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CS5201-1GST3 Datasheet(PDF) 6 Page - Cherry Semiconductor Corporation |
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CS5201-1GST3 Datasheet(HTML) 6 Page - Cherry Semiconductor Corporation |
6 / 7 page Figure 3. Protection diode for large output capacitors. Since the CS5201-1 is a three terminal regulator, it is not possible to provide true remote load sensing. Load regula- tion is limited by the resistance of the conductors connect- ing the regulator to the load. For the adjustable regulator, the best load regulation occurs when R1 is connected directly to the output pin of the regulator as shown in Figure 4. If R1 is connected to the load, RC is multiplied by the divider ratio and the effective resistance between the regulator and the load becomes RC ´ () RC = conductor parasitic resistance Figure 4. Grounding scheme for the adjustable output regulator to min- imize parasitic resistance effects. The CS5201-1 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 CS5201-1, 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 given package type based on an average die size. For a high current regulator such as the CS5201-1 the majority of the heat is generated 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 RC VIN conductor parasitic resistance CS5201-1 VIN Adj RLOAD R1 R2 R1 + R2 R1 Output Voltage Sensing VOUT VIN CS5201-1 VIN Adj R1 R2 C1 VOUT C2 CAdj IN4002 (optional) 6 Applications Information: continued |
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