LM1117 datasheet(11/21 Pages) NSC

11 / 21 page

Application Note (Continued)

The LM1117 regulators have internal thermal shutdown to

protect the device from over-heating. Under all possible

operating conditions, the junction temperature of the LM1117

must be within the range of 0˚C to 125˚C. A heatsink may be

required depending on the maximum power dissipation and

maximum ambient temperature of the application. To deter-

mine if a heatsink is needed, the power dissipated by the

regulator, P

D , must be calculated:

IN =IL +IG

D =(VIN-VOUT)I L +VINIG

Figure 6 shows the voltages and currents which are present

in the circuit.

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

R(max):

R(max) = TJ(max)-TA(max)

where T

J(max) is the maximum allowable junction tempera-

ture (125˚C), and T

A(max) is the maximum ambient tem-

perature which will be encountered in the application.

Using the calculated values for T

R(max) and PD, the maxi-

mum allowable value for the junction-to-ambient thermal

resistance (

JA) can be calculated:

JA =TR(max)/PD

If the maximum allowable value for

is found to be

≥136˚C/W for SOT-223 package or ≥79˚C/W for TO-220

package or

≥92˚C/W for TO-252 package, no heatsink is

needed since the package alone will dissipate enough heat

to satisfy these requirements. If the calculated value for

falls below these limits, a heatsink is required.

As a design aid, Table 1 shows the value of the

JA of

SOT-223 and TO-252 for different heatsink area. The copper

patterns that we used to measure these

JAs are shown at

the end of the Application Notes Section. Figure 7 and Figure

8 reflects the same test results as what are in the Table 1

Figure 9 and Figure 10 shows the maximum allowable power

dissipation vs. ambient temperature for the SOT-223 and

TO-252 device. Figures Figure 11 and Figure 12 shows the

maximum allowable power dissipation vs. copper area (in

for the SOT-223 and TO-252 devices. Please see AN1028

for power enhancement techniques to be used with SOT-223

and TO-252 packages.

*Application Note AN-1187 discusses improved thermal per-

formance and power dissipation for the LLP.

TABLE 1.

JA Different Heatsink Area

Layout

Copper Area

Thermal Resistance

Top Side (in

2)*

Bottom Side (in

2)(

JA,˚C/W) SOT-223

(

JA,˚C/W) TO-252

0.0123

136

103

0.066

123

0.3

0.53

0.76

0.2

115

0.4

0.6

0.8

0.066

125

0.175

10091937

FIGURE 5. Cross-sectional view of Integrated Circuit

Mounted on a printed circuit board. Note that the case

temperature is measured at the point where the leads

contact with the mounting pad surface

10091916

FIGURE 6. Power Dissipation Diagram

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