Texas Instruments | AN-178 Applications for an Adjustable IC Power Regulator (Rev. B) | Application notes | Texas Instruments AN-178 Applications for an Adjustable IC Power Regulator (Rev. B) Application notes

Texas Instruments AN-178 Applications for an Adjustable IC Power Regulator (Rev. B) Application notes
Application Report
SNVA513B – January 1977 – Revised May 2013
AN-178 Applications for an Adjustable IC Power Regulator
.....................................................................................................................................................
ABSTRACT
This application report provides applications for the Texas Instruments LM117 3-terminal adjustable
regulator.
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2
Contents
Basic Operation ............................................................................................................. 2
Applications .................................................................................................................. 4
List of Figures
1
Functional Schematic of the LM117 ...................................................................................... 3
2
Basic Voltage Regulator
3
Regulator and Voltage Reference ........................................................................................ 4
4
Regulator with Multiple Outputs ........................................................................................... 5
5
2-Terminal Current Regulator ............................................................................................. 5
6
Adjustable Regulator
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8
...................................................................................................
.......................................................................................................
Adjustable Current Regulator..............................................................................................
10 mA to 50 mA 2-Wire Current Transmitter ............................................................................
3
6
6
7
List of Tables
1
Typical Performance of the LM117 ....................................................................................... 2
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1
Basic Operation
1
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Basic Operation
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. Table 1 summarizes the typical performance of the LM117.
Table 1. Typical Performance of the LM117
Output Voltage Range
1.25V to 40V
Line Regulation
0.01%/V
Load Regulation IL = 1.5A
0.1%
Reference Voltage
1.25V
Adjustment Pin Current
50 μA
Minimum Load Current
(Quiescent Current)
3.5 mA
Temperature Stability
0.01%/°C
Current Limit
2.2A
Ripple Rejection
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 relatively constant with temperature. Over a −55°C to +150°C
temperature range, the current limit only shifts about 10%.
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.
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.
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.
2
AN-178 Applications for an Adjustable IC Power Regulator
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Basic Operation
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Figure 1. Functional Schematic of the LM117
A 1.2V reference voltage appears inserted between the non-inverting input of the op amp and the
adjustment terminal. About 50 μA is needed to bias the reference and this 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, 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:
(1)
The 50 μA 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.
Figure 2. Basic Voltage Regulator
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Applications
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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 1Ω
dynamic impedance and can operate over a range of 0.5 mA to 15 mA with virtually no change in
performance.
Figure 3. Regulator and Voltage Reference
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.
4
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Applications
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Figure 4. Regulator with Multiple Outputs
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 μA 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 5. 2-Terminal Current Regulator
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Applications
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Figure 6 shows an adjustable current regulator in conjunction with the voltage regulator from Figure 2 to
make constant voltage/constant current lab-type supply. Current sensing is done across R1, a 1Ω resistor,
while R2 sets the current limit point. When the wiper of R2 is connected, the 1Ω 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 1Ω 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 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.
Constant Voltage/Constant Current, 10 mA to 1.2A
Figure 6. Adjustable Regulator
Figure 7. Adjustable Current Regulator
6
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Applications
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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 current through R3 is delivered to the output of the regulator by Q1 and
Q2. For a 25Ω 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.
Figure 8. 10 mA to 50 mA 2-Wire Current Transmitter
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