The three-terminal TL431 voltage regulator is commonly found in switching power converters and instrumentation as a voltage reference. It is seemingly simple, yet has subtleties, including missing parameters in the specifications. This six-part series not only covers the TL431 as such but also demonstrates multiple depths of analog understanding. The parts of the series proceed from snorkeling, to SCUBA diving, to pressure suits and even bathyscaphs.
The TL431 voltage reference and shunt regulator was introduced long ago by TI and multiple sources exist. Its circuit diagram is shown below with its TO-92 and SMD pinouts.
The op-amp symbol is followed by an inverting transistor so that the output (pin 3) voltage is inverted from the op-amp input polarity. The input of the amplifier is differential and is the voltage difference between the reference of VR = 2.495 V ≈ 2.5 V (relative to the pin 2 common terminal) and the pin 1 voltage. The amplifier is a transconductance amplifier for which its output is the pin 3 current and is positive (into the pin 3 terminal, following both port convention and the actual total current direction; total = static + incremental). Consequently, a pin 1 increase in voltage at the + input of the amplifier symbol causes an increase in output current. If this current is dropped across a load resistor, RL , then the pin 3 output voltage change will be inverted relative to the pin 1 voltage change.
As an error amplifier in the forward path of a feedback loop, VR is the + input to the summing block, Σ , of the loop while υB is the feedback-path input voltage at the – input of the summer. Consequently, to simplify polarity considerations, it is convenient to use the following symbology: on the left is the TL431 circuit and on the right is the functional (block-diagram) equivalent.
The differential transconductance amplifier on the left has gain
The error voltage of the feedback loop is υ E when the differential input functions as the summer. Then
and the output current is
The output voltage developed across the resistance of the output node, rout , is thus
The full feedback block diagram is shown below, where G = Gm x rout . H is the feedback path and is υB /υO .
The TL431 output is also its positive supply terminal. Consequently, to provide adequate bias current, the output current range is limited to be above 1 mA (1.5 mA in some implementations) for linear response of the amplifier. So far, the part seems simple enough.