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SG117K Datasheet(PDF) 7 Page - Microsemi Corporation |
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SG117K Datasheet(HTML) 7 Page - Microsemi Corporation |
7 / 9 page 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 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, 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. 2 ADJ 1 2 REF OUT R I R R 1 V V • + = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ 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 |
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