LM317SSLASHNOPB datasheet(28/48 Pages) TI1

28 / 48 page

LM117, LM317-N

SNVS774P – MAY 2004 – REVISED OCTOBER 2015

www.ti.com

Layout Example (continued)

Figure 37. Layout Example (TO-220)

11.3 Thermal Considerations

11.3.1 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 38 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 (

θJA) can be calculated with Equation 5:

θJA = (TR(MAX) / PD)

(5)

Submit Documentation Feedback

Product Folder Links: LM117 LM317-N

LM317S/NOPB Datasheet(PDF) 28 Page - Texas Instruments