SG117A/SG117SERIES
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.
For convenience, a table of standard 1ꢀ resistor values is shown
below.
Table of ½ꢀ and 1ꢀ Standard Resistance Values
1.00
1.02
1.05
1.07
1.10
1.13
1.15
1.1±
1.21
1.24
1.27
1.30
1.33
1.37
1.40
1.43
1.47
1.50
1.54
1.5±
1.62
1.65
1.69
1.74
1.7±
1.±2
1.±7
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.±0
2.±7
2.94
3.01
3.09
3.16
3.24
3.32
3.40
3.4±
3.57
3.65
3.74
3.±3
3.92
4.02
4.12
4.22
4.32
4.42
4.53
4.64
4.75
4.±7
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.±1
6.9±
7.15
7.32
7.50
7.6±
7.±7
±.06
±.25
±.45
±.66
±.±7
9.09
9.31
9.53
9.76
FIGURE 17 - BASIC REGULATOR CIRCUIT
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.
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 ±4ꢀ. This tolerance is dangerously close to the ±5ꢀ
supply tolerance required in many logic and analog systems.
Further, many 1ꢀ resistors can drift 0.01ꢀ°C adding another 1ꢀ
to the output voltage tolerance.
For example, using 2ꢀ resistors and ±4ꢀ tolerance for VREF,
calculations will show that the expected range of a 5V regulator
design would be 4.66V ≤ VOUT ≤ 5.36V or approximately ±7ꢀ. If
the same example were used for a 15V regulator, the expected
tolerance would be ±±ꢀ. ꢁith these results most applications
require some method of trimming, usually a trim pot. This solution
is expensive and not conducive to volume production.
FIGURE 1± - CONNECTIONS FOR BEST LOAD REGULATION
One of the enhancements of Silicon General’s adjustable regu-
lators over existing devices is tightened initial tolerance. This
allows relatively inexpensive 1ꢀ or 2ꢀ film resistors to be used
for R1 and R2 while setting output voltage within an acceptable
tolerance range.
ꢁith a guaranteed 1ꢀ reference, a 5V power supply design,
using ±2ꢀ resistors, would have a worse case manufacturing
tolerance of ±4ꢀ. If 1ꢀ resistors were used, the tolerance would
drop to ±2.5ꢀ. A plot of the worst case output voltage tolerance
as a function of resistor tolerance is shown on the front page.
FIGURE 19 - 1.2V-25V ADJUSTABLE REGULATOR
2/93 Rev 1.2 10/02
LINFINITY Microelectronics Inc.
Copyright 1994
11861 Western Avenue ∞ Garden Grove, CA 92841
(714) 898-8121 FAX: (714) 893-2570
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MICRO-ELECTRONICS [ Micro Electronics ]
