The TL430 and TL431 are three-terminal ‘programmable’ shunt regulators. The devices are basically the same except the TL431 contains a diode connected between the emitter and collector of the output transistor. The standard symbol and block diagram are shown in Figure 6-39.
The circuit consists of a bipolar op amp driving an npn transistor. The reference on the TL430 is a band-gap reference (not temperature compensated). The TL431 has a true temperature compensated band-gap reference and is more stable and accurate than other shunt regulators. The TL431 also has a diode across the emitter-collector of the npn output transistor. If the cathode goes negative, the diode conducts around the transistor, emulating the
performance characteristics of a normal zener diode. The basic operating characteristics are shown in Figure 6-40. Their excellent thermal stability make these devices extremely attractive as a replacement for high cost, temperature-compensated zeners. As seen in Figure 6-41, the TL431 offers improved characteristics, even at low voltages. Since the TL431 operates as a shunt regulator, it can be used as either a positive or negative voltage reference. The TL431 has an equivalent full-range temperature coefficient of 50 ppm/°C (typical) and has low output noise voltage. Note in the graph (Figure 6-41) that for a nominal 2.495 V reference the curve is essentially flat from O°C to 70°C. Depending upon the zener voltage, the TL431 also has an extremely low dynamic impedance of about 0.2Ω
compared to a standard zener diode’s dynamic impedance of about 30 to 60Ω. A 2.5V reference voltage is developed across R2 as shown in Figure 6-42. Iref, the current input at the reference terminal, is about 10µA. To maintain a steady reference, it is advisable to allow 10µA of current flow through series resistors R1 and R2. This will assure a stable reference voltage independent of Iref variations. The TL431 is available in either the commercial temperature range of 0° – 70°C or the military temperature range of -55° to +125°C.
The circuit in Figure 6-43 uses a TL431 as a regulator to control the base drive to a TIP660 series pass transistor. For good reference stability, a current flow of about 1 mA (I2) though the resistor divider is recommended. A 2.5 V reference voltage is developed across R2, and R1 will develop a voltage drop of 21.5 V. The Darlington power transistor is used because of the reduced base drive requirement of the TIP660 which has a Vbe (max) of about 2 V. The hFE at 2.5 A IC is about 1000, so it would only require about 2.5 mA of base drive to produce 2.5 A of output current. In calculating the value of the current limit resistor, R3, we assume about 7.5 mA of current through the TL431. The value of R3, therefore, would be 600Ω and the current about 10 mA, so a 1/2W resistor will suffice. This is a simple method of designing a medium output current power-supply using only four components plus the series pass transistor.