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LM317L bảng dữ liệu(PDF) 6 Page - National Semiconductor (TI) |
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LM317L bảng dữ liệu(HTML) 6 Page - National Semiconductor (TI) |
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6 / 16 page  Application Hints In operation, the LM317L develops a nominal 1.25V refer- ence voltage, V REF, between the output and adjustment terminal. The reference voltage is impressed across pro- gram resistor R1 and, since the voltage is constant, a con- stant current I 1 then flows through the output set resistor R2, giving an output voltage of Since the 100µA current from the adjustment terminal repre- sents an error term, the LM317L was designed to minimize I ADJ and make it very constant with line and load changes. To do this, all quiescent operating current is returned to the output establishing a minimum load current requirement. If there is insufficient load on the output, the output will rise. EXTERNAL CAPACITORS An input bypass capacitor is recommended in case the regulator is more than 6 inches away from the usual large filter capacitor. A 0.1µF disc or 1µF solid tantalum on the input is suitable input bypassing for almost all applications. The device is more sensitive to the absence of input bypass- ing when adjustment or output capacitors are used, but the above values will eliminate the possibility of problems. The adjustment terminal can be bypassed to ground on the LM317L to improve ripple rejection and noise. This bypass capacitor prevents ripple and noise from being amplified as the output voltage is increased. With a 10µF bypass capaci- tor 80dB ripple rejection is obtainable at any output level. Increases over 10µF do not appreciably improve the ripple rejection at frequencies above 120Hz. If the bypass capaci- tor is used, it is sometimes necessary to include protection diodes to prevent the capacitor from discharging through internal low current paths and damaging the device. In general, the best type of capacitors to use is solid tanta- lum. Solid tantalum capacitors have low impedance even at high frequencies. Depending upon capacitor construction, it takes about 25µF in aluminum electrolytic to equal 1µF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies; but some types have a large decrease in capacitance at frequencies around 0.5MHz. For this reason, a 0.01µF disc may seem to work better than a 0.1µF disc as a bypass. Although the LM317L is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values be- tween 500pF and 5000pF. A 1µF solid tantalum (or 25µF aluminum electrolytic) on the output swamps this effect and insures stability. LOAD REGULATION The LM317L is capable of providing extremely good load regulation but a few precautions are needed to obtain maxi- mum performance. The current set resistor connected be- tween the adjustment terminal and the output terminal (usu- ally 240 Ω) should be tied directly to the output of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regulation. For example, a 15V regulator with 0.05 Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05 Ω xI L. If the set resistor is connected near the load the effective line resis- tance will be 0.05 Ω (1 + R2/R1) or in this case, 11.5 times worse. Figure 2 shows the effect of resistance between the regula- tor and 240 Ω set resistor. With the TO-92 package, it is easy to minimize the resis- tance from the case to the set resistor, by using two separate leads to the output pin. The ground of R2 can be returned near the ground of the load to provide remote ground sens- ing and improve load regulation. THERMAL REGULATION When power is dissipated in an IC, a temperature gradient occurs across the IC chip affecting the individual IC circuit components. With an IC regulator, this gradient can be es- pecially severe since power dissipation is large. Thermal regulation is the effect of these temperature gradients on output voltage (in percentage output change) per watt of power change in a specified time. Thermal regulation error is independent of electrical regulation or temperature coeffi- cient, and occurs within 5ms to 50ms after a change in power dissipation. Thermal regulation depends on IC layout as well as electrical design. The thermal regulation of a voltage regulator is defined as the percentage change of V OUT, per watt, within the first 10ms after a step of power is applied. The LM317L specification is 0.2%/W, maximum. In the Thermal Regulation curve at the bottom of the Typical Performance Characteristics page, a typical LM317L’s out- put changes only 7mV (or 0.07% of V OUT = −10V) when a 1W pulse is applied for 10ms. This performance is thus well inside the specification limit of 0.2%/W x 1W = 0.2% maxi- 00906407 FIGURE 1. 00906408 FIGURE 2. Regulator with Line Resistance in Output Lead www.national.com 6 |
Số phần tương tự - LM317L |
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Mô tả tương tự - LM317L |
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