SG117A/SG117 SERIES
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.
For convenience, a table of standard 1% resistor values is shown
below.
Table of �½% and 1% Standard Resistance Values
FIGURE 17 - BASIC REGULATOR CIRCUIT
1.00
1.02
1.05
1.07
1.10
1.13
1.15
1.18
1.21
1.24
1.27
1.30
1.33
1.37
1.40
1.43
1.47
1.50
1.54
1.58
1.62
1.65
1.69
1.74
1.78
1.82
1.87
1.91
1.96
2.00
2.05
2.10
2.15
2.21
2.26
2.32
2.37
2.43
2.49
2.55
2.61
2.67
2.74
2.80
2.87
2.94
3.01
3.09
3.16
3.24
3.32
3.40
3.48
3.57
3.65
3.74
3.83
3.92
4.02
4.12
4.22
4.32
4.42
4.53
4.64
4.75
4.87
4.99
5.11
5.23
5.36
5.49
5.62
5.76
5.90
6.04
6.19
6.34
6.49
6.65
6.81
6.98
7.15
7.32
7.50
7.68
7.87
8.06
8.25
8.45
8.66
8.87
9.09
9.31
9.53
9.76
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 regu-
lators 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.
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
2/93 Rev 1.2
10/02
Copyright
1994
6
11861 Western Avenue
∞
Garden Grove, CA 92841
(714) 898-8121
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FAX: (714) 893-2570
L
INFINITY
Microelectronics Inc.
MICRO-ELECTRONICS [ Micro Electronics ]
