SLCS144E July 2004 – October 2014 LM317L
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The two output resistors are the only components required to adjust VOUT.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Input voltage range||(Output Voltage + 2.5 V) to 32 V|
|Output voltage||VREF × (1 + R2 / R1) + IADJ × R2|
An input capacitor is not required, but it is recommended, particularly if the regulator is not in close proximity to the power-supply filter capacitors. A 0.1-µF ceramic or 1-µF tantalum provides sufficient bypassing for most applications, especially when adjustment and output capacitors are used.
An output capacitor improves transient response, but it not needed for stability.
The feedback resistor set the output voltage using Equation 2.
The optional adjustment pin capacitor will improve ripple rejection by preventing the amplification of the ripple. When capacitor is used and VOUT > 6 V, a protection diode from adjust to output is recommended.
Common Linear Regulator designs are concerned with the following parameters:
VO is calculated as shown in Equation 3.
Because IADJ typically is 50 µA, it is negligible in most applications.
CADJ is used to improve ripple rejection; it prevents amplification of the ripple as the output voltage is adjusted higher. If CADJ is used, it is best to include protection diodes.
If the input is shorted to ground during a fault condition, protection diodes provide measures to prevent the possibility of external capacitors discharging through low-impedance paths in the IC. By providing low-impedance discharge paths for CO and CADJ, respectively, D1 and D2 prevent the capacitors from discharging into the output of the regulator.
C2 helps to stabilize the voltage at the adjustment pin, which will help reject noise. Diode D1 exists to discharge C2 in case the output is shorted to ground.
In the 0-V to 30-V regulator circuit application, the output voltage is determined by Equation 4.
This application will limit the output current to the ILIMIT shown in Figure 5.
The tracking preregulator circuit application keeps a constant voltage across the second LM317L in the circuit.
The capacitor C1, in combination with the PNP transistor, helps the circuit to slowly start supplying voltage. In the beginning, the capacitor is not charged. Therefore, output voltage will start at 1.9 V, as determined by Equation 5. As the capacitor voltage rises, VOUT will rise at the same rate. When the output voltage reaches the value determined by R1 and R2, the PNP will be turned off.
The current-limit operation mode can be used to trickle charge a battery at a fixed current as determined by Equation 6. VI should be greater than VBAT + 3.75 V.
As the charge current increases, the voltage at the bottom resistor increases until the NPN starts sinking current from the adjustment pin. The voltage at the adjustment pin will drop, and consequently the output voltage will decrease until the NPN stops conducting.
This application allows higher currents at VOUT than the LM317L device can provide, while still keeping the output voltage at levels determined by the adjustment-pin resistor divider of the LM317L.