Electronic Circuits and Diagram-Electronics Projects and Design

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Three terminal voltage regulators

3 terminal ic voltage regulator

This series is typical of the three-terminal voltage regulators. The block diagram is shown in figure. The built-in reference voltage V_REF drives the non-inverting input of an op-amp. The operational amplifier consists of several stages of volt­age gain. Because of the high gain of the op-amp, the error voltage between the two input terminals of the op-amp approaches zero. So the inverting input voltage is approximately equal to V_REF. This means that current flowing through the potential divider

I = V_REF / R’₂

Incidentally, the prime attached to R’₂ indicates that this resistor is inside the IC itself, rather than being an external resistor. Since the same current flows through R’_1, the output voltage is given as

V_out = V_REF / R’₂ (R’₁ + R’₂)

Thus the output voltage of the regu­lator can be precisely controlled by set­ting up the desired values of R’₁ and R’_2.

The chip includes a pass transistor that can handle more than 1.5 A of load current provided that adequate heat sinking is used. Also included are thermal shutdown and current limiting. Thermal shutdown means that the chip will automatically turn itself off if the internal temperature becomes excessive, around 175° C. This is a precaution against excessive power dissipation that depends on the ambient temperature, type of heat sink­ing, and other variables. Because of thermal shutdown and current limiting, devices in the LM 340 series are almost indestructible.

LM 340-15 IC

Figure shows an LM 340-15 connected as a fixed voltage regulator, P_in 1 is the input, pin 2 is the output, and pin 3 is ground.

When an IC is more than a few cm from the filter capacitor of the unregulated power supply, the lead inductance may give rise to oscillations within the IC, Capacitor C₁ is required on pin 1 because of only this reason. Capacitor C₂ is sometimes used to improve the transient response of the circuit. Refer figure.

Any device in the LM 340 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.