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LM338 Folha de dados(PDF) 7 Page - Linear Technology |
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LM338 Folha de dados(HTML) 7 Page - Linear Technology |
7 / 14 page LT138A/LT338A LM138/LM338 7 138afd For more information www.linear.com/LT138A applicaTions inForMaTion 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 1.21Ω, 12.1Ω, 121Ω, 1.21k etc. Bypass Capacitors Input bypassing using a 1µF tantalum or 25µF electrolytic is recommended when the input filter capacitors are more than 5 inches from the device. Improved ripple rejection (80dB) can be accomplished by adding a 10µF capacitor from the ADJ pin to ground. Increasing the size of the capacitor to 20µF 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 pos- sibility of oscillation due to unknown reactive load, a 1µF 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 100µF 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. Figure 2 LT338A D1 1N4002 VIN R1 NOT NEEDED 138A/338A F02 VOUT ADJ VOUT VIN R2 CADJ 10μF COUT 100μF + Load Regulation Because the LT138A is a three-terminal device, it is not possible to provide true remote load sensing. Load regula- tion will be limited by the resistance of the wire connecting 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 connection, with the bottom of the output divider returned to the negative side of the load. Although it may not be immediately obvious, best load regulation is obtained when the top of the resis- tor divider, R1, is connected directly to the case not to the load. This is illustrated in Figure 3. If R1 were connected to the load, the effective resistance between the regulator and the load would be: RP R2+R1 R1 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟,RP = Parasitic Line Resistance Connected as shown, RP is not multiplied by the divider ratio.RPisabout0.004Ωperfootusing16gaugewire.This translates to 4mV/ft at 1A load current, so it is important to keep the positive lead between regulator and load as short as possible, and use large wire or PC board traces. |
Nº de peça semelhante - LM338 |
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Descrição semelhante - LM338 |
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