LM317SSLASHNOPB datasheet(11/38 Pages) TI1

11 / 38 page

LM117, LM317-N

www.ti.com

SNVS774L – MAY 2004 – REVISED FEBRUARY 2011

D1 protects against C1

D2 protects against C2

Figure 27. Regulator with Protection Diodes

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 should not exceed the rated maximum junction

temperature (TJ) of 150°C for the LM117, or 125°C for the LM317A and 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 calculated:

PD = ((VIN − VOUT) × IL) + (VIN × IG)

(2)

Figure 28 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):

TR(MAX) = TJ(MAX) − TA(MAX)

(3)

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

LM317A/LM317-N), and TA(MAX) is the maximum ambient temperature which 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:

θJA = (TR(MAX) / PD)

(4)

Figure 28. Power Dissipation Diagram

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no

additional work is needed. If the calculated maximum allowable thermal resistance is lower than the actual

package rating either the power dissipation (PD) needs to be reduced, the maximum ambient temperature TA(MAX)

needs to be reduced, the thermal resistance (

θJA) must be lowered by adding a heatsink, or some combination of

these.

If a heatsink is needed, the value can be calculated from the formula:

θHA ≤ (θJA - (θCH + θJC))

(5)

Submit Documentation Feedback

Product Folder Links: LM117 LM317-N

LM317S/NOPB 데이터시트(PDF) 11 Page - Texas Instruments