum or tantalum electrolytic are commonly used to provide improved impedance and rejection of transients. In addition, the adjustment terminal can be bypassed with C₃ to obtain very high ripple rejection ratios, which are difficult to achieve with standard three terminal regulators. With a 10 uF bypass capacitor C₃, 80 dB ripple rejection is obtainable at any output level.

When external capacitors are used with the LM317, it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. However, there is no need to use diodes for output capacitors of 25 uF or less. Thus, protection diodes are included for use with outputs greater than 25 V and high values of output capacitance [see Figure 4].

Adjustable negative voltage regulators

The LM337 series of adjustable voltage regulators is a complement to the LM317 series devices. These negative regulators are available in the same voltage and current options as the LM317 devices. Figure shows the different grades of regulators in the series.