When working with operational amplifiers, a common requirement is plus and minus supplies in the 15V range. A positive 15V supply is shown in Figure 6-68 and a negative 15 V supply is shown in Figure 6-69.
The positive supply, shown in Figure 6-68, receives +20 VDC from the rectifier/filter section. This is applied to pins 11 and 12 of the uA723 as well as to the collector of the 2N3055 series-pass transistor. The output voltage is sampled through R1 and R2 providing about 7V with respect to ground at pin 4.
The reference terminal (pin 6) is tied directly to pin 5, the non-inverting input of the error amplifier. For fine trimming of the output voltage, a potentiometer may be installed between R1 and R2. A 100-pF capacitor from pin 13 to pin 4 furnishes gain compensation for the amplifier.
Base drive to the 2N3055 pass transistor is furnished by pin 10 of the uA723. Since the desired output of the supply is 1A, maximum current limit is set to 1.5A by resistor RSC whose value is calculated as:
A 100µF electrolytic capacitor is used for ripple voltage reduction at the output. A 1 kΩ output resistor provides stability for the power supply under no-load conditions. The 2N3055 pass transistor must be mounted on an adequate heat sink since the 3.5W, 25°C rating of the device would be exceeded at 1 A load current.
The negative 15V version of this power supply is shown in Figure 6-69. The supply receives -20 V from the rectifier/filter which is fed to the collector of the Darlington pnp pass transistor, a TIP105. A different uA723 configuration is required when designing a negative regulator. The base drive to the TIP105 is supplied through resistor R5. The base of the TIP105 is driven from pin 9 (VZ terminal), which is the anode of a 6.2V zener diode that connects to the emitter of the uA723 output control transistor.
The method for providing the positive feedback required for fold back action is shown in Figure 6-69. This technique introduces positive feedback by increased current flow through resistors R1 and R2 under short-circuit conditions. This forward biases the base-emitter junction of the 2N2907 sensing transistor, which reduces base drive to the TIP105.
The final percentage of foldback depends on the relative contributions of the voltage drop across R2 and RSC to the base current of the 2N2907 sensing transistor. From the start of base-emitter conduction of the sense transistor to the full shut-off of the TIP105 pass transistor requires a 2µA base current.
The latch condition, or 100% positive feedback, is generated by any change in the input voltage which increases the voltage drop across R2 turning on the sense transistor (2N2907). It can only be reset by breaking the positive feedback path with switch S1. This allows the series pass device to once more be driven in a normal fashion.
R3 and R4 are equal in value and divide the 7.15V reference in half. The resulting 3.6 V reference is tied to the inverting input of the error amplifier. R6 and R7 are connected in series across the output of the power supply. The junction of R6 and R7 furnishes 3.6 V to the non-inverting input of the error amplifier. At this point the output is regulated at – 15V with respect to ground. Resistors R1 and R2 are calculated as follows:
Foldback limiting, as used in this circuit, is advantageous where excessive pass transistor power dissipation is a problem. The TIP105 can tolerate only 2W dissipation in free air at 25OC ambient, so adequate heat sinking is necessary.
*** VSENSE is defined as the base to emitter voltage needed to start turn-on of the 2N2907. From the data sheet this is about 0.5v.