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VCO using TL431 voltage reference

22 Oct 2013  | R O Ocaya

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The TL431 is a three-terminal programmable shunt regulator that implements Zener-like references with low temperature coefficients. Its output can be programmed from the internally set reference of about 2.5V to 36V using two external resistors. In addition, it exhibits a wide operating current range of 1.0mA to 100mA with a typical dynamic impedance of 0.22Ω. The characteristics of these references make them excellent replacements for zener diodes in many applications such as digital voltmeters, power supplies, and op-amp circuitry, where precision voltage references are needed. Today, they are ubiquitous in switching-mode power supplies.

The TL431 exhibits an interesting instability under certain conditions of input supply voltage and capacitive load, causing sustained oscillations that can range from 10kHz up to 1.5MHz, generally depending on the control input voltage. This occurs partly because of a negative resistance region under those conditions. As shown in this article, the instability neither arises from the presence of two internal poles, nor from a third pole introduced from an external capacitor in series with the load resistance. A single transistor output stage is added to provide buffering, producing a TTL output level over the entire range.

Figure 1: Circuit of the TL431 VCO with output buffered to TTL levels.

Operation of the VCO
The operation of the oscillator can be understood by considering the circuit in two aspects. The first aspect is the underlying action of the TL431 voltage reference. Consider the equivalent circuit of the oscillator shown in figure 2. Current I1 (figure 3) is a voltage-dependent constant current whose magnitude is approximately equal to (VCTRL-VKA)/R (VKA being the "Zener" voltage). Suppose initially that the capacitor is not charged, in which case VKA=0V. The capacitor gradually charges up from a current derived from I1 until it reaches the equilibrium value of the TL431, i.e., VKA=2.49V. Since charging current is still present, the capacitor continues to charge. A transient simulation of the circuit in figure 2 shows that the capacitor voltage need only exceed the equilibrium value of VKA by a few microvolts for the equilibrium restoring feedback of the device to kick in, as follows:

As the base of Q1 is directly connected to the capacitor, an increase in VKA also increases the emitter voltage of Q1 (which is also the base voltage of Q11), forcing Q11 to conduct more. Transistor Q9 and R8 form the collector load of Q11. A rising collector current in Q11 therefore causes the collector voltage of Q9 to fall. Since Q9 and Q10 are part of a current mirror, the two transistors have the same collector current as Q11, but Q10 has a dynamic collector load made up of Q6, which derives its base current through R5 from the second current mirror made up of the three transistors Q2, Q4, and Q12. This current mirror is configured such that the initial increase in the emitter voltage of Q1 also increases their VBE voltage. This affects Q6 by also increasing its collector current, reinforcing the rising collector current in Q10. Therefore the overall effect is an increase in its collector voltage, which is also the base voltage of the first transistor in the Darlington pair Q7 and Q8, forcing Q8 to conduct heavily, causing its collector-emitter voltage (VCE) which in effect is VKA to fall rapidly. In this particular application the reference terminal (R) to which the capacitor is connected is hardwired to the cathode terminal (K). Therefore, thus far, when the capacitor voltage exceeds the equilibrium, the device tends to rapidly lower its cathode-anode voltage to restore equilibrium.

Figure 2: Simulation circuit of the TL431. TL431 equivalent circuit for LTSpice.

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