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CS5203-3GDPR3 Datasheet(PDF) 5 Page - Cherry Semiconductor Corporation |
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CS5203-3GDPR3 Datasheet(HTML) 5 Page - Cherry Semiconductor Corporation |
5 / 6 page 5 Applications Information: continued Figure 2. Conductor parasitic resistance effects can be minimized with the above grounding scheme for fixed output regulators. The CS5203-3 linear regulator includes thermal shutdown and current limit circuitry to protect the device. High power regulators such as these usually operate at high junction temperatures so it is important to calculate the power dissipation and junction temperatures accurately to ensure that an adequate heat sink is used. The case is connected to VOUT on the CS5203-3, and electri- cal isolation may be required for some applications. Thermal compound should always be used with high cur- rent regulators such as these. The thermal characteristics of an IC depend on the follow- ing four factors: 1. Maximum Ambient Temperature TA (¡C) 2. Power dissipation PD (Watts) 3. Maximum junction temperature TJ (¡C) 4. Thermal resistance junction to ambient RQJA (C/W) These four are related by the equation TJ = TA + PD ´ RQJA (1) 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)ÐVOUT(min)}IOUT(max)+VIN(max)IQ (2) where VIN(max) is the maximum input voltage, VOUT(min) is the minimum output voltage, IOUT(max) is the maximum output current, for the application IQ is the maximum quiescent current at IOUT(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. Like series electrical resistances, these resistances are summed to determine RQJA, the total thermal resistance between the junction and the surrounding air. 1. Thermal Resistance of the junction to case, RQJC (¡C/W) 2. Thermal Resistance of the case to Heat Sink, RQCS (¡C/W) 3. Thermal Resistance of the Heat Sink to the ambient air, RQSA (¡C/W) These are connected by the equation: RQJA = RQJC + RQCS + RQSA (3) The value for RQJA is calculated using equation (3) and the result can be substituted in equation (1). The value for RQJC is 3.5ûC/W. For a high current regula- tor such as the CS5203-3 the majority of the heat is generat- ed in the power transistor section. The value for RQSA depends on the heat sink type, while RQCS depends on fac- tors such as package type, heat sink interface (is an insula- tor and thermal grease used?), and the contact area between the heat sink and the package. Once these calcula- tions 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 applica- tion note ÒThermal Management for Linear Regulators.Ó Calculating Power Dissipation and Heat Sink Requirements VOUT VIN CS5203-3 VIN RC RLOAD conductor parasitic resistance GND |
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