LM317SSLASHNOPB datasheet(28/51 Pages) TI

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LM117, LM317-N

SNVS774Q – MAY 2004 – REVISED JUNE 2020

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10 Power Supply Recommendations

The input supply to the LM117 and LM317-N must be kept at a voltage level such that its maximum input to

output differential voltage is not exceeded. The minimum dropout voltage must also be met with extra headroom

when possible to keep the LM117 and LM317-N in regulation. An input capacitor is recommended, especially

when the input pin is located more than 6 inches away from the power supply source. For more information

regarding capacitor selection, refer to External Capacitors.

11 Layout

11.1 Layout Guidelines

Some layout guidelines must be followed to ensure proper regulation of the output voltage with minimum noise.

Traces carrying the load current must be wide to reduce the amount of parasitic trace inductance and the

feedback loop from VOUT to ADJ must be kept as short as possible. To improve PSRR, a bypass capacitor can

be placed at the ADJ pin and must be located as close as possible to the IC. In cases when VIN shorts to ground,

an external diode must be placed from VOUT to VIN to divert the surge current from the output capacitor and

protect the IC. Similarly, in cases when a large bypass capacitor is placed at the ADJ pin and VOUT shorts to

ground, an external diode must be placed from ADJ to VOUT to provide a path for the bypass capacitor to

discharge. These diodes must be placed close to the corresponding IC pins to increase their effectiveness.

11.1.1 Thermal Considerations

11.1.1.1 Heatsink Requirements

The LM317-N regulators have internal thermal shutdown to protect the device from over-heating. Under all

operating conditions, the junction temperature of the LM317-N must not exceed the rated maximum junction

temperature (TJ) of 150°C for the LM117, or 125°C for the LM317-N. A heatsink may be required depending on

the maximum device power dissipation and the maximum ambient temperature of the application. To determine if

a heatsink is needed, the power dissipated by the regulator, PD, must be calculate with Equation 3:

PD = ((VIN − VOUT) × IL) + (VIN × IG)

(3)

Figure 36 shows the voltage and currents which are present in the circuit.

The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX) in Equation 4:

TR(MAX) = TJ(MAX) − TA(MAX)

(4)

where TJ(MAX) is the maximum allowable junction temperature (150°C for the LM117, or 125°C for the LM317-N),

and TA(MAX) is the maximum ambient temperature that will be encountered in the application.

Using the calculated values for TR(MAX) and PD, the maximum allowable value for the junction-to-ambient thermal

resistance (RθJA) can be calculated with Equation 5:

RθJA = (TR(MAX) / PD)

(5)

Figure 36. Power Dissipation Diagram

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