In this article we will study special regulators: voltage references and voltage inverters.
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.
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.
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.
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.
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 ≤+V ≤ 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.
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
VI-7660 Applications. Simple negative converter. Figure 3 shows typical connections when the VI-7660 is used as a simple negative converter.
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 ≤
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.
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
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