Shunt-type regulators are connected in parallel with the load by diverting the current to behave as a variable resistor and thus keep the voltage in the circuit constant. The circuit shown in the figure is suggested by Fairchild (www.fairchild.com) and can drain currents from 1 to 100 mA with a programmable output voltage up to 36 V. The output voltage is programmed by the resistive network formed by R1 and R2. The LM431AS integrated circuit is supplied in SOT89 enclosure. In this circuit, the Vref voltage is 2.5 V and the dynamic output impedance is 0.2 ohm.
