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CS5207A-1 データシート(PDF) 6 Page - Cherry Semiconductor Corporation |
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CS5207A-1 データシート(HTML) 6 Page - Cherry Semiconductor Corporation |
6 / 7 page 6 A rule of thumb useful in determining if a protection diode is required is to solve for current I = , where I is the current flow out of the load capacitance when VIN is shorted, C is the value of the load capacitance, V is the output voltage, and T is the time duration required for VIN to transition from high to being shorted. If the calculated current is greater than or equal to the typi- cal short circuit current value provided in the specifica- tions, serious thought should be given to including a pro- tection diode. Figure 3. Current Limit The internal current limit circuit limits the output current under excessive load conditions, and protects the regula- tor. Short Circuit Protection The device includes foldback short circuit current limit that clamps the output current at approximately two amperes less than its current limit value. Thermal Shutdown The thermal shutdown circuitry is guaranteed by design to become activated above a die junction temperature of 150¡C and to shut down the regulator output. This circuit- ry includes a thermal hysteresis circuit with 25¡C of typical hysteresis, thereby allowing the regulator to recover from a thermal fault automatically. High power regulators such the CS5207A-1 usually operate at high junction temperatures. Therefore, it is important to calculate the power dissipation and junction temperatures accurately to ensure that an adequate heat sink is used. Since the package tab is connected to Vout on the CS5207A-1, electrical isolation may be required for some applications. Also, as with all high power packages, ther- mal compound is necessary to ensure proper heat flow. For added safety, this high current LDO includes an internal thermal shutdown circuit The thermal characteristics of an IC depend on the follow- ing four factors. Junction temperature, ambient tempera- ture, die power dissipation, and the thermal resistance from the die junction to ambient air. The maximum junc- tion temperature can be determined by: TJ(max) = TA(max) + PD(max) ´ RQJA The maximum ambient temperature and the power dissi- pation are determined by the design while the maximum junction temperature and the thermal resistance depend on the manufacturer and the package type. The maximum power dissipation for a regulator is: PD(max) = (VIN(max) -VOUT(min))IOUT(max) + VIN(max) ´ IIN(max) A heat sink effectively increases the surface area of the package to improve the flow of heat away from the IC and into the surrounding air. Each material in the heat flow path between the IC and the outside environment has a thermal resistance which is measured in degrees per watt. Like series electrical resistances, these thermal resistances are summed to determine the total thermal resistance between the die junction and the surrounding air, RQJA. This total thermal resistance is comprised of three compo- nents. These resistive terms are measured from junction to case (RQJC), case to heat sink (RQCS), and heat sink to ambi- ent air (RQSA ). The equation is: RQJA = RQJC + RQCS + RQSA RQJC is rated @ 1.4¡C/W for the CS5207A-1. For a high cur- rent regulator such as the CS5207A-1 the majority of heat is generated in the power transistor section. The value for RQSA depends on the heat sink type, while the RQCS depends on factors such as package type, heat sink inter- face (is an insulator and thermal grease used?), and the contact area between the heat sink and the package. Once these calculations are complete, the maximum permissible value of RQJA can be calculated and the proper heat sink selected. For further discussion on heat sink selection, see our Cherry application note ÒThermal Management for Linear Regulators. Calculating Power Dissipation and Heat Sink Requirements V IN CS5207A-1 Adj V OUT C ´ V T Application Notes: continued |
同様の部品番号 - CS5207A-1 |
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同様の説明 - CS5207A-1 |
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