Lecture-3

# Lecture-3 - UNIVERSITY OF CALIFORNIA SAN DIEGO Department...

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UNIVERSITY OF CALIFORNIA, SAN DIEGO Department of Electrical and Computer Engineering Bang-Sup Song 1 ECE163 Lecture #3: Voltage Regulator Voltage regulator is a constant voltage source with a high current capacity to drive a low impedance load over process, temperature, and supply variations. A full-wave rectifier followed by a voltage regulator is shown in Fig. 3.1. The output of a full-wave rectifier has a ripple when loaded. The function of the regulator is to keep the output voltage constant. A key factor in the voltage regulator design is to keep its output resistance small so that the output voltage may stay constant regardless of the load resistance. Voltage Regulator Load 120V 60Hz Fig. 3.1: Voltage regulator function. Regulator Load R o R L V Thev V o Fig. 3.2: Equivalent circuit of a voltage regulator. In Fig. 3.2, V Thev is a Thevein equivalent voltage of the regulator, and R o is its output resistance. Ideally, R o = 0. However, due to finite output resistance R o , the actual output voltage V o is a little lower than V Thev . V o = R L R o + R L V Thev (3.1) Voltage regulator consists of two functional blocks. One is a voltage reference, and the other is an error amplifier. The former provides a constant voltage independent of any environmental factors. The latter is a feedback amplifier similar to an opamp to lower the output resistance of the regulator. It detects the difference between the reference and the actual output, and feeds the error back to correct it. Therefore, the regulator output resistance is reduced by the shunt feedback loop gain. There are two voltage references

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Department of Electrical and Computer Engineering Bang-Sup Song 2 ECE163 commonly used in integrated circuits. One is the Zener reference, and the other is the bandgap reference. Zener Reference Volatge If a PN junction diode is reverse-biased as shown in Fig. 3.3, a high current flows as the reverse-biased junction breaks down at 6 to 7V. In standard IC processes, the avalanche breakdown voltage is typically 6 ~ 7V range, and this reverse-biased diode is commonly called Zener diode. In fact, this high-field avalanche breakdown occurs in typical high-doping processes. The Zener breakdown is a tunneling effect in low-doping processes, and typically happens at 4 ~ 5V. Diodes based on both avalanche and Zener breakdowns are called as Zener diode. The avalanche breakdown voltage has a positive temperature coefficient (TC) of about 2.5mV/ o C while the Zener breakdown voltage has a negative TC of –2.5mV/ o C as sketched in Fig. 3.4. I V p+ n p - + - V + - I V V Z (6~7V) I s ~10 - 15 A Fig. 3.3: Zener diode and its I-V characteristic. V
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Lecture-3 - UNIVERSITY OF CALIFORNIA SAN DIEGO Department...

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