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LM-338K データシート(PDF) 6 Page - Linear Technology |
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LM-338K データシート(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LT138A/LT338A LM138/LM338 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 required some method of trimming, usually a trim pot. This solution is both expen- sive and not conductive to volume production. One of the enhancements of Linear Technology’s adjust- able regulators over existing devices is the tightened initial tolerance of VREF. This allows relatively inexpen- sive 1% or 2% film resistors to be used for R1 and R2 to set the output voltage within an acceptable tolerance. With a guaranteed 1% reference, a 5V power supply design, using ±2% resistors, would have a worst-case manufacturing tolerance of ±4%. If 1% resistors are used, the tolerance will 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 of this data sheet. For convenience, a table of standard 1% resistor values is shown in Table 1. Table 1. 0.5% 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 1.21 Ω, 12.1 Ω, 121Ω, 1.21k etc. Bypass Capacitors Input bypassing using a 1 µF tantalum or 25µf electrolytic is recommended when the input filter capacitors are more than 5 inches from the device. Improved ripple rejection APPLICATIONS INFORMATION (80dB) can be accomplished by adding a 10 µF capacitor from the ADJ pin to ground. Increasing the size of the capacitor to 20 µF will help ripple rejection at low output voltage since the reactance of this capacitor should be small compared to the voltage setting resistor, R2. For improved AC transient response and to prevent the possi- bility of oscillation due to unknown reactive load, a 1 µF capacitor is also recommended at the output. Because of their low impedance at high frequencies, the best type of capacitor to use is solid tantalum. Protection Diodes The LT138A/LT338A do not require a protection diode from the adjustment terminal to the output (see Figure 2). Improved internal circuitry eliminates the need for this diode when the adjustment pin is bypassed with a capaci- tor to improve ripple rejection. If a very large output capacitor is used, such as a 100 µF shown in Figure 2, the regulator could be damaged or destroyed if the input is accidentally shorted to ground or crowbarred, due to the output capacitor discharging into the output terminal of the regulator. To prevent this, a diode D1 as shown, is recommended to safely discharge the capacitor. LT338A D1 1N4002 VIN R1 NOT NEEDED 138A/338A F02 VOUT ADJ VOUT VIN R2 CADJ 10 µF COUT 100 µF + Figure 2 Load Regulation Because the LT138A is a three-terminal device, it is not possible to provide true remote load sensing. Load regu- lation will be limited by the resistance of the wire connect- ing the regulator to the load. The data sheet specification for load regulation is measured at the bottom of the package. Negative side sensing is a true Kelvin connec- tion, with the bottom of the output divider returned to the |
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同様の説明 - LM-338K |
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