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CS8281YDP5 Datasheet(PDF) 4 Page - Cherry Semiconductor Corporation |
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CS8281YDP5 Datasheet(HTML) 4 Page - Cherry Semiconductor Corporation |
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4 / 6 page  The CS8281 is a micropower dual 5V regulator. All bias required to operate the internal circuitry is derived from the standby output, VOUT2. If this output experiences an over current situation and collapses, then VOUT1 will also collapse (see timing diagrams). If there is critical circuitry that must remain active under most conditions it should be connected to VOUT2. Any cir- cuitry that is likely to be subjected to a short circuit, e.g., circuitry outside the module, should be connected to VOUT1. Output capacitors are required for stability with the CS8281. Without them, the regulator outputs will oscillate. Actual size and type may vary depending upon the application load and temperature range. Capacitor effective series resistance (ESR) is also a factor in the IC stability. Worst- case is determined at the minimum ambient temperature and maximum load expected. Output capacitors can be increased in size to any desired value above the minimum. One possible purpose of this would be to maintain the output voltages during brief con- ditions of negative input transients that might be character- istic of a particular system. Capacitors must also be rated at all ambient temperatures expected in the system. To maintain regulator stability down to -40ûC, capacitors rated at that temperature must be used. More information on capacitor selection for Smart Regulatorsª is available in the Smart Regulator applica- tion note, Compensation for Linear Regulators. The ENABLE function controls VOUT1. When ENABLE is high, VOUT1 is on. When ENABLE is low, VOUT1 is off. The maximum power dissipation for a dual output regula- tor (Figure 1) is: PD(max) = {VIN(max)ÐVOUT1(min)}IOUT1(max)+ {VIN(max)ÐVOUT2(min)}IOUT2(max)+VIN(max)IQ (1) where: VIN(max) is the maximum input voltage, VOUT1(min) is the minimum output voltage from VOUT1, VOUT2(min) is the minimum output voltage from VOUT2, IOUT1(max) is the maximum output current for the appli- cation, IOUT2(max) is the maximum output current for the appli- cation, and IQ is the quiescent current the regulator consumes at both IOUT1(max) and IOUT2(max). Once the value of PD(max) is known, the maximum per- missible value of RQJA can be calculated: RQJA = (2) The value of RQJA can then be compared with those in the package section of the data sheet. Those packages with RQJA's less than the calculated value in equation 2 will keep the die temperature below 150¡C. In some cases, none of the packages will be sufficient to dissipate the heat generated by the IC, and an external heat sink will be required. Figure 1: Dual output regulator with key performance parameters labeled. 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 will have a thermal resistance. Like series electrical resistances, these resistances are summed to determine the value of RQJA. RQJA = RQJC + RQCS + RQSA (3) where: RQJC = the junctionÐtoÐcase thermal resistance, RQCS = the caseÐtoÐheat sink thermal resistance, and RQSA = the heat sinkÐtoÐambient thermal resistance. RQJC appears in the package section of the data sheet. Like RQJA, it too is a function of package type. RQCS and RQSA are functions of the package type, heat sink and the interface between them. These values appear in heat sink data sheets of heat sink manufacturers. Heat Sinks VIN VOUT2 IIN IQ Control Features } IOUT2 VOUT1 IOUT1 Smart Regulator 150¡C - TA PD Calculating Power Dissipation in a Dual Output Linear Regulator ENABLE External Capacitors General 4 Application Notes |
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