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LMS8117A Datasheet(PDF) 9 Page - Texas Instruments |
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LMS8117A Datasheet(HTML) 9 Page - Texas Instruments |
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9 / 18 page  Application Note (Continued) When the adjustable regulator is used (Figure 3), the best performance is obtained with the positive side of the resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regu- lation. For example, a 5V regulator with 0.05 Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05 Ω xI L. If R1 (=125 Ω) is con- nected near the load, the effective line resistance will be 0.05 Ω (1+R2/R1) or in this case, it is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to provide remote ground sens- ing and improve load regulation. 4.0 PROTECTION DIODES Under normal operation, the LMS8117A regulators do not need any protection diode. With the adjustable device, the internal resistance between the adjust and output terminals limits the current. No diode is needed to divert the current around the regulator even with capacitor on the adjust ter- minal. The adjust pin can take a transient signal of ±25V with respect to the output voltage without damaging the device. When a output capacitor is connected to a regulator and the input is shorted to ground, the output capacitor will discharge 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 LMS8117A regulators, 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 regu- lator 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 current, its junction temperature is elevated. It is important to quantify its thermal limits in order to achieve acceptable performance and reliability. This limit is determined by sum- ming the individual parts consisting of a series of tempera- ture rises from the semiconductor junction to the operating environment. A one-dimensional steady-state model of con- duction heat transfer is demonstrated in Figure 5. The heat generated 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 affect the thermal resistance and in turn the need for a heatsink. Rθ JC(Component Variables) Rθ CA (Application Variables) 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 10119619 FIGURE 3. Best Load Regulation using Adjustable Output Regulator 10119615 FIGURE 4. Regulator with Protection Diode www.national.com 8 |
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Similar Description - LMS8117A |
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