ECEstream: Voltage Regulators

Special Regulators

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In this article we will study special regulators: voltage references and voltage inverters.</div>
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Voltage references: Voltage references are a special type of voltage regulator that is used for reference purposes as reference voltages. Typical applications of voltage references include D/A and A/D converters that do not have internal references, amplifier biasing, low-temperature-coefficient zener replacements, high-stability current refetences, comparator circuits, and voltmeter system references. Typical examples of voltage references include the following: The ICL8069 is a 1.2 V temperature-compensated voltage reference; the 9495 is a Teledyne 5 V reference; and the MC1403 is a Motorola 2.5 V reference.</div>
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MC1403 Used as a Voltage Reference for DAC MC1408: Figure 1 shows a D/A converter using the MC1403 voltage reference. A stable current reference of nominally 2.0 mA required for the MC1408 is obtained from the MC1403 with the addition of series resistors R1 and R2. Also, resistor R3 improves temperature performance and capacitor Co decouples any noise present on the reference line. A single MC1403 can provide the required current input for up to five of the D/A converters. 
 
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Voltage inverter: Datel's VI-7660 is a monolithic CMOS voltage inverter that provides -1.5 to -10 V from +1.5 to +10V supplies with the addition of only two non critical external capacitors. The block diagram of the VI-7660 is shown in Figure 2, which contains a dc voltage regulator, RC oscillator, voltage-level translator, four output-power MOS switches, and a logic network. The logic network senses the most negative voltage in the device and ensures that the output N-channel switches are not forward biased. This assures latch-up free operation. When unloaded, the oscillator oscillates at a nominal frequency of 10 kHZ for an input supply voltage of 5V. Because of noise or other considerations, it may be desirable to increase the oscillator frequency in some applications. This may be done by overdriving the oscillator frequency in some applications. This may be done by overdriving the oscillator from an external clock. On the other hand, to maximize the conversion efficiency it may be necessary to lower the oscillator frequency. This is achieved by connecting an additional capacitor (typically 100 pF) between pins 7 and 8. 
 
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Typical applications for the VI-7660 include data acquisition and microprocessor-based systems in which a positive supply is available and an additional negative supply is required. The VI-7660 is also ideally suited as on-board negative supply for up to 64 dynamic RAMs (random-access memory ICs). 
 
VI-7660 Applications. Simple negative converter. Figure 3 shows typical connections when the VI-7660 is used as a simple negative converter. 
 
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To improve the low-voltage (LV) operation. that is, when supply voltage + V < 3.5 V, the LV pin may be grounded. However, for + V ≥ 3.5 V, the LV pin is left open to prevent device latch-up. Also, an additional diode D1 must be included for proper operation at higher supply voltages (+ V > 6.5 V) and/or elevated temperatures. When capacitor COSC is used to maximize the conversion efficiency of the device, the oscillator frequency decreases and hence the reactances of C1 and C2 will increase. Therefore, to overcome the increase in their reactances, the values of C1 and C2 must be increased by the same factor that the frequency has been reduced. For example, the addition of COSC = 100 pF between pins 7 and 8 will lower the oscillator frequency from 10 to 1 kHz and will thereby call for an increase in the values of C1 and C2 from 10 to 100 uF. The output voltage equation is 
 
Vo = - (+V)                    for 1.5 ≤ <complete id="goog_118404273">+V </complete>≤ 6.5 V 
Vo = - (+V - Vdi)           for 6.5 ≤ +V ≤ 10 V 
 
where VD1 = forward voltage drop of diode D1 
                   = 0.7 V typically 
 
Negative-voltage multiplier. To produce larger negative multiplication of the initial supply voltage, the VI-7660s may be cascaded as shown in Figure 4. 
 
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The practical limit to the number of devices that can be cascaded is 10 for light loads. The output voltage is given by the equation 
 
Vo = -(n)(+V) 
 
where n = number of devices cascaded and must be ≤ 10 
         +V = input supply voltage</div>
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Special Regulators

In this article we will study special regulators: voltage references and voltage inverters. Voltage references: Voltage references ar...

Switching Regulators

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An example of a general-purpose regulator is Motorola's MC1723. It can be used in many different ways, for example, as a fixed positive or negative output voltage regulator, variable output voltage regulator, or switching regulator. Because of its flexibility, it has become a standard type in electronic industry. Although it is designed to deliver load current to 150 mA, the current capability can be increased to several amperes through the use of one or more external pass transistors. Figure (a) shows the connections for switching-regulator action of the 723-type regulator for positive output. The regulator requires an external transistor and a 1 mH choke. To minimize its power dissipation during switching, the external transistor used must be a switching power transistor. The 1 mH choke smooths out the current pulses delivered to the load while capacitor C holds output voltage at constant dc level. 
 
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Fixed voltage regulators such as the 78XX and 79XX and the adjustable regulators such as the LM317 are all called series dissipative regulators. This is because these regulators simulate a variable resistance between the input voltage and the load, and hence function in a linear mode. In fact, for a specified range of variation in the input voltage and load current, the linear regulator maintains a constant output voltage by dissipating the excess power as heat. In a series dissipative regulator, conversion efficiency decreases as the input/output voltage differential increases, or vice versa. For this reason, the linear series regulator is well suited for medium current applications with a small voltage differential, where the power dissipation can be handled with heat sinks. 
 
To improve the efficiency of a regulator, the series-pass transistor is used as a switch (alternatively turned on and off) rather than as a variable resistor as in the linear mode. A regulator constructed to operate in this manner is called a series switching regulator. In such regulators the series-pass transistor is switched between cutoff and saturation at a high frequency, which produces a pulse-width-modulated (PWM) square wave output. This output is then filtered through a low pass LC filter to produce an average dc output voltage. Thus the output voltage is proportional to the pulse width and frequency. The efficiency of a series switching regulator is independent of the input/output differential and can approach 95%. 
 
Switching regulators come in various circuit configurations including the flyback, feed-forward, push-pull, and non isolated single-ended or single-polarity types. Also, the switching regulators can operate in any of three modes: step-down, step-up, or polarity inverting.</div>
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Switching Regulators

An example of a general-purpose regulator is Motorola's MC1723. It can be used in many different ways, for example, as a fixed positiv...

Adjustable Voltage Regulators

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Adjustable positive voltage regulators</h4>
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The LM317 series adjustable three-terminal positive voltage regulators. The different grades of regulators in the series are available with output voltage of 1.2 to 57 V and output current from 0.10 to 1.5 A as shown in Figure 1. 
 
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The LM317 series regulators are available in standard transistor packages that are easily mounted and handled [see Figure 2]. 
 
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The three terminals are Vin, Vout and ADJUSTMENT (ADJ). Figure 3 shows a typical connection diagram for the LM317 regulator. From this diagram it is obvious that the LM317 requires only two external resistors to set the output voltage. When configured as shown in Figure 3, the LM317 develops a nominal 1.25 V, referred to as the reference voltage VREF, between the output and adjustable terminal. This reference voltage is impressed across resistor R1 and, since the voltage is constant, the current IADJ is also constant for a given value of R1. Because resistor R1 sets current I1, the current IADJ from the adjustment terminal also flows through the output set resistor R2. 
 
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The LM317 is designed such that IADJ is very small and constant with line and load changes. The maximum value of adjustment pin current IADJ is 100 uA. Thus, referring to Figure 3, the output voltage Vo is 
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Vo = R1I1 + R2(I1 + IADJ) 
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where I1 = VREF / R1</div>
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         R1 = current (I1) set resistor</div>
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         R2 = current (Vo) set resistor</div>
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         IADJ = adjustment pin current</div>
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Substituting the value of I1 in above equation and rearranging, we get</div>
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Vo = VREF (1 + R2 / R1) + IADJ R2</div>
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where VREF = 1.25 V = reference voltage between the output and adjustment terminals.</div>
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However, the current IADJ is very small (100 uA) and constant. Therefore, the voltage drop across R2 due to IADJ is very small and can be neglected. In short,</div>
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Vo = 1.25 (1 + R2 / R1)</div>
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Equation indicates that the output voltage Vo is a function of R2 for a given value of R1 and can be varied by adjusting the value of R2. The current set resistor R1 is usually 240 ohm, and to achieve good load regulation it should be tied directly to the output of the regulator rather than near the load.</div>
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Normally, no capacitors are needed unless the LM317 is situated far from the power supply filter capacitors, in which case an input bypass capacitor C1 is needed [see Figure 4]. A 0.1 uF disc or 1 uF tantalum capacitor C2 can be added to improve transient response. Output capacitors in the range of 1 to 1000 uF of aluminum or tantalum electrolytic are commonly used to provide improved impedance and rejection of transients. In addition, the adjustment terminal can be bypassed with C3 to obtain very high ripple rejection ratios, which are difficult to achieve with standard three terminal regulators. With a 10 uF bypass capacitor C3, 80 dB ripple rejection is obtainable at any output level. 
 
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When external capacitors are used with the LM317, it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. However, there is no need to use diodes for output capacitors of 25 uF or less. Thus, protection diodes are included for use with outputs greater than 25 V and high values of output capacitance [see Figure 4].</div>
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Adjustable negative voltage regulators</h4>
 
The LM337 series of adjustable voltage regulators is a complement to the LM317 series devices. These negative regulators are available in the same voltage and current options as the LM317 devices. Figure shows the different grades of regulators in the series.</div>
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Adjustable Voltage Regulators

Adjustable positive voltage regulators The LM317 series adjustable three-terminal positive voltage regulators. The different grades ...

Fixed Voltage Regulators

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Positive voltage regulator series with seven voltage options:</h4>
 
The 7800 series consists of three-terminal positive voltage regulators with seven voltage options. [see Figure 1 (a).] 
 
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These ICs are designed as fixed voltage regulators and with adequate heat sinking can deliver output currents in excess of 1 A. Although these devices do not require external components, such components can be used to obtain adjustable voltages and currents. These ICs also have internal thermal overload protection and internal short-circuit current limiting. 
 
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As shown in Figure 1 (c), proper operation requires a common ground between input and output voltages. In addition, the difference between input and output voltages (Vin - Vout), called dropout voltage, must be typically 2.0 V even during the low point on the input ripple voltage. Furthermore, the capacitor Ci is required if the regulator is located an appreciable distance from a power supply filter. Even though Co is not needed, it may be used to improve the transient response of the regulator. 
 
Typical performance parameters for voltage regulators are line regulation, load regulation, temperature stability, and ripple rejection. Line or input regulation is defined as the change in output voltage for a change in the input voltage and is usually expressed in millivolts or as a percentage of output voltage for a change in load current and is also expressed in millivolts or as a percentage of output voltage Vo. Temperature stability or average temperature coefficient of output voltage (TCVo) is the change in output voltage per unit change in temperature and is expressed in either millivolts/oC or parts per million (ppm)/oC. Ripple rejection is the measure of a regulator's ability to reject ripple voltages. It is usually expressed in decibels. The smaller the values of line regulation, load regulation, and temperature stability, the better the regulator. 
 
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The 7800 regulators can also be used as current sources. Figure 1 (d) shows a typical connection diagram of the 7805C as a 0.5 A current source. The current supplied to the load is given by the equation 
 
IL = VR / R + IQ 
 
where IQ = quiscent current (amperes) 
               = 4.3 mA typically for the 7805C 
 
Referring to Figure 1 (d), VR = V23 = 5 V and R = 10 ohm; therefore, 
 
IL = 0.5 A 
 
The output voltage Vo with respect to ground is 
 
Vo = VR + VL 
 
where VL = ILRL. The load resistance RL = 10 ohm; hence VL = 5 V. Therefore, Vo = 10 V. Since the dropout voltage for the 7805C is 2 V, the minimum input voltage required is given by equation 
 
Vin = Vo + (dropout voltage) 
       = 12 V 
 
In short, a current source circuit using a voltage regulator can be designed for a desired value of load current (IL) simply by selecting an appropriate value for R. Note, however, that Vin depends on size of RL and also the dropout voltage of the regulator. 
 
<h4>
Negative voltage regulator series with nine voltage options:</h4>
 
The 7900 series of fixed output negative voltage regulators are complements to the 7800 series devices. These negative regulators are available in the same seven- voltage options as the 7800 devices. In addition, two extra voltage options, -2 V and - 5.2 V, are also available in the negative 7900 series, as shown in Figure 2 (a). 
 
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Figure 2 (b) shows the package types in which the 7900 series voltage regulators are available. 
 
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Fixed Voltage Regulators

Positive voltage regulator series with seven voltage options: The 7800 series consists of three-terminal positive voltage regulators wi...

Voltage Regulators

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A voltage regulator is a circuit that supplies a constant voltage regardless of changes in load currents. Although voltage regulators can be designed using op-amps, it is quicker and easier to use IC voltage regulators. Furthermore, IC voltage regulators are versatile and relatively inexpensive and are available with features such as a programmable output, current/voltage boosting, internal short-circuit current limiting, thermal shutdown, and floating operation for high-voltage applications. IC voltage regulators are of the following types:</div>
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<li><a href="http://www.ecestream.com/2014/12/fixed-voltage-regulators.html" target="_blank">Fixed output voltage regulators: positive and/or negative output voltage</a></li>
<li><a href="http://www.ecestream.com/2014/12/adjustable-voltage-regulators.html" target="_blank">Adjustable output voltage regulators: positive or negative output voltage</a></li>
<li><a href="http://www.ecestream.com/2014/12/switching-regulators.html" target="_blank">Switching regulators </a></li>
<li><a href="http://www.ecestream.com/2014/12/special-regulators.html" target="_blank">Special regulators</a></li>
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Except for the switching regulators, all other types are of regulators are called linear regulators. The impedance of linear regulator's active element may be continuously varied to supply a desired current to the load. On the other end, in the switching regulator a switch is turned on or off at a rate such that the regulator delivers the desired average current in periodic pulses to the load. Because the switching element dissipates negligible power in either the on or off state, the switching regulator is more efficient than the linear regulator. Nevertheless, in switching regulators the power dissipation is substantial during the switching intervals (on to off or off to on). In addition, most loads cannot accept the average current in periodic pulses. Therefore, most practical regulators are of linear type. 
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Voltage regulators are commonly used for on-card regulation and laboratory-type power supplies. Voltage regulators, especially the switching type, are used as control  circuits in pulse width modulation (PWD), push-pull bridges, and series type switch mode supplies. Almost all power supplies use some type of voltage regulator IC because voltage regulators are simple to use, reliable, low in cost, and, above all, available in a variety of voltage and current ratings.</div>
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Voltage Regulators

A voltage regulator is a circuit that supplies a constant voltage regardless of changes in load currents. Although voltage regulators can...

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Special Regulators

Switching Regulators

Adjustable Voltage Regulators

Fixed Voltage Regulators

Voltage Regulators

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