National Semiconductor Application Note 178 January 1977 A new 3-terminal adjustable IC power regulator solves many of the problems associated with older, fixed regulators. The LM117, a 1.5A IC regulator is adjustable from 1.2V to 40V with only 2 external resistors. Further, improvements are made in performance over older regulators. Load and line regulation are a factor of 10 better than previous regulators. Input voltage range is increased to 40V and output characteristics are fully specified for load of 1.5A. Reliability is improved by new overload protection circuitry as well as 100% burn-in of all parts. The table below summarizes the typical performance of the LM117. relatively constant with temperature. Over a -55C to +150C temperature range, the current limit only shifts about 10%. TABLE 1. Since the LM117 is a floating voltage regulator, it sees only the input-to-output voltage differential. This is of benefit, especially at high output voltage. For example, a 30V regulator nominally operating with a 38V input can have 70V input transient before the 40V input-to-output rating of the LM117 is exceeded. Output Voltage Range Line Regulation 1.25V-40V 0.01%/V Load Regulation IL = 1.5A 0.1% Reference Voltage 1.25V Adjustment Pin Currrent 50 A Minimum Load Current (Quiescent Current) Temperature Stability 3.5 mA 0.01%/C Current Limit Ripple Rejection 2.2A 80 dB The overload protection circuitry on the LM117 includes current limiting, safe-area protection for the internal power transistor and thermal limiting. The current limit is set at 2.2A and, unlike presently available positive regulators, remains At high input-to-output voltage differentials the safe-area protection decreases the current limit. With the LM117, full output current is available to 15V differential and, even at 40V, about 400 mA is available. With some regulators, the output will shut completely off when the input-to-output differential goes above 30V, possibly causing start-up problems. Finally, the thermal limiting is always active and will protect the device even if the adjustment terminal should become accidentally disconnected. Basic Operation The operation of how a 3-terminal regulator is adjusted can be easily understood by referring to Figure 1, which shows a functional circuit. An op amp, connected as a unity gain buffer, drives a power Darlington. The op amp and biasing circuitry for the regulator is arranged so that all the quiescent current is delivered to the regulator output (rather than ground) eliminating the need for a separate ground terminal. Further, all the circuitry is designed to operate over the 2V to 40V input-to-output differential of the regulator. Applications for an Adjustable IC Power Regulator Applications for an Adjustable IC Power Regulator 00733401 FIGURE 1. Functional Schematic of the LM117 (c) 2002 National Semiconductor Corporation AN007334 current comes out of the adjustment terminal. In operation, the output of the regulator is the voltage of the adjustment terminal plus 1.2V. If the adjustment terminal is grounded, www.national.com AN-178 A 1.2V reference voltage appears inserted between the non-inverting input of the op amp and the adjustment terminal. About 50 A is needed to bias the reference and this AN-178 Basic Operation stant voltage/constant current lab-type supply. Current sensing is done across R1, a 1 resistor, while R2 sets the current limit point. When the wiper of R2 is connected, the 1 sense resistor current is regulated at 1.2A. As R2 is adjusted, a portion of the 1.2V reference of the LM117 is cancelled by the drop across the pot, decreasing the current limit point. At low output currents, current regulation is degraded since the voltage across the 1 sensing resistor becomes quite low. For example, with 50 mA output current, only 50 mV is dropped across the sense resistor and the supply rejection of the LM117 will limit the current regulation (Continued) the device acts as a 1.2V regulator. For higher output voltages, a divider R1 and R2 is connected from the output to ground as is shown in Figure 2. The 1.2V reference across resistor R1 forces 10 mA of current to flow. This 10 mA then flows through R2, increasing the voltage at the adjustment terminal and therefore the output voltage. The output voltage is given by: The 50 A biasing current is small compared to 5 mA and causes only a small error in actual output voltages. Further, it is extremely well regulated against line voltage or load current changes so that it contributes virtually no error to dynamic regulation. Of course, programming currents other than 10 mA can be used depending upon the application. Since the regulator is floating, all the quiescent current must be absorbed by the load. With too light of a load, regulation is impaired. Usually, a 5 mA programming current is sufficient; however, worst case minimum load for commercial grade parts requires a minimum load of 10 mA. The minimum load current can be compared to the quiescent current of standard regulators. 00733402 FIGURE 2. Basic Voltage Regulator Applications An adjustable lab regulator using the LM117 is shown in Figure 2 and has a 1.2V to 25V output range. A 10 mA program current is set by R1 while the output voltage is set by R2. Capacitor C1 is optional to improve ripple rejection so that 80 dB is obtained at any output voltage. The diode, although not necessary in this circuit since the output is limited to 25V, is needed with outputs over 25V to protect against the capacitors discharging through low current nodes in the LM117 when the input or output is shorted. The programming current is constant and can be used to bias other circuitry, while the regulator is used as the power supply for the system. In Figure 3, the LM117 is used as a 15V regulator while the programming current powers an LM127 zener reference. The LM129 is an IC zener with less than 1 dynamic impedance and can operate over a range of 0.5 mA to 15 mA with virtually no change in performance. Another example of using the programming current is shown in Figure 4 where the output setting resistor is tapped to provide multiple output voltage to op amp buffers. An additional transistor is included as part of the overload protection. When any of the outputs are shorted, the op amp will current limit and a voltage will be developed across its inputs. This will turn "ON" the transistor and pull down the adjustment terminal of the LM117, causing all outputs to decrease, minimizing possible damage to the rest of the circuitry. Ordinary 3-terminal regulators are not especially attractive for use as precision current regulators. Firstly, the quiescent current can be as high as 10 mA, giving at least 1% error at 1A output currents, and more error at lower currents. Secondly, at least 7V is needed to operate the device. With the LM117, the only error current is 50 A from the adjustment terminal, and only 4.2V is needed for operation at 1.5A or 3.2V at 0.5A. A simple 2-terminal current regulator is shown in Figure 5 and is usable anywhere from 10 mA to 1.5A. Figure 6 shows an adjustable current regulator in conjunction with the voltage regulator from Figure 2 to make con- www.national.com 00733403 FIGURE 3. Regulator and Voltage Reference to about 3% for a 40V change across the device. An alternate current regulator is shown in Figure 7 using an additional LM117 to provide the reference, rather than an LM113 diode. Both current regulators need a negative supply to operate down to ground. Figure 8 shows a 2-wire current transmitter with 10 mA to 50 mA output current for a 1V input. An LM117 is biased as a 10 mA current source to set the minimum current and provide operating current for the control circuitry. Operating off the 10 mA is an LM108 and an LM129 zener. The zener provides a common-mode voltage for operation of the LM108 as well as a 6.9V reference, if needed. Input signals are impressed across R3, and the curent through R3 is delivered to the output of the regulator by Q1 and Q2. For a 25 resistor, this gives a 40 mA current change for a 1V input. This circuit can be used in 4 mA to 20 mA applications, but the LM117 must be selected for low quiescent current. Minimum operating voltage is about 12V. 2 AN-178 Applications (Continued) 00733404 FIGURE 4. Regulator with Multiple Outputs 00733405 FIGURE 5. 2-Terminal Current Regulator 3 www.national.com AN-178 Applications (Continued) 00733406 FIGURE 6. Adjustable Regulator. Constant Voltage/Constant Current, 10 mA to 1.2A 00733407 FIGURE 7. Adjustable Current Regulator 00733408 FIGURE 8. 10 mA to 50 mA 2-Wire Current Transmitter www.national.com 4 Applications for an Adjustable IC Power Regulator Notes LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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