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CS8371ET7 Scheda tecnica(PDF) 7 Page - Cherry Semiconductor Corporation |
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CS8371ET7 Scheda tecnica(HTML) 7 Page - Cherry Semiconductor Corporation |
7 / 8 page 7 Applications Notes: continued Calculating Power Dissipation in a Dual Output Linear Regulator The maximum power dissipation for a dual output regu- lator (Figure 1) is PD(max) = {VIN(max) Ð VOUT1(min)}IOUT1(max) + {VIN(max) Ð VOUT2(min)}IOUT2(max) + VIN(max)IQ, (1) where VIN(max) is the maximum input voltage, VOUT 1(min) is the minimum output voltage from VOUT1, VOUT 2(min) is the minimum output voltage from VOUT2, IOUT 1(max) is the maximum output current, for the appli- cation, IOUT 2(max) is the maximum output current, for the appli- cation, IQ is the quiescent current the regulator consumes at IOUT(max). Once the value of PD(max) is known, the maximum permis- sible value of R QJA can be calculated: R QJA = (2) The value of R QJA can then be compared with those in the package section of the data sheet. Those packages with R QJA'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 heatsink will be required. Heatsinks A heatsink 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 R QJA: R QJA = RQJC + RQCS + RQSA, (3) where R QJC = the junctionÐtoÐcase thermal resistance, R QCS = the caseÐtoÐheatsink thermal resistance, and R QSA = the heatsinkÐtoÐambient thermal resistance. R QJC appears in the package section of the data sheet. Like R QJA, it too is a function of package type. RQCS and RQSA are functions of the package type, heatsink and the inter- face between them. These values appear in heatsink data sheets of heatsink manufacturers. 150¡C - T A PD VIN VOUT2 IIN IQ Control Features } IOUT2 VOUT1 IOUT1 Smart Regulator Figure 1: Dual output regulator with key performance parameters labeled. |
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