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Voltage Reference Fundamentals: The Shunt Reference

Maxim Integrated Application Note 4003 continues and explains the fundamentals of the shunt voltage reference:

A shunt reference has two terminals, OUT and GND. It is similar in concept to a zener diode, but has much better specifications. Like a zener diode, it requires an external resistor and operates in parallel with its load (Figure 2). It can be regarded as a voltage-controlled current source between the OUT and GND terminals. Regulation is achieved by adjusting the current level so that VSUPPLY minus the drop across R1 equals the reference voltage at OUT. As an alternate description, the shunt reference maintains a constant voltage at OUT by forcing the sum of load current and current through the reference to be constant. Shunt references have these characteristics:

  • Given an R1 appropriately sized for power dissipation, the shunt reference imposes no limit on the maximum power-supply voltage.
  • The power supply delivers the same maximum current regardless of load. Supply current flows through load and reference, dropping just the right voltage across R1 to maintain the OUT reference voltage.
  • As a simple 2-terminal device, the shunt regulator can be used in novel circuit configurations such as negative regulators, floating regulators, clipping circuits, and limiting circuits.
  • Shunt references generally have lower operating currents than do series references.

Figure 2

Block diagram of the two-terminal shunt voltage reference.

Block diagram of the two-terminal shunt voltage reference.

This is the third fundamental on references leading up to our Ask the Experts session on Wednesday, May 21, at 1:00 p.m. EDT (10:00 a.m. PDT). You can sign in using this link on May 21, and you can even enter questions ahead of time.

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