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LM317M Datasheet(PDF) 8 Page - ON Semiconductor |
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LM317M Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 13 page LM317M http://onsemi.com 8 APPLICATIONS INFORMATION Basic Circuit Operation The LM317M is a three−terminal floating regulator. In operation, the LM317M develops and maintains a nominal 1.25 V reference (Vref) between its output and adjustment terminals. This reference voltage is converted to a programming current (IPROG) by R1 (see Figure 19), and this constant current flows through R2 to ground. The regulated output voltage is given by: Vout + Vref 1 ) R2 R1 ) IAdj R2 Since the current from the terminal (IAdj) represents an error term in the equation, the LM317M was designed to control IAdj to less than 100 mA and keep it constant. To do this, all quiescent operating current is returned to the output terminal. This imposes the requirement for a minimum load current. If the load current is less than this minimum, the output voltage will rise. Since the LM317M is a floating regulator, it is only the voltage differential across the circuit which is important to performance, and operation at high voltages with respect to ground is possible. + Vref Adjust Vin Vout LM317M R1 IPROG Vout R2 IAdj Vref = 1.25 V Typical Figure 19. Basic Circuit Configuration Load Regulation The LM317M is capable of providing extremely good load regulation, but a few precautions are needed to obtain maximum performance. For best performance, the programming resistor (R1) should be connected as close to the regulator as possible to minimize line drops which effectively appear in series with the reference, thereby degrading regulation. The ground end of R2 can be returned near the load ground to provide remote ground sensing and improve load regulation. External Capacitors A 0.1 mF disc or 1.0 mF tantalum input bypass capacitor (Cin) 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 mF capacitor should improve ripple rejection about 15 dB at 120 Hz in a 10 V 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 1.0 mF tantalum or 25 mF aluminum electrolytic capacitor on the output swamps this effect and insures stability. Protection Diodes 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 20 shows the LM317M with the recommended protection diodes for output voltages in excess of 25 V or high capacitance values (CO > 25 mF, CAdj > 5.0 mF). Diode D1 prevents CO from discharging thru the IC during an input short circuit. Diode D2 protects against capacitor CAdj discharging through the IC during an output short circuit. The combination of diodes D1 and D2 prevents CAdj from discharging through the IC during an input short circuit. D1 Vin Cin 1N4002 LM317M Vout R1 + CO D2 R2 CAdj 1N4002 Adjust Vout Figure 20. Voltage Regulator with Protection Diodes |
Similar Part No. - LM317M_06 |
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Similar Description - LM317M_06 |
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