LM317EMPRG

LM317 is an adjustable 3-terminal positive voltage regulator capable of supplying more than 1.5A over an output voltage range of 1.2V to 37V. This regulator is very easy to use, requiring only two external resistors to set the output voltage. It also incorporates internal current limiting, thermal shutdown, and safe operating area compensation, making it essentially immune to blown fuses. The LM317 is a floating 3-terminal regulator. In operation, the LM317 establishes and maintains a nominal 1.25V reference voltage Vref between the output and adjustment terminals. This reference voltage is converted by R1 into a programming current IPROG, which flows through R2 to ground. The regulated output voltage is given by: Vout = Vref(1 + R2/R1) + Iadj × R2. Since the adjustment pin current Iadj represents an error term in the equation, the LM317 is designed to keep Iadj below 100uA and maintain it constant. To achieve this, all quiescent operating current is returned to the output terminal. This requires a minimum load current; if the load current falls below this minimum, the output voltage will rise. Because the LM317 is an adjustable regulator, only the voltage differential across the device is critical to performance, making operation at high voltages above ground possible. The LM317 provides good load regulation, but several points should be noted to achieve optimal performance: Programming resistor R1 should be connected as close to the regulator as possible to minimize line voltage drops in series with the reference voltage and avoid degraded regulation. The ground end of R2 can be returned near the load ground to provide remote ground sensing and improve load regulation. A 0.1uF ceramic capacitor or 1.0uF capacitor is recommended as the input bypass capacitor (Cin) to reduce sensitivity to input supply impedance. Ripple rejection can be improved by bypassing the adjustment terminal to ground. Capacitor CAdj prevents ripple amplification when the output voltage is raised; in a 10V application, a 10uF capacitor can improve ripple rejection by approximately 15dB at 120Hz. Although the LM317 is stable without an output capacitor, certain values of external capacitance, as with other feedback circuits, can cause excessive oscillation. A 1.0uF capacitor or 25uF aluminum electrolytic capacitor used as output capacitor Co will eliminate this problem and ensure stability. When external capacitors are used with any basic circuit regulator, protection diodes must sometimes be added to prevent the capacitors from discharging into the regulator at low current levels. Diode D1 prevents Co from discharging through the IC during an input short circuit. Diode D2 prevents CAdj from discharging through the IC during an input short circuit. The combination of diodes D1 and D2 prevents CAdj from discharging through the IC during an input short circuit.

Applications

• Local regulation
• On-card regulation
• Precision current regulator
• Output voltage regulator

Key features

• Adjustable output voltage down to 1.2V.
• Maximum 1.5A output current.
• Output short-circuit protection.
• Overcurrent and overtemperature protection.
• Pass transistor safe operating area protection.
• Standard 3-terminal transistor package.