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LM1117-ADJ-220 Scheda tecnica(PDF) 9 Page - First Components International |
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LM1117-ADJ-220 Scheda tecnica(HTML) 9 Page - First Components International |
9 / 14 page APPLICATION NOTE (Continued) into the output of the regulator. The discharge current de- pends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of V IN. In the LM1117 regu- lators, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor ( ≥1000 µF), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an external diode is recommended between the output and input pins to protect the regulator, as shown in Figure 4. 5.0 Heatsink Requirements When an integrated circuit operates with an appreciable cur- rent, its junction temperature is elevated. It is important to quantify its thermal limits in order to achieve acceptable per- formance and reliability. This limit is determined by summing the individual parts consisting of a series of temperature rises from the semiconductor junction to the operating envi- ronment. A one-dimensional steady-state model of conduc- tion heat transfer is demonstrated in Figure 5. The heat gen- erated at the device junction flows through the die to the die attach pad, through the lead frame to the surrounding case material, to the printed circuit board, and eventually to the ambient environment. Below is a list of variables that may af- fect the thermal resistance and in turn the need for a heat- sink. RθJC(Component Vari- ables) RθCA (Application Vari- ables) Leadframe Size & Material Mounting Pad Size, Material, & Location No. of Conduction Pins Placement of Mounting Pad Die Size PCB Size & Material Die Attach Material Traces Length & Width Molding Compound Size and Material Adjacent Heat Sources Volume of Air Ambient Temperatue Shape of Mounting Pad The LM1117 regulators have internal thermal shutdown to protect the device from over-heating. Under all possible op- erating 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: I IN =IL +IG P D =(VIN-VOUT)I L +VINIG Figure 6 shows the voltages and currents which are present in the circuit. FIGURE 4. Regulator with Protection Diode 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 FIGURE 6. Power Dissipation Diagram 9 LM1117 800mA Low-dropout Linear Regulator |
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