LM1117/LM1117I 800mA Low-Dropout Linear Regulator

Application Note (Continued)LM1117/LM1117I1009193710091916FIGURE 5. Cross-sectional view of Integrated CircuitMounted on a printed circuit board. Note that the casetemperature is measured at the point where the leadscontact with the mounting pad surfaceThe LM1117 regulators have internal thermal shutdown toprotect the device from over-heating. Under all possibleoperating conditions, the junction temperature of the LM1117must be within the range of 0˚C to 125˚C. A heatsink may berequired depending on the maximum power dissipation andmaximum ambient temperature of the application. To determineif a heatsink is needed, the power dissipated by theregulator, P D , must be calculated:I IN =I L +I GP D =(V IN -V OUT )I L +V IN I GFigure 6 shows the voltages and currents which are presentin the circuit.FIGURE 6. Power Dissipation DiagramThe next parameter which must be calculated is the maximumallowable temperature rise, T R (max):T R (max) = T J (max)-T A (max)where T J (max) is the maximum allowable junction temperature(125˚C), and T A (max) is the maximum ambient temperaturewhich will be encountered in the application.Using the calculated values for T R (max) and P D , the maximumallowable value for the junction-to-ambient thermalresistance (θ JA ) can be calculated:θ JA =T R (max)/P DIf the maximum allowable value for θ JA is found to be≥136˚C/W for SOT-223 package or ≥79˚C/W for TO-220package or ≥92˚C/W for TO-252 package, no heatsink isneeded since the package alone will dissipate enough heatto satisfy these requirements. If the calculated value for θ JAfalls below these limits, a heatsink is required.As a design aid, Table 1 shows the value of the θ JA ofSOT-223 and TO-252 for different heatsink area. The copperpatterns that we used to measure these θ JA s are shown atthe end of the Application Notes Section. Figure 7 and Figure8 reflects the same test results as what are in the Table 1Figure 9 and Figure 10 shows the maximum allowable powerdissipation vs. ambient temperature for the SOT-223 andTO-252 device. Figures Figure 11 and Figure 12 shows themaximum allowable power dissipation vs. copper area (in 2 )for the SOT-223 and TO-252 devices. Please see AN1028for power enhancement techniques to be used with SOT-223and TO-252 packages.*Application Note AN-1187 discusses improved thermal performanceand power dissipation for the LLP.TABLE 1. θ JA Different Heatsink AreaLayout Copper Area Thermal ResistanceTop Side (in 2 )* Bottom Side (in 2 ) (θ JA ,˚C/W) SOT-223 (θ JA ,˚C/W) TO-2521 0.0123 0 136 1032 0.066 0 123 873 0.3 0 84 604 0.53 0 75 545 0.76 0 69 526 1 0 66 477 0 0.2 115 848 0 0.4 98 709 0 0.6 89 6310 0 0.8 82 5711 0 1 79 5712 0.066 0.066 125 8911www.national.com