SG117 / 217 / 317 / 117A / 217A / 317A
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
Copyright © 1994
Rev. 1.3, 2006-02-08
WWW.Microsemi .COM
1.5A Three Terminal Adj. Voltage Regulator
TM
®
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, R
1
between these two terminals, a
constant current is caused to flow through R
1
and down
through R
2
to set the overall output voltage, Normally this
current is the specified minimum load current of 5mA or
10mA.
2ADJ
1
2
REFOUT
RI
R
R
1VV •+=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
Figure 17 – Basic Regulator Circuit
Because I
ADJ
is very small and constant when compared
with the current through R
1
, 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 V
REF
. 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 <
V
OUT
< 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 R
1
and R
2
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
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
S
S
