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SG317

Positive Adjustable Linear Voltage Regulators

Microsemi Corporation 

SG117 / 217 / 317 / 117A / 217A / 317A

TM

®

1.5A Three Terminal Adj. Voltage Regulator

APPLICATION INFORMATION

GENERAL

The SG117A develops a 1.25V reference voltage

between the output and the adjustable terminal (see Figure

1). By placing a resistor, R1 between these two terminals, a

constant current is caused to flow through R1 and down

through R2 to set the overall output voltage, Normally this

current is the specified minimum load current of 5mA or

10mA.

V

OUT

=

V

REF

⎜⎜⎝⎛

1

R

R

2

1

⎟⎟⎠⎞ + IADJ

•R

2

Figure 17 – Basic Regulator Circuit

Because IADJ is very small and constant when compared

with the current through R1, it represents a small error and

can usually be ignored.

It is easily seen from the above equation, that even if the

resistors were of exact value, the accuracy of the output is

limited by the accuracy of VREF. Earlier adjustable

regulators had a reference tolerance of ±4%. This tolerance

is dangerously close to the ±5% supply tolerance required

in many logic and analog systems. Further, many 1%

resistors can drift 0.01%°C adding another 1% to the output

voltage tolerance.

For example, using 2% resistors and ±4% tolerance for

VREF, calculations will show that the expected range of a

5V regulator design would be 4.66V < VOUT < 5.36V or

approximately ±7%. If the same example were used for a

15V regulator, the expected tolerance would be ±8%. With

these results most applications require some method of

trimming, usually a trim pot. This solution is expensive and

not conducive to volume production.

One of the enhancements of Silicon General’s adjustable

regulators over existing devices is tightened initial

tolerance. This allows relatively inexpensive 1% or 2% film

resistors to be used for R1 and R2 while setting output

voltage within an acceptable tolerance range.

With a guaranteed 1% reference, a 5V power supply

design, using ±2% resistors, would have a worse case

manufacturing tolerance of ±4%. If 1% resistors were used,

the tolerance would drop to ±2.5%. A plot of the worst case

output voltage tolerance as a function of resistor tolerance

is shown on the front page.

For convenience, a table of standard 1% resistor values

is shown below

Table of ½% and 1% Standard Resistance Values

1.00 1.47 2.15 3.16 4.64 6.81

1.02 1.50 2.21 3.24 4.75 6.98

1.05 1.54 2.26 3.32 4.87 7.15

1.07 1.58 2.32 3.40 4.99 7.32

1.10 1.62 2.37 3.48 5.11 7.50

1.13 1.65 2.43 3.57 5.23 7.68

1.15 1.69 2.49 3.65 5.36 7.87

1.18 1.74 2.55 3.74 5.49 8.06

1.21 1.78 2.61 3.83 5.62 8.25

1.24 1.82 2.67 3.92 5.76 8.45

1.27 1.87 2.74 4.02 5.90 8.66

1.30 1.91 2.80 4.12 6.04 8.87

1.33 1.96 2.87 4.22 6.19 9.09

1.37 2.00 2.94 4.32 6.34 9.31

1.40 2.05 3.01 4.42 6.49 9.53

1.43 2.10 3.09 4.53 6.65 9.76

Standard Resistance Values are obtained from the

Decade Table by multiplying by multiples of 10. As an

example: 1.21 can represent 1.21Ω, 12.1Ω, 121Ω, 1.21kΩ,

etc.

Figure 18 – Connections for Best Load Regulation

Figure 19 – 1.2V – 25V Adjustable Regulator

Copyright © 1994

Rev. 1.3, 2006-02-08

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 7

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