Power Regulator Notes - National Semiconductor

Applications for an

Adjustable IC Power

Regulator

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 charac-

teristics 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.

TABLE 1.

Output Voltage Range 1.25V–40V

Line Regulation 0.01%/V

Load Regulation I

= 1.5A 0.1%

Reference Voltage 1.25V

Adjustment Pin Currrent 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 dif-

ferential goes above 30V, possibly causing start-up prob-

lems. 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 regu-

lator nominally operating with a 38V input can have 70V

input transient before the 40V input-to-output rating of the

LM117 is exceeded.

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.

A 1.2V reference voltage appears inserted between the

non-inverting input of the op amp and the adjustment termi-

nal. 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,

00733401

FIGURE 1. Functional Schematic of the LM117

National Semiconductor

Application Note 178

January 1977

Applications for an Adjustable IC Power Regulator AN-178

Basic Operation (Continued)

the device acts as a 1.2V regulator. For higher output volt-

ages, 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 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,a5mAprogramming current is suffi-

cient; however, worst case minimum load for commercial

grade parts requires a minimum load of 10 mA. The mini-

mum load current can be compared to the quiescent current

of standard regulators.

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 1Ωdynamic 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 addi-

tional 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. Sec-

ondly, 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 6 shows an adjustable current regulator in conjunc-

tion with the voltage regulator from Figure 2 to make con-

stant voltage/constant current lab-type supply. Current sens-

ing is done across R1, a 1Ωresistor, while R2 sets the

current limit point. When the wiper of R2 is connected, the

1Ωsense 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 de-

graded since the voltage across the 1Ωsensing 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 alter-

nate current regulator is shown in Figure 7 using an addi-

tional 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

25Ωresistor, 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.

00733402

FIGURE 2. Basic Voltage Regulator

00733403

FIGURE 3. Regulator and Voltage Reference

AN-178

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Applications (Continued)

00733404

FIGURE 4. Regulator with Multiple Outputs

00733405

FIGURE 5. 2-Terminal Current Regulator

AN-178

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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

AN-178

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Applications for an Adjustable IC Power Regulator AN-178

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.