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LM317M Folha de dados(PDF) 5 Page - STMicroelectronics |
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LM317M Folha de dados(HTML) 5 Page - STMicroelectronics |
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5 / 11 page  APPLICATION INFORMATION The LM317M provides an internal reference voltage of 1.25V between the output and adjustments terminals. This is used to set a constant current flow across an external resistor divider (see fig. 3), giving an output voltage VO of: VO =VREF (1 + R2 R1 ) +IADJ R2 The device was designed to minimize the term IADJ (100 µA max) and to maintain it very constant with line and load changes. Usually, the error term IADJ ⋅R2 can be neglected. To obtain the previous requirement, all the regulator quiescent current is returned to the output terminal, imposing a minimum load current condition. If the load is insufficient, the output voltage will rise. Since the LM317M is a floating regulator and ”sees” only the input-to-output differential voltage, supplies of very high voltage with respect to ground can be regulated as long as the maximum input-to-output differential is not exceeded. Furthermore, programmable regulator are easily obtainable and, by connecting a fixed resistor between the adjustment and output, the device can be used as a precision current regulator. In order to optimise the load regulation, the current set resistor R1 (see fig. 3) should be tied as close as possible to the regulator, while the ground terminal of R2 should be near the ground of the load to provide remote ground sensing. EXTERNAL CAPACITORS (Fig.5) Normally no capacitors are needed unless the device is situated far from the input filter capacitors; in which case an input bypass is needed. A 0.1 µF disc or 1µF tantalium input bypass capacitor (CI) is recommended to reduce the sensitivity to input line impedance. The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor (Cadj) prevents ripple from being amplified as the output voltage is increased. A 10 µF capacitor should improve ripple rejection about 80dB at 120Hz an a 10V application. Although the LM317M is stable with no output capacitance like any feedback circuit, certain values of external capacitance can cause excessive ringing. An output capacitance (CO)in the form of a 1 µF tantalium or 25µF aluminium electrolytic capacitor on the output swamps this effect and insures stability. PROTECTION DIODES (Fig.5) When external capacitors are used with any IC regulator it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Figure 5 show the LM317M with the recommended protection diodes for output voltages in excess of 25V or high capacitance values (C3 >25 µF, C2 >10µF). Diode D1 prevents C3 from discharging through the IC during an input short-circuit. The combination of diodes D1 and D2 prevents C2 from discarging through the regulator during an input or output short-circuit. START-UP BLOCK It’s not guaranteed the Re-Boot of the device when the junction temperature is over 85 oC. LM217M/LM317M 5/11 |
Nº de peça semelhante - LM317M |
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Descrição semelhante - LM317M |
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